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Characterization of RScO3, LuFe2O4 and M72Fe30 based molecules by x-ray spectroscopic techniques Dissertation zur Erlangung des Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) dem Fachbereich Physik der Universität Osnabrück vorgelegt von Christine Derks, Dipl. Phys. Osnabrück im Dezember 2012 ii Supervisor, first reviewer: apl. Prof. Prof. h.c. Dr. Dr. h.c. Manfred Neumann Second reviewer: Prof. Dr. Joachim Wollschläger Universität Osnabrück Fachbereich Physik AG Elektronenspektroskopie Barbara Str. 7 D-49069 Osnabrück iii iv ”Wie konnte uns das Alles nur passieren!” vi Table of Contents Introduction 1 Experimental Methods and Theory 1.1 Basics of X-ray Spectroscopy . . . . . . . . . . . . . . . . . . . . 1.1.1 X-ray Photoelectron Spectroscopy (XPS) . . . . . . . . . 1.1.2 Effects in Electron Spectroscopy . . . . . . . . . . . . . . 1.1.2.1 Exchange Splitting . . . . . . . . . . . . . . . . 1.1.2.2 Spin-orbit Coupling . . . . . . . . . . . . . . . 1.1.2.3 Satellites . . . . . . . . . . . . . . . . . . . . . 1.1.2.4 Multiplet Splitting . . . . . . . . . . . . . . . . 1.1.2.5 Auger Electrons . . . . . . . . . . . . . . . . . 1.1.3 X-ray Absorption Spectroscopy (XAS) . . . . . . . . . . 1.1.4 X-ray Emission Spectroscopy (XES) . . . . . . . . . . . . 1.1.4.1 Resonant X-ray Emission Spectroscopy (RXES) 1.1.5 X-ray Magnetic Circular Dichroism (XMCD) . . . . . . . 1.2 Principles of Multiplet Theory . . . . . . . . . . . . . . . . . . . 1.2.1 Single-particle Approximation . . . . . . . . . . . . . . . 1.2.2 Multiplet Effects . . . . . . . . . . . . . . . . . . . . . . 1.2.2.1 Atomic Multiplet Theory . . . . . . . . . . . . 1.2.2.2 Ligand-field Multiplet Theory . . . . . . . . . . 1.2.2.3 Charge-transfer Multiplet Theory . . . . . . . . 1.3 Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 The Photoelectron Spectrometer PHI 5600ci . . . . . . . 1.3.2 The Advanced Light Source (ALS) . . . . . . . . . . . . 1.3.2.1 Beamline 8.0.2 at the ALS . . . . . . . . . . . . 1.3.2.2 Beamline 4.0.2 at the ALS . . . . . . . . . . . . 1.3.3 The Swiss Light Source (SLS) . . . . . . . . . . . . . . . 1.3.3.1 TBT-XMCD Endstation at the SLS . . . . . . 1.3.4 Bessy II . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.4.1 Russian German Dipole Beamline . . . . . . . . 1.3.5 European Synchrotron Radiation Facility (ESRF) . . . . 1.3.5.1 ID12 Circular Polarisation Beamline . . . . . . 1.3.6 Superconducting Quantum Interference Device - SQUID 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5 5 8 8 9 10 10 11 12 13 14 14 17 17 18 18 19 20 21 21 22 24 25 25 25 25 25 26 26 26 vii Table of Contents 2 RScO3 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Basic Properties and Preparation of RScO3 (R=Pr, Nd, Sm, Eu, Gd, Tb and Dy) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Core Level XPS of Rare-Earth, Scandium and Oxygen . . . . . . . . . 2.3.1 XPS of Scandium and Oxygen . . . . . . . . . . . . . . . . . . . 2.3.2 XPS of the Rare-Earth . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 XAS and XES/RIXS of R, Sc and O . . . . . . . . . . . . . . . . . . . 2.4.1 R M4,5 -Edges XAS and R 4f → 3d XES . . . . . . . . . . . . . 2.4.2 Sc L2,3 -Edges XAS and Sc 3d → 2p XES . . . . . . . . . . . . . 2.4.3 O K-Edge XAS and O 2p → 1s XES . . . . . . . . . . . . . . . 2.4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 27 3 LuFe2 O4 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . 3.2 Basic Properties of LuFe2 O4 . . . . . . . . . . . . . 3.3 Preparation of LuFe2 O4 . . . . . . . . . . . . . . . 3.4 XMCD of Iron L2,3 -edges in LuFe2 O4 . . . . . . . . 3.5 XAS and XMCD of Iron K-edge in LuFe2 O4 . . . . 3.6 XAS and XMCD of Lutetium L2,3 -edges in LuFe2 O4 3.7 SQUID and XMCD Hysteresis . . . . . . . . . . . . 3.8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 51 53 53 53 55 59 60 60 . . . . . . . . . . . . . . . . . 63 63 64 64 64 66 67 68 70 70 71 71 74 77 77 79 79 79 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Iron-Based Magnetic Polyoxometalates 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Core Level XPS of Iron, Oxygen, Molybdenum and Wolfram . . . . 4.2.1 Specific Experimental Details . . . . . . . . . . . . . . . . . 4.2.2 Iron Core Levels . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 Oxygen Core Levels . . . . . . . . . . . . . . . . . . . . . . . 4.2.4 Wolfram and Molybdenum Core Levels . . . . . . . . . . . . 4.2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 XAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Specific Experimental and Theoretical Details . . . . . . . . 4.3.2 Iron L2,3 -edges . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2.1 Mo72 Fe30 acetate and W72 Fe30 sulfate at the ALS . 4.3.2.2 Mo72 Fe30 acetate and Mo72 Fe30 sulfate at the Bessy 4.3.3 Oxygen K-edge . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Magnetic Measurements on W72 Fe30 sulfate . . . . . . . . . . . . . 4.4.1 Specific Experimental and Theoretical details . . . . . . . . 4.4.2 SQUID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii . . . . . . . . . . . . . . . . . . . . . . . . . 28 30 30 32 34 36 36 42 45 49 Table of Contents 4.4.3 4.4.4 XMCD at the Iron L2,3 -edges of W72 Fe30 sulfate . . . . . . . . . 80 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 5 Conclusion 83 Fazit 85 Danksagung 87 References 89 List of Publications 101 ix x List of Figures 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10 1.11 Schematic representation of XPS and UPS processes . Principle of XPS . . . . . . . . . . . . . . . . . . . . Energy level diagram for an XPS experiment . . . . . Principle of the Auger electron emission . . . . . . . Schematic representation of XAS . . . . . . . . . . . Schematic representation of XES . . . . . . . . . . . Schematic representation of REXS . . . . . . . . . . Schematic representation of XMCD . . . . . . . . . . Scheme of the XPS spectrometer . . . . . . . . . . . Scheme of the ALS . . . . . . . . . . . . . . . . . . . Scheme of the ALS beamline . . . . . . . . . . . . . . 2.1 Orthorhombic RScO3 crystal structure; there is an octahedron tilting about [001]p , [110]p and [111]p respectively. . . . . . . . . . . . . . . . . O 1s XPS spectra of RScO3 (R=Pr, Nd, Sm, Eu, Gd, Tb and Dy) . . . Sc 2p XPS spectra of RScO3 (R=Pr, Nd, Sm, Eu, Gd, Tb and Dy) (red); the reference Sc2 O3 (black) was taken from Chastain et al. [1] . . Pr 3d XPS spectra of PrScO3 . . . . . . . . . . . . . . . . . . . . . . . Nd 3d XPS spectra of NdScO3 (red); the reference Nd2 O3 (black) was taken from Suzuki et al. [2] . . . . . . . . . . . . . . . . . . . . . . . . Sm 3d XPS spectra of SmScO3 . . . . . . . . . . . . . . . . . . . . . . Eu 3d XPS spectra of EuScO3 . . . . . . . . . . . . . . . . . . . . . . . Gd 3d XPS spectra of GdScO3 (red); the reference Gd2 O3 (black) was taken from Lütkehoff [3] . . . . . . . . . . . . . . . . . . . . . . . . . . Tb 3d XPS spectra of TbScO3 . . . . . . . . . . . . . . . . . . . . . . . Dy 3d XPS spectra of DyScO3 . . . . . . . . . . . . . . . . . . . . . . . XAS at the Pr M4,5 -edges of PrScO3 . . . . . . . . . . . . . . . . . . . Pr 4f → 3d of PrScO3 . . . . . . . . . . . . . . . . . . . . . . . . . . . XAS at the Nd M4,5 -edges of NdScO3 . . . . . . . . . . . . . . . . . . . Nd 4f → 3d of NdScO3 . . . . . . . . . . . . . . . . . . . . . . . . . . XAS at the Sm M4,5 -edges of SmScO3 taken from [4] . . . . . . . . . . Sm 4f → 3d XES of SmScO3 taken from [4] . . . . . . . . . . . . . . . XAS at the Eu M4,5 -edges of EuScO3 . . . . . . . . . . . . . . . . . . . Eu 4f → 3d XES of EuScO3 . . . . . . . . . . . . . . . . . . . . . . . 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 7 8 11 12 13 14 15 21 23 24 29 31 31 32 32 33 33 34 34 35 37 37 38 38 39 39 40 40 xi List of Figures 2.19 2.20 2.21 2.22 2.23 2.24 2.25 2.26 2.27 2.28 2.29 2.30 2.31 2.32 2.33 2.34 2.35 2.36 3.1 3.2 3.3 3.4 3.5 3.6 xii XAS at the Gd M4,5 -edges of GdScO3 taken from [4] . . . . . . . . . . Gd 4f → 3d XES of GdScO3 taken from [4] . . . . . . . . . . . . . . . XAS at the Tb M4,5 -edges of TbScO3 . . . . . . . . . . . . . . . . . . . Tb 4f → 3d of TbScO3 . . . . . . . . . . . . . . . . . . . . . . . . . . XAS at the Dy M4,5 -edges of DyScO3 taken from [4] . . . . . . . . . . . Dy 4f → 3d of DyScO3 taken from [4] . . . . . . . . . . . . . . . . . . XAS spectra taken at Sc L2,3 -edges of RScO3 (R=Pr, Nd, Sm, Eu, Gd, Tb and Dy) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XES spectra of the Sc L2,3 -edges of PrScO3 and EuScO3 with EExc = 419.3 eV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XES spectra of the Sc L2,3 -edges of NdScO3 and TbScO3 with EExc = 403.7 eV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XES spectra of the Sc L2,3 -edges of SmScO3 , GdScO3 and DyScO3 with EExc = 420.2 eV taken from [4] . . . . . . . . . . . . . . . . . . . . . . O K-edge XAS and O 2p → 1s XES of PrScO3 . . . . . . . . . . . . . O K-edge XAS and O 2p → 1s XES of NdScO3 . . . . . . . . . . . . . O K-edge XAS and O 2p → 1s XES of SmScO3 . . . . . . . . . . . . . O K-edge XAS and O 2p → 1s XES of EuScO3 . . . . . . . . . . . . . O K-edge XAS and O 2p → 1s XES of GdScO3 . . . . . . . . . . . . . O K-edge XAS and O 2p → 1s XES of TbScO3 . . . . . . . . . . . . . O K-edge XAS and O 2p → 1s XES of DyScO3 . . . . . . . . . . . . . a) Tolerance factor (Goldschmidt factor) for RScO3 , b) Sc−O−Sc bond angles of RScO3 , c) Distances between Sc and the two oxygen spectra, d) Sc−O mean distance and experimental band gaps. All structural parameters have been extracted from Liferovich et al. [5]. . . . . . . . . 40 40 41 41 41 41 Multiferroic triangle . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crystal structure of LuFe2 O4 with Lu (large dark-gray spheres), Fe (small black spheres) and O (large white spheres). Taken from Subramanian et al. [6]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LuFe2 O4 Fe L2,3 -edges XMCD performed at 150K (black), the belonging dichroic signal is green (a). It is compared to multiplet calculations considering different possible spin orderings (b and c). The data are taken from Kuepper et al. [7]. . . . . . . . . . . . . . . . . . . . . . . . XMCD measurement (dark green) and XAS measurement (light green) of the Fe K-edge on LuFe2 O4 performed at 125K and 6T. . . . . . . . . XMCD measurements of the Fe K-edge on LuFe2 O4 at different temperatures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XMCD Pre-edge measurements (green) at the Fe K-edge of LuFe2 O4 compared to multiplet-calculation (black). . . . . . . . . . . . . . . . . 51 42 44 44 44 45 45 46 46 47 47 48 50 54 55 56 57 57 List of Figures 3.7 XMCD (orange) and XAS (green) measurements on the Lu L2,3 -edges of LuFe2 O4 . The temperature was 150K and the external applied magnetic field had 9T. The k was parallel to the c-axis of the crystal. . . . . . . . 59 3.8 SQUID measurements (blue) compared to XMCD measurements (green), both measurements are recorded at 150K. . . . . . . . . . . . . . . . . 61 4.1 XPS measurements of Fe 2p (left panel) and Fe 3s (right panel) of W72 Fe30 sulfate (red) in comparison with Fe2+ to Fe3+ reference compounds (black). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 4.2 XPS measurements of Fe 2p of Mo72 Fe30 sulfate (left panel, red) and Mo72 Fe30 acetate (right panel, red) in comparison with Fe2+ to Fe3+ reference compounds (black). . . . . . . . . . . . . . . . . . . . . . . . 66 4.3 XPS measurements of O 1s of W72 Fe30 sulfate (a), Mo72 Fe30 sulfate (b) and Mo72 Fe30 acetate (c). . . . . . . . . . . . . . . . . . . . . . . . . . 67 4.4 XPS measurement of W 4f of W72 Fe30 sulfate. . . . . . . . . . . . . . . 68 4.5 XPS measurement of Mo 3d of Mo72 Fe30 acetate (left) and Mo72 Fe30 sulfate (right). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 4.6 Fe L2,3 -edges XAS series of 1 (left) and 2 (right). The experimental data are green and the black lines represent the corresponding simulated spectra, which were obtained by superimposing corresponding fractions of the simulated Fe3+ and Fe2+ spectra. . . . . . . . . . . . . . . . . . . 73 4.7 Fraction of Fe3+ versus the percentage x-ray photon flux for the XAS series shown in figure 4.6. . . . . . . . . . . . . . . . . . . . . . . . . . 73 4.8 First (left) and second (right) series of Fe L2,3 -edges of molecule 3. The green lines represent the experimental data and the black lines represent the corresponding simulated spectra that were obtained by superimposing corresponding fractions of simulated Fe2+ and Fe3+ spectra. . . . . 74 4.9 Series of Fe L2,3 -edges of molecule 1 in green and simulated spectra in black. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 4.10 Fraction of Fe3+ versus the percentage x-ray photon flux for the XAS series shown in figure 4.9 and 4.8. . . . . . . . . . . . . . . . . . . . . . 75 4.11 Top: O K edge XAS of 1 (left) and 3 (right). The spectra were taken at a low photon flux and a fresh spot of the corresponding sample. Bottom: Calculated unoccupied densities of states for Mo72 Fe30 acetate (left) and W72 Fe30 sulfate (right). . . . . . . . . . . . . . . . . . . . . . 78 4.12 SQUID measurements of W72 Fe30 sulfate from -6T up to 6T at 2 (orange),5 (green) and 15K (black) . . . . . . . . . . . . . . . . . . . . . . 80 xiii List of Figures 4.13 XMCD measurement of the Fe L2,3 -edges on W72 Fe30 sulfate. The experimental results are plotted on the top of the graph. The paddle in the middle shows the experimental dichroic signal (green) compared to CT multiplet simulations (black). In the panel on the bottom the experimental XAS signal (green) is compared to CT multiplet simulations (black). The measurements were made at 6.5T and 0.7K. . . . . . . . . 82 xiv List of Tables 2.1 2.2 2.3 Structural parameters of RScO3 taken from Liferovich et al. [5] . . . . 29 Measured inter peak separations (in eV ± 0.2 eV) in the Sc L2,3 XAS spectra of Fig. 2.25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Band gaps of rare-earth scandates (in eV) as found in the present work (the upper line) in comparison with previously reported values. Since we applied the identical equivalent experimental conditions, the relative error bars are ± 0.1-0.2 eV; the absolut error could be larger. . . . . . . 47 3.1 Slater integrals (in eV) used for the Fe2+ and Fe3+ charge-transfer multiplet simulations of the Fe K-edge XAS. . . . . . . . . . . . . . . . . . 58 4.1 Slater integrals (in eV) used for the Fe2+ and Fe3+ -charge-transfer multiplet simulations of the Fe L2,3 -edges XAS. The spin-orbit parameters were not reduced, whereas the d-d and p-d integrals were reduced to 80% of the Hartree-Fock values for the subsequent simulation of the spectra. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 Fraction of Fe3+ after x-ray radiation with different intensities. . . . . . 72 Fraction of Fe3+ after x-ray radiation at the Bessy . . . . . . . . . . . . 76 4.2 4.3 xv xvi List of Abbreviations ALS Advanced Light Source BESSY Berliner Elektronen Speicherring CT Charge-Transfer DOS Density Of States ESRF European Synchrotron Radiation Facility R Rare-Earth REXS Resonant Elastic X-ray Scattering RIXS Resonant Inelastic X-ray Scattering RXES Resonant X-ray Emission Spectroscopy SLS Swiss Light Source SXF Soft X-ray Fluorescence SXPS Soft X-ray Photoelectron Spectroscopy TEY Total Electron Yield TFY Total Fluorescence Yield UPS Ultraviolet Photoeletron Spectroscopy XAS X-ray Absorption Spectroscopy XES X-ray Emission Spectroscopy XMCD X-ray magnetic circular dichroism XPS X-ray Photoelectron Spectroscopy xvii xviii Introduction Over the last few decades computers and related electronic equipment have become more and more powerful and faster. Simultaneously their basic submits, i.e. transistors and hard discs (in particular the magnetic domain size) have shrunken dramatically. As a consequence the CPU speed and the hard disc storage capability both gained several magnitudes of order. Nowadays engineers as well as researchers seek new advanced materials in order to continue the miniaturization of electronic devices down to the nano- or molecular scale. Among several others, two specific material classes appear to be of special interest for potential future nanoelectric devices, namely: (i) (Transition) metal oxide based solid state electronic devices and (ii) (Transition) metal oxide based molecular electronic devices. Both kind of materials are well promising candidates for a number of potential applications in novel (nano)electric devices since several electron quantum and correlation effects dominate their intrinsic properties down to the atomic scale. Transition metal based oxides and molecules display an enormous range of electronic transport phenomena, leading to a unique variety of electrical, magnetic and optical properties. This includes fascinating collective ordering phenomena such as superconductivity, colossal magneto resistance, or the simultaneous existence of more than one ferroic phase, e.g. ferroelectricity and ferromagnetism. These exceptional electronic properties are dominated by the intricate interplay between the 3d electrons degrees of freedom (spin, charge, and orbital), and their interaction with the 2p ligand states. A detailed knowledge of the underlying electronic structure and the subsequent electron correlation effects is of utmost importance for the interpretation and understanding of a transition metal based oxide or molecule as a functional part in a potential new electronic device. The detailed description of the electronic properties of transition metal based compounds is one of the challenges in nowadays state of the art condensed matter physics and chemistry since the results of first principles band structure calculations are often inconsistent with experimental findings due to the mentioned 3d-3d and 3d-2p electron correlation effects. However, from experimental point of view, x-ray spectroscopic techniques are extremely powerful tools for electronic structure investigations of a compound in question. Namely, X-ray Photoelectron Spectroscopy (XPS) and X-ray Emission Spectroscopy (XES) are well established techniques in order to tackle the occupied densities of states (valence band) as well as chemical valence states of transition metal ions, for 1 Introduction instance. At the other side, X-ray Absorption Spectroscopy (XAS) at transition metal L2,3 -edges allows to probe the local coordination (crystal field) of transition metal ions in ionic compounds, and O K-edge spectroscopy gains information about the unoccupied densities of states (conduction band), respectively. Furthermore, X-ray Magnetic Circular Dichroism (XMCD) can be used for an element specific investigation of the magnetic properties, including the possibility to separate the magnetic moments into their spin and orbital contributions. This thesis deals with a detailed electronic structure study of three particularly interesting transition metal based compounds of both above mentioned material classes, which exhibit fascinating properties from fundamental as well as from potential applicative point of view. The materials investigated in depth within this thesis are the rare earth scandates RScO3 (R=Pr, Nd, Sm, Eu, Gd, Tb and Dy) the potential multiferroic and “ferroelectric” layered ferrite LuFe2 O4 , and magnetically frustrated molecules with Mo72 Fe30 and W72 Fe30 cores. For all three kinds of compound various complementary x-ray spectroscopic techniques have been applied, and the results are compared with other experimental probes as well as with different theoretical approaches, namely ab initio electronic structure calculations and full (charge transfer) multiplet simulations. An advanced understanding of the underlying electronic and magnetic structure is developed and discussed for each of the above mentioned kind of compounds. This thesis is structured as following: • Chapter 1 introduces in the details and features of the employed experimental techniques. This part is meant to give a rather complete but descriptive introduction to any reader which is not familiarized with this kind of measurements. The cited references account for the state of the art development in this field and can serve as important further literature. Additionally the basics of multiplet theory are presented and in the end the used experimental facilities are shown. • In Chapter 2 the electronic structure of rare-earth scandates of type RScO3 (R=Pr, Nd, Sm, Eu, Gd, Tb and Dy) is investigated in deep detail. These compounds are well promising candidates in order to replace SiO2 as a high-k gate dielectric on future metal-oxide-semiconductor field effect transistors (MOSFETs). Within this thesis a complete electronic structure investigation of a series of seven RScO3 (R=Pr, Nd, Sm, Eu, Gd, Tb and Dy) has been performed. XPS, XES and XAS have been used, and the experimental results are compared with ab initio band structure calculations. • An example of magneto electric coupling is presented in Chapter 3. The layered oxide LuFe2 O4 exhibits ferroelectric and ferrimagnetic properties due to its crystalline structure. A large response of the dielectric constant by applying relatively small magnetic fields has been reported, opening a potential avenue toward novel magneto electric devices. Here XMCD has been used as a probe 2 at the Fe L2,3 , Lu L2,3 and Fe K-edges in order to investigate the fine details of the relationship between electronic structure and magnetic properties in this compound. The results of the Fe L and Fe K-edges are compared to each other and partly simulated by means of full multiplet calculations. XMCD at the Lu L-edges is used to study the Lu 5d-induced magnetic moment. Finally this magnetic structure study is completed by SQUID measurements. • In the last part of this thesis the focus moves toward the second class of material mentioned above. In Chapter 4 a complete picture of the electronic structure of molecules with Mo72 Fe30 and W72 Fe30 cores is developed. From fundamental point of view these molecules are hosting a highly symmetric array of 30 magnetic Fe ions, building up a frustrated magnetic system. This type of molecule is furthermore an interesting prototype for potential applications in molecular spintronics and quantum computing. Within this thesis XPS, XAS, and XMCD have been used as experimental techniques and compared to complementary SQUID measurements and theoretical results of the above mentioned approaches. Furthermore, an x-ray induced Fe3+ to Fe2+ photoreduction process has been investigated. • Chapter 5 summarizes the main findings of this thesis. 3 4 1 Experimental Methods and Theory In this chapter the experimental methods to analyse the samples and their theoretical background are presented. The introduced methods are X-ray Phototelectron Spectroscopy (XPS 1.1.1), X-ray Absorption Spectroscopy (XAS 1.1.3), X-ray Emission Spectroscopy (XES 1.1.4) with the specification of Resonant X-ray Emission Spectroscopy (RXES 1.1.4.1) as well as X-ray Magnetic Circular Dichroism (XMCD 1.1.5). In section 1.2 the basic principles of multiplet calculations are presented. The setup of the used experimental equipment is presented in this chapter’s last section (1.3). 1.1 Basics of X-ray Spectroscopy The basic principle of photoelectron spectroscopy is the photoelectric effect which was discovered in 1887 by H. Hertz [8]. He noticed that a metal plate discharges much faster when irritated with light. The following year his assistant W. Hallwachs [9] found out that the velocity of the discharge depends on the used material, the wavelength and the intensity of the light. An explanation for this phenomena was given by A. Einstein in 1905 when he published his quantum hypothesis for electromagnetic radiation [10]. This was the beginning of the theory’s development of photoelectron spectroscopy. 1.1.1 X-ray Photoelectron Spectroscopy (XPS) The photoelectron spectroscopy is based on the photo effect described in section 1.1. The basic configuration of photoelectron spectroscopy consists of a light source, a sample and a detector. This basic configuration can be seen in figure 1.2. The x-ray source sends radiation toward a sample. The sample’s electrons absorb this energy and if it is large enough an electron will be animated to leave the atom. The kinetic energy Ekin of this so called photoelectron will be detected and allows a conclusion from the binding energy of the electron to the kind of the atom. The quantum light hypothesis leads to equation 1.1. Ekin = hν − Φsolid (1.1) In this case h is the Planck’s constant, ν the frequency of light and Φ describes the work function which is the energy an electron has to overcome to leave the atom. The equation 1.1 only works for electrons at the Fermi level. For electrons near the 5 1 Experimental Methods and Theory Figure 1.1: Schematic representation of XPS and UPS processes nucleus and with higher binding energy the equation 1.1 needs to be expanded with the effective binding energy EB,eff . That leads to the equation 1.2 Ekin = hν − EB,eff − Φsolid (1.2) One is interested in EB,eff , the effective binding energy of the emitted electron, so the equation 1.2 will be transformed to equation 1.3. EB,eff = hν − Ekin − Φsolid (1.3) In figure 1.3 the following used symbols are shown, so it will be much easier to understand the next explanations by looking at the figure. There are two different cases. The first one is that the spectrometer limits the kinetic energy and the second is that the solid limits. The first case is the one which will be employed throughout this thesis, so it will be described in detail. The explanations are for a conductive sample, which are connected to the spectrometer. So the Fermi levels of sample and spectrometer are equal. 0 The detected kinetic energy of the emitted electron is Ekin and depends on Ekin due to the following equations: ∆Φ = Φsolid − Φspectrometer 6 (1.4) 1.1 Basics of X-ray Spectroscopy electron energy analyzer X-ray source X-rays electron counter (detector) photoelectron sample Figure 1.2: Principle of XPS 0 Ekin = = = 0 =⇒ Ekin = Ekin + ∆Φ Ekin + (Φsolid − Φspectrometer ) (hν − EB,eff − Φsolid ) + (Φsolid − Φspectrometer ) hν − EB,eff − Φspectrometer Here Φspectrometer is the work function of the spectrometer and is well known in contrast to the work function of the solid (Φsolid ). That makes it possible to calculate the effective binding energy of the electron. The binding energy of an electron depends on the number of protons and electrons of an atom. Fortunately the binding energy of an electron is like a fingerprint, so it is possible to calculate from which element and shell the electron comes. The valency of an atom leads to a chemical energy shift so that the valency can also be determined. There are several different methods based on the photoemission principle. When the excitation energy hν is smaller than 100 eV the method is called Ultraviolet Photoelectron Spectroscopy (UPS), when hν is between 100 eV and 1000 eV it is Soft X-ray Photoelectron (SXPS). In this work X-ray Photoelectron Spectroscopy (XPS) with hν > 1000 eV is used. The used excitation energies in this work are 1486.6 eV (Al Kα ) and 1253.6 eV (Mg Kα ). For more details see section 1.3.1. The penetration depth of these photons in a solid sample is in the order of 110 micrometers, although XPS is a surface sensitive method because the electron 7 1 Experimental Methods and Theory emission depth is around 10 Å. Due to the small mean free path XPS measurements have to be in ultra-high vacuum (UHV) otherwise the photoelectrons could not reach the analyzer and the surface would be polluted. Further information about the used UHV techniques and the analyser are given in section 1.3.1. XPS is one of the x-ray spectroscopic methods which gives the total density of states (tDOS). Figure 1.3: Energy level diagram for an XPS experiment 1.1.2 Effects in Electron Spectroscopy The principles of electron spectroscopy are described in section 1.1.1. But during an experiment there are several effects which one has to bear in mind to come to the correct conclusions. In the following section typical effects in electron spectroscopy and their origin are described. 1.1.2.1 Exchange Splitting Two atoms bond by interaction of their valence electrons. This bonding leads to a change in the electric environment of the atoms and changes the electric potential. Due to this change the binding energy of the electrons including the core level electrons 8 1.1 Basics of X-ray Spectroscopy change. This changed binding energy can be measured and information about the chemical bonding of the atoms can be achieved. To get information about the kind of binding reference measurements from known materials are used. It is very difficult to determine the so called chemical shift theoretically, because there are too many interacting factors. It is mainly influenced by the kind of binding and the neighboring atoms. An example for the chemical shift of Fe 2p is shown at the PhD Thesis of Küpper [11] in section 2.3.2.3. One theoretical possibility to describe the chemical shift is the chargepotential model. In this model the effective binding energy EB,eff depends on the potentials created by the valence electrons of the observed atom and the environmental electrons. EB,eff is described in equation 1.5. EB,eff = EB,atom + ∆(Echem + EMad ) ∆EMad = P B6=A qB RAB (1.5) is the so called Madelung term, which describes the influence of the surrounding atoms in the bulk. ∆Echem = KqA is the chemical shift which is connected to the potentials of the valence electrons of the observed atom. The interaction between the valence and the core electrons is here called K and the shift relative to a reference state is denoted by qA . Thus the overall chemical shift can be represented by: EB,eff = EB, atom + KqA + X qB . R AB B6=A (1.6) 1.1.2.2 Spin-orbit Coupling There are several quantum numbers to describe electronic levels of an electron. First there is the quantum number n [n=1,2,...]. n declares the shell the electron is placed on. The electron spin s [s= 12 , − 12 ] and the angular momentum l[l=0,1,2,...,n-1]. The total angular momentum j (j > 0) is the sum of the spin s and the angular momentum l. The interaction between spin and orbital angular momentum is called Spin-orbit coupling and so each core level line is a doublet in the XPS-spectra for l > 0, because j = l + s or j = l − s. Instead of l = 0, 1, 2, 3 one often uses s,p,d,f. The official nomenclature is nlj . For example a level with n = 2, l = 1 and s = 21 leads to j = 1 + 12 = 32 and is called 2p 3 . The relative intensity of the two peaks of a doublet 2 is given by: Ij=l+s l+1 = Ij=l−s l (1.7) 9 1 Experimental Methods and Theory 1.1.2.3 Satellites During a perfect photoemission process a photoelectron leaves the atom so fast, that the remaining electrons do not have time to readjust. But during a real experiment it is possible that the photoelectron interacts with the exited state (N-1 electron) of an atom, that leads to additional lines in the spectra, called satellites. There are two different types of satellites. On the one hand the extrinsic satellites which come from inter-atomic excitations and on the other hand the intrinsic satellites are the result of intra-atomic relaxations. During a photoemission process it is possible that a second electron will be excited. If it stays bound to the atom but on a higher level it is called shake up satellite, and if the electron leaves the atom it is called shake off satellite. This shake off electron has a lower kinetic energy than for a direct excitation because of the higher binding energy in an (N-1) electron system. Transition metal oxides have an extra satellite due to charge-transfer. In this case an electron from the ligand 2p level will be transfered to the metal 3d shell: 3dn L −→ 3dn+1 L−1 . The required energy is given in 1.8 ∆ = E(3dn+1 L−1 ) − E(3dn L) (1.8) 1.1.2.4 Multiplet Splitting A core-level electron which will be released from a system with unpaired electrons in the valence level keeps information about the spins of the electrons which have left the system. The 3s level is currently the only system, in which one can interpret the information because other systems have too many additional interactions. By the 3s core-level the unpaired electron left in this shell interacts with the spin of the electrons in the valence band levels. They couple parallel or anti parallel. The binding energy of the photoelectron depends on the coupling. This causes a splitting of the core level line and the exchange splitting (∆Es ) can be written according to the van Vleck theorem [12]: ∆Es = S+1 2 G (3s, 3d) 2l + 1 (1.9) G2 (3s, 3d) is the Slater exchange integral and l the orbital quantum number (l=2). The binding energy of the state with (S + 12 ) is lower than the binding energy corresponding to (S − 12 ). This leads to a doublet in the spectrum and to the intensity ratio of the two peaks is given by: IS+ 1 2 IS− 1 2 = S+1 S (1.10) In 1970 Fadley et al. found that the van Vleck theorem does not work in all cases. The ratio of the intensity (equation 1.10) was about two times smaller than expected 10 1.1 Basics of X-ray Spectroscopy [13, 14]. Bagus et al. [15] associated this to intra atomic near-degeneracy correlation effects. Today the treatment of the 3s multiplet splitting is based upon full multiplet calculations [16]. For l 6= 0 the multiplet splitting is more complex because an additional spin-orbit splitting occurs in the spectra. 1.1.2.5 Auger Electrons An emitted photoelectron leaves a hole in the atom, this hole will be refilled with an electron of a higher energy shell. The excess energy of the filling electron can be transmitted to an electron which uses this energy to leave the atom. This electron will be detected with a low kinetic and a high binding energy. This is a secondary process and the detected electrons are called Auger electrons. To identify the origin of the Auger electron the ABC form is used. A stands for the shell the photoelectron was actuated from, B for the shell the refilling electron comes from and C for the shell the Auger electron comes from. For example an photoelectron is excited from the K-shell (1s level) an electron of the L1 (2s) level recombines with th hole in the K shell and the resulting photon excites an electron of the L2,3 (2p1/2 or 2p3/2 ) level. The Auger electron is then called KL1 L23 . Figure 1.4: Principle of the Auger electron emission 11 1 Experimental Methods and Theory 1.1.3 X-ray Absorption Spectroscopy (XAS) Figure 1.5: Schematic representation of XAS X-ray Absorption Spectroscopy (XAS) is a method to determine the unoccupied states of an atom. For this purpose a core electron is excited to an unoccupied state above the Fermi level. The energy which is needed for this excitation can be disposaled by an x-ray source, which emits different energies. It is important that one can change the excitation energy (Eexc ) because one needs the exact energy difference between the core level and the unoccupied states. With different excitation energies one can reache different unoccupied states. The energy of the core level is called initial state Einitial and the energy of the unoccupied state above the Fermi level is called final state Efinal . The energy difference 1.11 between this both states is the energy which is needed to excite an electron. Eexc = hν = Efinal − Einitial (1.11) Due to the dipole selection rules there are only special transitions allowed. The angular momentum quantum number l has to be changed by one (∆l = ±1). The spin s has to be fixed (∆s = 0) and the z-component of the orbital momentum m can be equal or changed by one (∆m = 0, ±1). The first method to detect an XAS-signal is to measure the intensity of the transmitted light. Because if one excites a transition, a photon is absorbed and the intensity of the light is reduced. Unfortunately this method can only be employed for very thin samples. The alternative and often used method is to measure the total electron 12 1.1 Basics of X-ray Spectroscopy yield (TEY). This is the drain current to the sample to refill the empty core level. The total electron yield is proportional to the XAS-signal. The measurement of the TEY is suitable for conductive samples. If the sample is an insulator the intensity of radiant recombination called partial- or total fluorescence yield (PFY or TFY) should be measured. 1.1.4 X-ray Emission Spectroscopy (XES) Figure 1.6: Schematic representation of XES X-ray emission spectroscopy based on the recombination which is shown in figure 1.6. X-rays excite a core electron and the resulting hole will be refilled with an electron from the valence band. Naturally the system prefers a state of minimum energy. The excess energie will be released by a photon. The energy of the emitted photon can be explained by equation 1.12. Eem = Einitial − Efinal (1.12) Eem is the energy of the emitted light, Efinal is the binding energy of the electron in the valence band and Einitial is the effective binding energy of the hole. To determine the intensity of the emitted light a CCD camera is often used. There are dipole selection rules: ∆l = ±1 and ∆j = ±1; 0. Electrical quadrupole transitions and magnetic dipole selection rules are negligible. With XES one detects the partial density of states (pDOS), because a special excitation energy excite only one specific element. Therefore this method is element specific. The occupied states will be measured 13 1 Experimental Methods and Theory because the recombining electrons come from occupied states. It is also a bulk-sensitive method due to the mean free path of the photons which is much bigger than the mean free path of the electrons by XPS or XAS. XES is a very good technique to determine the occupied states of the valence band. XES is a so called photon in - photon out spectroscopy. 1.1.4.1 Resonant X-ray Emission Spectroscopy (RXES) Figure 1.7: Schematic representation of REXS Resonant X-ray Emission Spectroscopy (RXES) is the generic term for two similar spectroscopy methods. The so called Resonant Elastic X-ray Scattering (REXS) and the Resonant Inelastic X-ray Scattering(RIXS). In the first case the excited electron recombines in the initial state. In the second case an electron from any occupied state above the core hole recombines into this core hole. This leads to characteristic loss features. An great advantage of this method is that the charging of the sample has no influence on the emitted photon in contrast to the emitted electrons by XPS. 1.1.5 X-ray Magnetic Circular Dichroism (XMCD) X-ray Magnetic Circular Dichroism (XMCD) is a method to get magnetic information of a sample. With the sum rules invented by Thole and Carra et al. [17, 18] one can determine the element specific spin and orbital momentum of a sample. These equations were modified by Chen et al. [19]. XMCD is element specific and the signal can be separated into spin and orbital moments. The x-ray magnetic circular dichroism 14 8 Basics of x-ray spectroscopy 1.1 Basics of X-ray Spectroscopy sample because the excited electron remains localized on the excited atom. XES is a so called photon in − photon out spectroscopy. theory was first approved by Schütz et al. [20]. Today this technique became an often magnetic dichroism (XMCD) used method2.4 for theX-ray characterization of circular magnetic materials. An absorption spectrum is measured with left and with right circularly polarized By means of x-ray magnetic circular dichroism (XMCD) one can analyze the light. During this spins to be aligned magnetic field magnetic measurements moments element the specific and have also separated into their in spina and orbital moments. wasx-ray first verified in 1987 by Schütz et al. [8]. the parallel to the light. To getItan magnetic circular dichroism theInspectra have to following decade it became an because often usedof method for the characterization of of electrons be subtracted. There is a difference, the preferential excitation magnetic materials. The XMCD is the difference between spectra measured with different spin directions with respect to the helicity of the light and the magnetic with left and right circularly polarized x-rays. For this effect the spins have field. to be aligned in a magnetic field. Dependent on the light helicity one spin direction is preferentially excited into the unoccupied 3d states. 3d states sz = - 1 2 EF L2 absorption of left circular pol. x-rays L2 absorption of right circular pol. x-rays 2p3/2 ¯¯ 2p1/2 o o ¯ Figure 1.8: Schematic representation of XMCD Figure 2.3: Schematic representation of the XMCD process in the one electron picture [9]. The XMCD In process is the schematically in figure 1.8.The The excitation to the figure 2.3 XMCD processpresented is schematically presented. d-band is 3d band is taken as an example. The d-band is split into spin up and spin down bands. First the absorption of circularly polarized x-ray photons leads to a spin polarization of the photoelectrons due to the spin orbit coupling (j=l±s). In the second step the d valence band acts as a spin detector. At the L3 edge (j=l+s) left hand circularly polarized x-rays mainly probe the unoccupied spin up d states with respect to the direction of the magnetization. The effect reverses at the L2 edge due to the opposite sign of the spin orbit coupling (j=l-s). X-ray absorption spectroscopy can be used for the XMCD technique. For 2p → 3d transitions in XAS the magnetic moments can be calculated by using the so called XMCD sum rules, which are shown in equation 1.13. They are devised by Thole and Carra et al. [17, 18] and modified by Chen et al. [19] as mentioned before. 15 1 Experimental Methods and Theory R (µ+ − µ− )dω morb = − RL3 +L2 (10 − n3d ) 3 L3 +L2 (µ+ + µ− )dω R R 6 L3 (µ+ − µ− )dω − 4 L3 +L2 (µ+ − µ− )dω R mspin = − (µ+ + µ− )dω L3 +L2 7 hTz i ×(10 − n3d ) 1 + 2 hSz i 4 (1.13) (1.14) morb is the orbital magnetic moment and mspin is the spin magnetic moment in units of µB /atom. The indices L3 and L2 refer to the integrals of the L3 and L2 peaks. (µ+ − µ− ) is the XMCD spectrum and (µ+ + µ− ) is the sum of the XAS spectra excited with left and right polarized light. n3d is the number of 3d electrons in the corresponding ion. hTz i is the ground state expectation value of the magnetic dipole term originating from the expectation value of the magnetic dipole operator and hSz i 7hTz i << 1 and can is the corresponding spin operator. Usually for bulk cubic crystals 2hS zi be neglected. There are some simplifications made during the invention of the sum rules. It is considered that the 2p → 3d transitions take place between free atoms. Secondly the L3 and L2 -edges should have a complete energetic separation for the purpose of getting exact results for the integrals. Problems occur for less than half filled 3d transition metals. Hence the sum rules can be used for the late and intermediate 3d transition metal ions. Teramura et al. [21] found a deviation from the XMCD spin sum rule, due to the Coulomb interaction between electrons, which mixes the so called L3 and L2 regions with each other. In this paper [21] a list of the deviations for various ionic states for the elements Ni, Co, Fe and Mn is presented, which may serve as a correction factor in the estimation of expectation values of magnetic quantities from XMCD data. The correction factor for iron is used in this thesis. For 2p → 5d transitions in XAS the magnetic moments can be calculated by using slightly different XMCD sum rules, which are shown in equation 1.15. They are although devised by Thole and Carra et al. [17, 18], but the here presented version with its necessary adjustments for 2p → 5d transitions were taken from Chaboy et al. [22]. morb mspin R 2 L3 +L2 (µ+ − µ− )dω (10 − n5d ) = R (µ+ + µ− )dω L3 +L2 R R 3( L3 (µ+ − µ− )dω − 2 L2 (µ+ − µ− )dω) R = 2( L3 +L2 (µ+ + µ− )dω) ×(10 − n5d ) − 16 7 hTz i 2 (1.15) (1.16) 1.2 Principles of Multiplet Theory For the calculations at the lutetium L-edge the following assumptions were R made acR 3 cording to Chaboy et al. [22]: (i) L3 +L2 (µ+ + µ− )dω is approximated by 2 L3 +L2 (µ+ + µ− )dω ; (ii) hTz i is assumed to be negligible in the spin sum rules; (iii) estimates of both morb and mspin moments have been derived by considering n5d =0. 1.2 Principles of Multiplet Theory In this work some x-ray absorption measurements and x-ray magnetic circular dichroism measurements are compared to so called multiplet calculations. In this section the basic principles are shown, starting with a theoretical description of x-ray absorption. 1.2.1 Single-particle Approximation X-ray absorption is, as mentioned before in section 1.1.3, an excitation of a core electron to an unoccupied state. The resulting x-ray absorption spectrum can be described with the so called Fermi Golden rule (equation 1.17). IXAS ∝ |hΦf |ê · r| Φi i|2 δEf −Ei −~ω (1.17) One gets the absorption intensity IXAS by coupling the initial state (Φi ) and the final state (Φf ) with a dipole matrix element (ê · r). The conservation of the energy is given by the delta function (δ). Now the assumptions which lead to the single-particle approximation will be mentioned. First the final state can be described as initial state plus an excited continuum electron () minus a core electron (c); Φf = cΦi . The second assumption is that all electrons which are not actively involved in the transition are removed from the matrix element. As a result only the excited core electron stays in the initial state (Φi ) and only the continuum electron remains in the final state (Φf ). The delta function will be replaced by the density of states (ρ). This results in equation 1.18 from Muller et al. [23]. IXAS ∝ |h |ê · r| ci|2 ρ (1.18) Due to the matrix element (ê · r) the density of states have an orbital moment that differs by 1 from the core state (∆L = ±1) and the spin is conserved (∆S = 0). For x-ray excitation the quadrupole transitions are some hundred times weaker than the dipole transitions, so they are usually neglected. The density functional theorem (DFT) is well-suited to describe for example the 1s x-ray absorption (K-edge) [24], but there is to mention that the simulation of the iron K-pre-edge of the XMCD measurement of LuFe2 O4 in chapter 3 is not possible with this theory. At the iron K-pre-edge the quadrupole transitions are very important and can not be neglected. 17 1 Experimental Methods and Theory 1.2.2 Multiplet Effects The single-particle approximation does not work for x-ray absorption measurements which include 2p→3d transitions, because in this case the p core wave function overlaps with the d valence wave function. The results are so called multiplet effects. The multiplet theory of ionic transition metal (TM) compounds is based also on the Fermi Golden rule, but the core hole and free electron of equation 1.18 are, in equation 1.19, replaced by a 2p core hole (2p) and a 3d electron. For the calculations from the valence electrons only the 3d electrons are taken into account, see equation 1.20. 2 IXAS ∝ Φi 2p3d |ê · r| Φi δEf −Ei −~ω (1.19) 2 IXAS ∝ 2p3dN +1 |ê · r| 3dN δEf −Ei −~ω (1.20) 1.2.2.1 Atomic Multiplet Theory The atomic multiplet (AM) theory was founded inter alia by Cowan [25] and Weissbluth [26]. They were faced with the problem of calculating the electronic states of a solid measured with x-ray spectroscopies. So on the one hand they had extended valence states and on the other hand a localized core hole. Over the last three decades it was found out that a complete localized approach, based on the atomic multiplet theory is suited for calculating core-level spectroscopy measurements. The basic assumptions for the analysis of x-ray absorption will be shown in this section. The basic equation to describe an N-electron atom in the atomic multiplet theory is the Schrödinger equation for free atoms, without any influence of the surrounding atoms. That leads to the Hamiltonian in 1.21. H= P p2i 2m X p2 X −Ze2 X e2 X i + + + ζ(ri )li · si 2m ri r pairs ij N N N is the kinetic energy of N electrons, P N−Ze2 describes the Coulomb attraction of the nucleus ri N P e2 = Hee is the electron-electron repulsion and rij pairs P N (1.21) with the atomic number Z, ζ(ri )li · si = Hls is the spin-orbit coupling of each electron. The average of a certain state will be defined as Hav , due to the kinetic energy and the interaction with the nucleus which are the same for all electrons in a certain atomic configuration. Hls and Hee still have to be solved. Hee is problematic to solve, the solution was a central field approximation by separating the spherical average of the electron-electron interaction hHee i from the non spherical part. The modified electron0 electron Hamiltonian Hee (equation 1.22) is the result of the difference between Hee 18 1.2 Principles of Multiplet Theory and hHee i. This leads to a simplified Hamiltonian in equation 1.23. So there are only 0 and Hls left to be solved. the two interactions Hee * + X e2 X e2 0 Hee = Hee − hHee i = − (1.22) r r pairs ij pairs ij 0 + Hls H = Hav + Hee (1.23) An calculated example of Ti4+ can be found in the thesis of Taubitz [27] in section “Atomic multiplet theory”. 1.2.2.2 Ligand-field Multiplet Theory A weakness of the atomic multiplet theory is that the influence of surrounding atoms is not considered. For this reason the ligand-field multiplet (LFM) theory, or also called crystal field multiplet theory, extends the atomic multiplet theory by adding a crystal field (equation 1.24). This extension was developed by Thole and co-workers for core level spectroscopy [28]. HLFM = H + HCF (1.24) H is already know from equation 1.21 and HCF is the crystal field, which consists of the electronic charge e times a potential Φ(r) that describes the surroundings (1.25). HCF = −eΦ(r) (1.25) The potential Φ(r) can be explained as series expansion of spherical harmonics YLM (equation 1.26), they can be seen as disruption to the atomic energy states. Φ(r) = ∞ X L X f L ALM YLM (ψ, φ) (1.26) L=0 M =−L The matrix element of Φ(r) will be determined regarding the atomic 3d orbitals. The resulting matrix elements h3d |Φ(r)| 3di will be splitted into a radial part and a spherical part. In this case the radial part gives the strength of the crystal field. The spherical part of the matrix element will be written in YLM symmetry. For 3d electrons the crystal field potential is reduced to: Φ(r) = A00 Y00 + 2 X M =−2 2 r A2M Y2M + 4 X r4 A4M Y4M (1.27) M =−4 The crystal field is usually defined by the three parameters X400 , X420 and X220 . Butler [29] defined this notation by indices {ijk}, due to the symmetry properties of electron orbitals. In optical spectroscopy the crystal field is described by Dq, Ds and 19 1 Experimental Methods and Theory Dt. There is a possibility to compare the different parameters, but it is not possible to determine the real crystal field splitting only by absorption measurements. A good overview about the crystal field theory can be found by Moffit and Ballhausen [30]. One often used crystal field is the cubic ligand-field, it has also the strongest effect on the symmetry. It is described in detail in the textbooks of Sugano et al. [31], Butler [29] and Fontaine [32]. In principle ligand-field multiplet calculations can be compared to experiments and they show quite good results [33, 34]. The soft x-ray edges have a high resolution, due to the long lifetime of the core states. That makes the spectra sensitive to details of the electronic structure, like valence, symmetry, spin state and crystal-field values [35, 36]. 1.2.2.3 Charge-transfer Multiplet Theory Finally one has to take into account the different itinerant electronic features. This will be done by the charge-transfer multiplet theory. It is close to the ligand field multiplet theory but it uses more than one configuration. For example to the 3dn ground state a 3dn+1 L configuration is added, and a 3dn L2 configuration and so on. Mostly two configurations are enough to explain an x-ray absorption spectrum, but in special cases, like higher valence states, more configurations can be useful. The charge-transfer effect adds a second dipole transition, second initial, and second final states: 2 IXAS,2 ∝ 3dn+1 L |p| 2p5 3dn+2 L (1.28) HINIT,2 = 3dn+1 L |HLF M | 3dn+1 L (1.29) HFINAL,2 = 2p5 3dn+2 L |HLF M | 2p5 3dn+2 L (1.30) The two initial states and two final states are coupled by monopole transitions, like hybridization. MI1,I2 = 3dn |HM IX | 3dn+1 L (1.31) MF1,F2 = 2p5 3dn+1 |HM IX | 2p5 3dn+2 L (1.32) P + The mixing Hamiltonian is defined as HM IX = ν V (Γ)(a+ dν aν +aν adν ). V (Γ) is the + hybridization strength and aν,dν are electron creation operators. The x-ray absorption spectrum can be calculated by solving the above given equations. The analysis of the effects of charge-transfer shows that the charge-transfer influences the spectral shape by contracting the multiplet structure and by small satellites. These observations are comparable to experimental results from Okada, Kotani and van der Laan [37–41]. Hu 20 1.3 Experimental Details et al. [42, 43] found out that for many systems charge-transfer multiplet calculations fit much better than ligand-field multiplet calculations. More detailed information about multiplet calculations can be found at the paper of deGroot [44] and the book of deGroot and Kotani [45]. 1.3 Experimental Details 1.3.1 The Photoelectron Spectrometer PHI 5600ci For the XPS measurements a PHI 5600ci multitechnique spectrometer produced by the Perkin Elmer Cooperation [46] was used. A schematic construction of this system is shown in figure 1.9. Al/Mn anodes monochromator ion gun electron gun hemisferical analyzer electronic lens X−rays multi−channel detector mono X−rays sample Al X−ray anode Figure 1.9: Scheme of the XPS spectrometer A preparation chamber is attached to the PHI 5600ci. This chamber is produced by the fine mechanical workshop of the department of physics. The preparation chamber afford to file or cleave the sample in vacuum with the integrated diamond file or the pincer. As described in section 1.1.1 XPS is a very surface sensitive method. Unfortunately the samples contaminate very fast if kept in atmosphere. This contamination adulterates the measurements and due to that it is very important to have the possibility to prepare the sample in vacuum. Another or additional way to clean the surface is to sputter with an ion gun. There is an argon ion gun in the main chamber. The argon ions are accelerated with a maximum voltage up to 4.5 kV. When they hit the sample surface the contamination 21 1 Experimental Methods and Theory will be excluded. Not every material is suitable for sputtering because the ions can destroy the structure and stoichiometry of the sample. Especially oxides are difficult to clean by sputtering but this method is appropriate for metals and alloys. The PHI 5600ci is provided with two x-ray sources. On the one hand there is a dual Mg/Al x-ray anode and on the other hand a monochromatized Al anode. The radiation energies are 1253.6 eV for the Mg Kα with a half-width of 0.7 eV and 1486.6 eV for the Al Kα . The half-width by the dual anode for Al Kα is 0.85 eV and for the monochromatized 0.3 eV. The monochomatized Al anode is used in most cases. The small half-width of 0.3 eV will be achieved by a quartz crystal and based upon the Bragg equation nλ = 2d · sin(θ). For XPS measurements an ultra high vacuum (UHV) is necessary for two reasons. First the mean free path of the photo electrons increases so they can reach the analyser without being scattered in the atmosphere. The second reason is that the UHV keeps the surface of the sample clean during the measurements, because there are less molecules to adsorb on the surface. The UHV is reached by a number of different types of vacuum-pumps. First rotation pumps create a pressure in that turbo molecular pumps can work. They reach a pressure around 1 × 10−8 mbar. An ion getter pump and a sublimation pump can then be used to achieve a pressure about 1 × 10−9 mbar. To analyze the excited photoelectrons an 11 inch hemispherical analyser is used. First the incoming electrons are focused by a lens system. Afterward their kinetic energy is reduced to a certain pass energy Ep to ensure that the absolute resolution is constant for the hole spectrum. The constant analyser transition (CAT) mode allows only electrons with an energy Ep ± δE to pass the analyser, δE denotes the absolute energy resolution. For a higher energy resolution of the recorded spectra the pass energy has to be reduced, but that also leads to a smaller overall intensity of the XPS signal. By measuring insulating samples, like the rare-earth scandates investigated in this thesis, local charges can occur at the surface. They lead to a movement of the spectra during the measurements because the resolved photoelectrons seem to get higher binding energies due to the electric field between sample and analyser. To avoid this a neutraliser, a low-energy electron gun, can be used to compensate the charging. The accelerating potential can be chosen between 0 V and 10 V at a maximal current of 25 µA. 1.3.2 The Advanced Light Source (ALS) The Advanced Light Source (ALS), Berkeley, California USA, is a research facility used by scientists to explore the properties of materials, analyze samples for trace elements, probe the structure of atoms and molecules as well as other targets. Exemplarily for synchrotrons in general, the composition of the Synchrotron will be described in the following passage. 22 1.3 Experimental Details Figure 1.10: Scheme of the ALS The ALS produces light, principally x-rays, with special qualities. In figure 1.10 a simplified diagram of the ALS is shown, which will give a look at the most important parts of the ALS. Number 1 shows the linear accelerator, or linac. It is the electromagnetic catapult that brings electrons from a standing start to relativistic velocity. This velocity close to the speed of light. A linac long enough to accelerate electrons to the energy needed by the ALS would not fit inside the building. Instead, a circular booster synchrotron (number 2) is used, in which the electrons receive a boost from an accelerating chamber each time they go around. In less than one second, the electrons make 1,300,000 revolutions (and travel 98,000 kilometers) and reach 99.999994% of the speed of light. Once the electrons reach their target energy in the booster synchrotron, an injection system transfers them from the booster to the storage ring (number 3) where they circulate for hours. The storage ring is roughly circular with 12 arc-shaped sections (about 10 meters long) joined by 12 straight sections (about 6 meters long). Hundreds of precision electromagnets focus and bend the electron beam as it circles the storage ring more than a million times a second. Electrons curving through the ring’s 12 arc sections emit fanlike beams of photons. Between these curves there are straight sections where multi-magnet devices, called undulators and wigglers, shake 23 1 Experimental Methods and Theory the electrons to form a narrow beam of light 100 million times brighter than conventional x-ray sources. The synchrotron light emitted by the electrons is directed to beamlines through the round beam ports. The brightest synchrotron light at the ALS comes from undulators (number 4) which contain over one hundred magnetic poles lined up in rows above and below the electron beam. The magnets force the electrons into a snake-like path, so that the light from all the curves adds together. As the electrons travel in their circular orbit in the storage ring, they emit synchrotron light in the ultraviolet and x-ray range of the spectrum. The beamlines (number 5) deliver the light down an optical obstacle course from the storage ring to the experiment stations. A detailed scheme of the beamline is shown in figure 1.11. Number 6 shows the position of the end stations. The end stations used for this thesis are presented below. Figure 1.11: Scheme of the ALS beamline This information is taken from the official homepage of the ALS, for further information see reference [47]. 1.3.2.1 Beamline 8.0.2 at the ALS The used endstation on beamline 8.0.1 at the Advanced Light Source (ALS) was the soft x-ray fluorescence (SXF) endstation of the University of Knoxville, Texas. This one was used to make the XAS and XES measurements on the rare-earth scandates, which are presented in this work. The used detection modes were total fluorescence yield (TFY) and total electron yield (TEY). The spot size was 100 µm horizontal and 50 to 3000 µm vertical. The maximal size of the sample was up to 2 cm. The measurements made on this endstation were made by a temperature around 293 K. 24 1.3 Experimental Details 1.3.2.2 Beamline 4.0.2 at the ALS At the Beamline 4.0.2 the used endstation is the XMCD chamber (6T, 2K). This endstation is from the University of California, Davis/Berkeley Lab, California. The XMCD measurements on the Fe L-edge of LuFe2 O4 were made on this endstation. The temperature was between 260 K and 280 K and at magnetic field of 6 T. The spot size is 1 mm horizontal and 0.1 mm vertical. The maximal spot size is up to 5x5 mm. More details about this enstation can be found on the official homepage of the ALS [47]. 1.3.3 The Swiss Light Source (SLS) The Swiss Light Source (SLS) at the Paul Scherrer Institute in Villingen, Switzerland is a third-generation synchrotron light source. It was built to get a facility with high brightness, a wide wavelength spectrum and very stable temperature conditions for the primary electron beam and the secondary photon beams. It has an energy of 2.4 GeV and is used for research in material science, biology and chemistry. The SLS has some special features, like a very large spectrum of synchrotron light ranging from infrared light to hard x-rays or the so called top-up injection which produces a constant beam intensity for experiments. The here presented information is taken from the official homepage of the Paul Scherrer Institute [48]. 1.3.3.1 TBT-XMCD Endstation at the SLS The XMCD experiments for the Fe L-edge of W72 Fe30 sulfate were performed at the surface and interface microscopy (SIM) beamline [49]. The used endstation was the TBT-XMCD endstation from the Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS) [50]. This experimental setup is unique in Europe. The XMCD experiments can be made by temperatures down to 300mK in a magnetic field up to 7T. For the investigated Fe L-edge XMCD measurements the magnetic field had 6.5T and the temperature was 0.7K. 1.3.4 Bessy II Bessy II is a synchrotron of the Helmholtz Zentrum Berlin (HZB) [51]. It offers a large variety of methods and experimental techniques involving a multitude of experimental stations. Detailed information about the available endstations can be found on the home page of the BESSY II [52]. 1.3.4.1 Russian German Dipole Beamline The Russian German Dipole Beamline (RGBL), has an energy range from 30 eV to 1500 eV. The used endstation was called Mustang. It was used for photoelectron 25 1 Experimental Methods and Theory spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) measurements. On this endstations the XAS measurements for Mo72 Fe30 acetate and Mo72 Fe30 sulfate were made at room temperature. 1.3.5 European Synchrotron Radiation Facility (ESRF) The European Synchrotron Radiation Facility (ESRF) is an international x-ray light source in Grenoble, France, supported by 19 countries. More information about the ESRF can be found at the official homepage [53]. 1.3.5.1 ID12 Circular Polarisation Beamline The ID12 Polarisation-dependent X-ray Spectroscopy Beamline is a unique instrument worldwide that offers users full control of the polarization state of the x-ray beam over a wide energy range (2-15 keV) and is devoted to research at the ultimate limits of xray spectroscopy [54]. On this endstation the XMCD measurements on the Fe K-edge and Lu L-edges were made. The endstation had an electromagnet to which enabled magnetic fields up to 15T and an electromagnet which could offer a magnetic fields up to 6T. The used temperatures varied from 125K up to 300K. 1.3.6 Superconducting Quantum Interference Device - SQUID The here presented magnetometry measurements on LuFe2 O4 and W72 Fe30 sulfate were performed at a Quantum Design MPMS at the University of Ulm, Germany. The maximum field which was applicable had 7T. The temperature could be varied from 1.9K up to 400K. The sensitivity was smaller than 10−8 emu. 26 2 RScO3 In this chapter the aim is to present a complete study about the RScO3 series including PrScO3 , NdScO3 , SmScO3 , EuScO3 , GdScO3 , TbScO3 and DyScO3 . The first subsection describes the basic properties of the crystals. After this the x-ray photoelectron (XPS) core levels will be shown. Afterward an x-ray absorption and x-ray emission study will be presented. These measurements were used to determine band gaps and opens up a discussion about the influence of structural parameters on the size of the optical band gaps of these charge-transfer compounds. Some of the results of NdScO3 , SmScO3 , GdScO3 , TbScO3 and SmScO3 were already presented in my diploma thesis [55] and the PhD thesis of M. Raekers [4]. Further some experimental results were published by Raekers et al. [56] and Derks et al. [57]. The latest publication by Postnikov et al. [58] is in print. 2.1 Introduction Transition metal based perovskites exhibit a vast variety of unique physical characteristics, e.g., transport properties [59]. There were already a lot of investigations on these materials in the nineties. Among the best known examples of such systems during this time are cuprates, they show superconductivity at high-temperature [60]. Another group are manganites, they are well known because of the colossal magneto resistance effect [61–63]. Cobaltes have a rich magnetic phase diagram and highspin to low-spin transitions [64]. More recently, superconducting iron pnictides [65] and multiferroic perovskites like BiFeO3 [66] have attracted much attention. BiFeO3 shows ferromagnetic and ferroelectric ordering phenomena. On the other hand, dielectric and ferroelectric “d0 -perovskites” like BaTiO3 , LaTiO3 or SrTiO3 are subject to intense research activities due to their remarkable dielectric properties and the possibility to control the electrical polarization, which is a required pre-requisite for constructing a ferroelectric memory (FeRAM) [67, 68]. The investigations during the last thirty years changed the fundamental understanding of electron correlation effects. Transition-metal oxides can be parted into two types of insulators. On the one hand Mott-Hubbard insulators, their band gap is determined by the repulsive potential Udd between the 3d electrons. On the other hand there are charge transfer insulators, in which the energy energy gap (∆pd ) spans between the filled ligand p bands and the unoccupied 3d conduction band states [69]. In this relation, there are two issues of interest. First to get more information about the nature of correlation in mate- 27 2 RScO3 rial, by analyzing detailed trends in the band gap variation. The second intention could be to use the detected trends for tailoring the optical band gap in wide-gap oxidically insulators to desired values for possible applications. For example to use it as a high-k gate dielectric [70, 71], or a transparent conducting oxide [72]. One of these wide gap oxides are the scandates of type RScO3 (R = Pr, Nd, Sm, Eu, Gd, Tb and Dy). They are particularly promising candidates for replacing SiO2 as gate dielectric [73–76] in electrical devices. Moreover they can be used as a model system for applications in the terahertz regime [77]. Another purpose of applying these scandium-based perovskites is the use as thin-film substrates. They belong to the best available substrates for the epitaxial growth of high-quality thin films. So they can be used for strain tailoring of ferroelectric, ferromagnetic, or multiferroic perovskite thin films by choosing a suitable RScO3 [78–83]. Including a forecast of the band gap reduction at the interface of DyScO3 and SrTiO3 [84]. The already known properties of the RScO3 require a detailed description of the electronic structure to understand the complex properties of RScO3 itself and its interaction to other materials. Up to now there is only a limited knowledge about their electronic structure. Delugas et al. [76] performed ab initio investigations on DyScO3 . Luckovsky et al. [74] published some oxygen x-ray absorption spectra. Raekers et al. [56] investigated the electronic structure of SmScO3 , GdScO3 , and DyScO3 . In the following chapter, the aim is to extend the electronic structure studies on the RScO3 series onto further rare-earth scandates, namely PrScO3 , NdScO3 , EuScO3 and TbScO3 . 2.2 Basic Properties and Preparation of RScO3 (R=Pr, Nd, Sm, Eu, Gd, Tb and Dy) The RScO3 compounds crystallize in an orthorhombic perovskite structure RMO3 p (space group Pbnm (no. 62)) with a ≈ b ≈ 2ap , and c ≈ 2ap , and four formula units per units per cell [5]. The lattice constances are shown in table 2.1. R represents a trivalent rare earth metal and M a trivalent or mixed valent transition metal. [5]. The lattice constances are shown in table 2.1 and a drawing of the crystal is presented in figure 2.1. Uecker et al. [78] published the exact growing procedure. The perovskitetype melt congruently, so the crystals could be grown by the conventional Czochralski technique with RF-heating (25 kW microwave generator) and automatic diameter control. Flowing nitrogen or argon was used for the growth atmosphere. The pulling rate was 0.5-1.5 mm h−1 and the rotation rate was 8-15 rpm. To achieve [110] R scandate substrates, the R scandate crystals were grown along the [110] direction. The here presented and investigated single crystals were grown by R. Uecker at the Institute for Crystal Growth in Berlin. 28 2.2 Basic Properties and Preparation of RScO3 (R=Pr, Nd, Sm, Eu, Gd, Tb and Dy) [001] [111] RE [110] ScO6 c b a Figure 2.1: Orthorhombic RScO3 crystal structure; there is an octahedron tilting about [001]p , [110]p and [111]p respectively. PrScO3 NdScO3 SmScO3 EuScO3 GdScO3 TbScO3 DyScO3 a (Å) 5.6118(1) 5.5809(1) 5.5343(1) 5.5109(1) 5.4862(1) 5.4654(1) 5.4494(1) b (Å) 5.7802(1) 5.7765(1) 5.7622(1) 5.7565(1) 5.7499(1) 5.7292(1) 5.7263(1) c (Å) 8.0276(1) 8.0072(1) 7.9674(1) 7.9515(1) 7.9345(1) 7.9170(1) 7.9132(1) Table 2.1: Structural parameters of RScO3 taken from Liferovich et al. [5] 29 2 RScO3 2.3 Core Level XPS of Rare-Earth, Scandium and Oxygen X-ray photoelectron spectroscopy (XPS) was used to investigate the rare-earth scandate single crystals. Information about the valence states of the compounds is given by chemical shifts and the shape of the spectra. The measurements were performed at the PHI 5600ci multitechnique spectrometer 1.3.1 at the University of Osnabrueck. The crystals were cleaved in situ in a preparation chamber at a pressure of around 10−7 mbar. The main chamber had a pressure of around 10−9 mbar during the measurements. The temperature was 293K. The samples were neutralized by an electron flood gun, so that the charging of the insulating samples could be avoided. The spectra were measured with a pass energy of 5.3 eV of the electron energy analyzer which resulted in an overall spectral resolution of 80 meV. 2.3.1 XPS of Scandium and Oxygen In figure 2.2 are the seven oxygen 1s spectra plotted. All seven XPS spectra have their maximum at 530 eV, which is an indicator for O2− . The satellite at around 532 eV results from OH-groups which can adsorb on oxygen defects on the surface. PrScO3 , TbScO3 and DyScO3 show a larger satellite so either the crystals have more defects or the cleaved surface was not perfectly broken. PrScO3 , SmScO3 and DyScO3 show a second, small but visible satellite at around 535 eV. This signal originates from a carbonate (H2 CO3 ) contamination on the surface. The 2p signals of the trivalent Sc atoms are shown in figure 2.3. The valence is trivalent in all seven crystals. As a reference for Sc3+ the spectrum of Sc2 O3 is plotted at the bottom of figure 2.3. The 2p3/2 main peak is located at 402 eV with a spin-orbit splitting of around 4.5 eV. The 2p1/2 peak is visible around 406 eV. There are satellites around 414 eV and 418 eV which result from an excitation in the valence band from the oxygen 2p states to unoccupied scandium 3d states. O 2p and Sc 3d are hybridized. This hybridization will be very useful in the following chapter to determine the band gaps. 30 2.3 Core Level XPS of Rare-Earth, Scandium and Oxygen Figure 2.2: O 1s XPS spectra of RScO3 (R=Pr, Nd, Sm, Eu, Gd, Tb and Dy) Figure 2.3: Sc 2p XPS spectra of RScO3 (R=Pr, Nd, Sm, Eu, Gd, Tb and Dy) (red); the reference Sc2 O3 (black) was taken from Chastain et al. [1] 31 2 RScO3 2.3.2 XPS of the Rare-Earth The 3d core levels of the rare-earth ions are located at binding energies between 900 eV and 1350 eV. The praseodymium (Pr) 3d spectrum is shown in figure 2.4. The 3d5/2 peak is located at around 933.2 eV and the Pr 3d3/2 peak at 953.5 eV. The location of the peaks is an indicator for trivalent ions, because Pr4+ has higher binding energies [1]. The double peak structure of 3d5/2 and 3d3/2 could not be resolved. The spin-orbit splitting is 20.3 eV. At this point it should be mentioned that the real spin-orbit splitting has to take into account all satellites which belong to a certain energy level. Sometimes this is hard to determine because of the small intensities of the satellites and the large distances between them. So the here shown spin-orbit splittings belong to the distances between the main peaks. Deviations are not excluded. The second rare-earth ion in this row is neodymium (Nd). The Nd 3d core levels are plotted on top of figure 2.5, in red. At the bottom of this graph an XPS measurement of Nd2 O3 , taken from Suzuki et al. [2], is shown (black). Both spectra have a similar shape and the Nd 3d5/2 peak is located at 982.5 eV and the 3d3/2 at 1005 eV, respectively. The spin-orbit splitting is 22.5 eV. Nd2 O3 is a reference for trivalent neodymium, so it can be shown, that Nd ions in NdScO3 are trivalent. The shoulders are typical for oxide bondings. Figure 2.4: Pr 3d XPS spectra of PrScO3 32 Figure 2.5: Nd 3d XPS spectra of NdScO3 (red); the reference Nd2 O3 (black) was taken from Suzuki et al. [2] 2.3 Core Level XPS of Rare-Earth, Scandium and Oxygen The next spectrum discussed is the samarium (Sm) 3d core level spectrum. It is shown in figure 2.6. The main 3d5/2 peak at 1083 eV indicates a trivalent state [1]. The Sm 3d3/2 peak is located at 1110 eV. The spin-orbit splitting of Sm 3d is 27 eV. The small shoulders at both peaks and the small structures around the main peaks are due to the complex multiplet splitting of the Sm 3d states. Europium (Eu) 3d has two sharp peaks, they are shown in figure 2.7. The 3d5/2 is located at 1135.6 eV and the 3d3/2 peak at 1165.5 eV, resulting in a spin-orbit splitting of 29.9 eV. The position of the peaks is a good indicator for trivalent europium. Cho et al. [85] showed, that the peaks emerge at lower binding energies for divalent europium. Figure 2.6: Sm 3d XPS spectra of SmScO3 Figure 2.7: Eu 3d XPS spectra of EuScO3 Figure 2.8 contains two gadolinium (Gd) 3d XPS core level spectra, from GdScO3 (top, red) and Gd2 O3 (bottom, black). It is known, that the Gd in Gd2 O3 is trivalent and it can be seen, that the spectrum is similar to the GdScO3 spectrum. The Gd 3d5/2 peak is located at around 1189 eV and the 3d3/2 at around 1221.5 eV. The positions of the peak are also a clear indicator for Gd3+ . A satellite is located at 1199 eV. The XPS spectrum of the terbium (Tb) 3d core levels is plotted in figure 2.9. The main peak at 1241.8 eV belongs to the Tb 3d5/2 level. The Tb 3d3/2 is located at 1276 eV. This leads to a spin-orbit splitting of 34.2 eV for Tb 3d. The last rare-earth 3d spectrum from dysprosium (red) is plotted in figure 2.10 together with the Dy 3d spectrum of Dy2 O3 (black), where Dy is trivalent. Both spectra look similar regarding their shape and have the Dy 3d5/2 peak at a binding energy of 1295.8 eV and the Dy 3d3/2 peak at 1334.7 eV. The spin-orbit splitting is 33 2 RScO3 Figure 2.8: Gd 3d XPS spectra of GdScO3 (red); the reference Gd2 O3 (black) was taken from Lütkehoff [3] Figure 2.9: Tb 3d XPS spectra of TbScO3 about 39 eV. The intensity of the signal is rather low, because the excitation energy of an Al Kα anode is 1486.6 eV and the excited electrons have a low kinetic energy before they reach the analyser. 2.3.3 Conclusion The photoelectron spectroscopy measurements of the rare-earth scandates showed clear valence states for the ions in the investigated crystals. The oxygen is in all seven crystals divalent (O2− ). The oxygen spectra make obvious that the crystals have some defects, which lead to adsorptions of OH-groups and carbonates. The scandium spectra confirm the expected, trivalent state of scandium. The hybridization between O 2p and Sc 3d states, which is known from Raekers et al. [56], can be confirmed due to the satellites with 12 eV distances from the main peaks. Core-level spectra from R 3d show that the spin-orbit splitting increases from 20.3 eV for Pr to 39 eV for Dy. Additionally all spectra confirm a trivalent state of the rare-earth ions. 34 2.3 Core Level XPS of Rare-Earth, Scandium and Oxygen Figure 2.10: Dy 3d XPS spectra of DyScO3 35 2 RScO3 2.4 XAS and XES/RIXS of R, Sc and O X-ray Absorption Spectroscopy (XAS) is a method to determine the element specific density of unoccupied states above the Fermi level. X-ray Emission Spectroscopy (XES) is used to examine the element specific density of occupied states below the Fermi level. The XAS and XES measurements were performed at the Advanced Light Source in Berkely, USA, at beamline 8.0.1. It is described in details in section 1.3.2. In the last part of this section the band gaps of the rare-earth scandates are determined by combining XAS and XES at the O K-edge. All rare earth emission spectra were measured in second order of the spectrometer and therefore the spectra are at the half energy value of the first order rare earth spectra. Measurements in second order for the rare-earth ions were necessary because the first order energies were outside the detection limit of the detector. All following x-ray absorption and emission spectra in the chapter are detected in total fluorescence yield. 2.4.1 R M4,5 -Edges XAS and R 4f → 3d XES The praseodymium M4,5 -edges XAS is dominated by two main peaks at the M5 -edge at 932 eV and the M4 -edge at 952 eV. The splitting of 20 eV is due to the spin-orbit splitting of Pr 3d. The spectrum is plotted in figure 2.11. The XES spectrum of praseodymium is shown in figure 2.12. The spectrum was measured in the second order with an excitation energy of 1000 eV. The two main peaks at around 465.6 eV (M4 -edge) and 476.5 eV (M5 -edge) in second order are the equivalents to peaks at 931.2 eV and 953 eV in first order. There is a spin-orbit splitting of 22 eV. The neodymium M4,5 -edges XAS (shown in figure 2.13) is dominated by two main peaks at the M5 -edge at 982 eV and the M4 -edge at 1004 eV. The splitting is due to the spin-orbit splitting of Nd 3d, and there are pre-peaks at around 977 eV and 999 eV. The XES spectrum of neodymium is plotted in figure 2.14. The spectrum was measured in the second order with an excitation energy of 1016 eV. That way the elastic peak, which is used for calibration, can be seen at 508 eV. The two main peaks at around 486 eV and 497 eV are the energy pendants to 972 eV and 994 eV of the first order. There is a spin-orbit splitting of 22 eV. The peak at around 495 eV is the Sc 2s. If the measurements were detected in the first order of neodymium, the Sc 2s would not be visible. The Sm M4,5 -edges XAS is plotted in figure 2.15. The XAS spectrum consists of two intense features at the M5 -edge at 1080.6 eV and the M4 -edge at 1104 eV due to the spin-orbit splitting of Sm 3d states. The M5 -edge is splitted in a second peak at 1078.6 eV and has a pre-peak at 1072.8 eV. The M4 -edge peak has a pre-peak at 1098 eV. The excitation energies of the resonant x-ray emission measurements are plotted in figure 2.16 close to the relevant spectrum. The first order oxygen K emission at around 525 eV is present in each spectrum. The elastic peaks in the lower three spectra are clearly visible at 552 eV, 541 eV and 538 eV in the corresponding 36 2.4 XAS and XES/RIXS of R, Sc and O Figure 2.11: XAS at the Pr M4,5 -edges of PrScO3 Figure 2.12: Pr 4f → 3d of PrScO3 spectrum. In the spectrum excited with 1076.5 eV there is only a small inelastic feature at 537 eV. The resonantly excited inelastic part of the spectrum excited with 1082.5 eV is more intense than the elastic peak in this spectrum. The resonant excitation at the Sm M4 -edge results in a three peak structure centered at around 550 eV, with the aforementioned elastic peak at 552 eV, and inelastic features at 550 eV and 549 eV, respectively. The normal XES spectra at the top of figure 2.16 shows the mentioned O Kα emission and just small inelastic peaks from the rare earth states at 537 eV and 550 eV. In the resonant x-ray emission spectra no loss feature is observable that would always appear in the same distance from the elastic peak. Therefore no small excitation in the valence band region takes place. This is due to the large band gap which suppresses such effects. The x-ray absorption spectrum of Eu M4,5 -edges is plotted in figure 2.17. There are two main peaks. The M5 peak is splitted in two and the peak positions are 1139.2 eV and 1143 eV. There is a pre-peak at 1134.2 eV and a shoulder at 1145.8 eV. The M4 peak is located at 1167.8 eV with a shoulder at 1171.4 eV. The x-ray emission spectrum was measured in second order and is plotted in figure 2.18. It is dominated by a peak at 577.7 eV which belongs to the Eu 3d3/2 state with an energy of 1155 eV. The second peak at 563 eV is due to the Eu 3d5/2 -state which is in the first order at 1126 eV. The gadolinium x-ray absorption spectrum plotted in figure 2.19 has a sharp M5 edge peak at 1185 eV and two obvious shoulders at 1189.4 eV and 1192.9 eV. The M4 -edge has two nearly equal tips on one peak at 1215 eV and 1216.8 eV. Next to 37 2 RScO3 Figure 2.13: XAS at the Nd M4,5 edges of NdScO3 Figure 2.14: Nd 4f → 3d of NdScO3 this spectrum, in figure 2.20, the x-ray emission spectra of the Gd M4,5 -edges are plotted. The x-ray emission spectra were excited with various energies. To excite the Gd M5 -edge in resonance the excitation energies 1181.51 eV and 1183.8 eV were used. The complete inelastic emission structure of the Gd M5 -edge at 590 eV is visible in the resonant emission spectrum excited with 1215.7 eV. In this spectrum the elastic peak and the inelastic Gd M4 -edge emerge at 608 eV and 605 eV, respectively. In the normal XES excited with 1263.5 eV the M5 - and the M4 -edges are located at 590 eV and 605 eV, respectively. No energy loss features are present. The Tb M4,5 -edges XAS of TbScO3 is shown in figure 2.21. There are, comparable to the previous x-ray absorption spectra of the rare-earths, two peaks. The M5 -edge is located at 1244.6 eV and the M4 -edge at 1276 eV. In figure 2.22 the XES spectrum of terbium is plotted. The excitation energy was 1265 eV. Due to the measurement in second order the peak at 632.5 eV is the elastic peak and is used for calibration. The terbium peaks are located at 615.5 eV and 620.5 eV. In first order measurements this would correspond to peaks at 1231 eV and 1241 eV. So there is a spin-orbit splitting of 10 eV. The M5 -edge of dysprosium has its maximum at 1290 eV with a pre-peak at 1293.5 eV and a shoulder around 1298.3 eV. The XAS M4 -edge is located at 1329.3 eV. The spectrum is plotted in figure 2.23 next to the x-ray emission spectra, which is shown in figure 2.24 and is also measured in second order. There are different excitation energies used. The values are connected to the spectra. The three lower spectra excited resonantly at the M5 -edge comprise the elastic peak and a small shoulder to 38 2.4 XAS and XES/RIXS of R, Sc and O Figure 2.15: XAS at the Sm M4,5 edges of SmScO3 taken from [4] Figure 2.16: Sm 4f → 3d XES of SmScO3 taken from [4] lower photon energies which is due to inelastic features excited in resonance. The resonant emission spectrum excited at the M4 -edge at 1328.1 eV shows the weak inelastic structure of the M5 -edge from 642 eV to 652 eV. The elastic peak and resonantly excited inelastic features are present around 663 eV. The pure inelastic structure of the Dy M4,5 -edges is visible in the normal emission spectra excited with 1363.6 eV. No energy loss features are visible. 39 2 RScO3 Figure 2.17: XAS at the Eu M4,5 edges of EuScO3 Figure 2.18: Eu 4f → 3d XES of EuScO3 Figure 2.19: XAS at the Gd M4,5 edges of GdScO3 taken from [4] Figure 2.20: Gd 4f → 3d XES of GdScO3 taken from [4] 40 2.4 XAS and XES/RIXS of R, Sc and O Figure 2.21: XAS at the Tb M4,5 edges of TbScO3 Figure 2.22: Tb 4f → 3d of TbScO3 Figure 2.23: XAS at the Dy M4,5 edges of DyScO3 taken from [4] Figure 2.24: Dy 4f → 3d of DyScO3 taken from [4] 41 2 RScO3 Figure 2.25: XAS spectra taken at Sc L2,3 -edges of RScO3 (R=Pr, Nd, Sm, Eu, Gd, Tb and Dy) 2.4.2 Sc L2,3 -Edges XAS and Sc 3d → 2p XES To get further information about the internal fields of the crystals, the x-ray absorption measurements of the scandium L2,3 -edges of all seven investigated rare-earth scandates are considered. The spectra are shown in figure 2.25. They show nearly the same peak distances. The first two peaks belong to the 2p3/2 states or in another notation L3 edge. The third and fourth peak correspond to 2p1/2 states or the so called L2 -edge. Higuchi et al. [86] wrote that the first peak of the double peak shows states of the t2g sub band and the second peak, which is somewhat larger, results from the eg sub band. The first peak is located at 401 eV, the second peak around 403 eV. The crystal-field splitting (10Dq) amounts to 1.9-2 eV for all seven rare-earth scandates. The distance between peak three and four is between 1.9 and 2 eV, with one exception. PrScO3 shows a distance of 1.8 eV. As a result the distances between peak one and three and between two and four lay between 4.2 eV and 4.4 eV. This more than 4 eV splitting is a result of the spin-orbit splitting of the Sc 2p core levels. The 2 eV splitting between the first and second and third and fourth peak is caused by the crystal field of the oxygen octahedral which surrounds the scandium atoms. The distances are equal for all seven crystals, so it is clear that there is no local field splitting. Otherwise there has to be a change in the peak positions and splittings in the absorption spectra of different rare-earth ions. 42 2.4 XAS and XES/RIXS of R, Sc and O a-b c-d a-c b-d PrScO3 1.9 1.8 4.3 4.2 NdScO3 2.0 1.9 4.3 4.2 SmScO3 2.0 1.9 4.4 4.3 EuScO3 1.9 1.9 4.2 4.2 GdScO3 2.0 2.0 4.4 4.4 TbScO3 1.9 2.0 4.2 4.3 DyScO3 1.9 2.0 4.3 4.4 Table 2.2: Measured inter peak separations (in eV ± 0.2 eV) in the Sc L2,3 XAS spectra of Fig. 2.25. The occupied Sc 3d states were probed by XES. There were different excitation energies used and that leads to quite different spectra. NdScO3 and TbScO3 were excited with 403.7 eV and the probed occupied Sc 3d states can be seen in figure 2.27. Due to the 3d0 configuration, the occupied Sc 3d states are nearly empty. But there is a strong hybridization with O 2p states, that leads to a small signal. There are two peaks, which have different intensities for NdScO3 and TbScO3 , but are located at the same energies. The first peak is located at 392.3 eV and the second one is located at 394.3 eV. The peak at 394.3 eV refers to Sc 3d states. The different ratio of the two peaks in both samples suggests an influence of the presence of the rare-earth. This influence can originate from different distortions of the oxygen octahedral and from a more complex hybridization of Sc 3d, O 2p and R 5d states. A complex hybridization was already mentioned by Lucovsky et al. [87] and Liferovich et al. [5]. For PrScO3 and EuScO3 , the excitation energy was set to 419.3 eV (figure 2.26). SmScO3 , GdScO3 and DyScO3 were excited by 420.2 eV and are plotted in figure 2.28. The spectra comprise one common main feature at 394 eV. Also common is a shoulder at 391 eV which is a result of the Sc 2p spin-orbit splitting. The different intensities of the shoulder can not be explained this way, so there could be an influence by the rare-earth atom. The different rare-earth ions could lead to different distortions of the oxygen octahedral and to a more complex hybridization of Sc 3d, O 2p and R 5d states. Such a complex hybridization was also mentioned by Lucovsky et al. [87] and Liferovich et al. [5]. PrScO3 , EuScO3 and GdScO3 show additional peaks at 399 eV, 400.5 eV, 402 eV and 406.7 eV. A shoulder at 496 eV appears for SmScO3 , GdScO3 and DyScO3 . 43 2 RScO3 Figure 2.26: XES spectra of the Sc L2,3 -edges of PrScO3 and EuScO3 with EExc = 419.3 eV Figure 2.27: XES spectra of the Sc L2,3 -edges of NdScO3 and TbScO3 with EExc = 403.7 eV Figure 2.28: XES spectra of the Sc L2,3 -edges of SmScO3 , GdScO3 and DyScO3 with EExc = 420.2 eV taken from [4] 44 2.4 XAS and XES/RIXS of R, Sc and O Figure 2.29: O K-edge XAS and O 2p → 1s XES of PrScO3 Figure 2.30: O K-edge XAS and O 2p → 1s XES of NdScO3 2.4.3 O K-Edge XAS and O 2p → 1s XES In this section the x-ray absorption and x-ray emission spectra of oxygen of the rareearth scandates are shown in figure 2.29, 2.30, 2.31, 2.32, 2.33, 2.34 and 2.35. The two measurements of each single crystal are plotted in a common graph in order to obtain and estimate the size of the band gap. The x-ray absorption spectra are shifted by the binding energy of the O 1s XPS peak (530 eV), so that the results are comparable with the band structure calculations already published by Raekers et al. [56]. In that publication the measurements were compared with ab initio calculations to determine the band gap of the investigated materials. Similar methods were already published elsewhere. Dong et al. [88] used the onset of O K XAS and XES spectra in order to determine the band gap of ZnO. Hüfner et al. [89] used the separation between the Fermi level of photoelectron spectra and the maximum of the first peak of bremsstrahlung isochromat spectroscopy (BIS). By using this methods it must be taken into account that the final states are different. On the one hand there are partial occupied, delocalised bands and on the other hand there are complete occupied bands. The Fermi levels are pinned differently in both methods and thus they have to be used very carefully. The advantages of XAS and XES compared with band structure calculations are, that small densities of states at the edge of the band gap are taken into account, the experiments are made under very similar conditions in a short time 45 2 RScO3 Figure 2.31: O K-edge XAS and O 2p → 1s XES of SmScO3 Figure 2.32: O K-edge XAS and O 2p → 1s XES of EuScO3 range, and the precise relative calibration of XAS and XES. A disadvantage is the element-specific band gap, but due to the delocalization and hybridization of the O 2p electrons this method is applicable. The x-ray absorption spectra are shifted, as mentioned before, by the binding energy of the O 1s XPS peak (530 eV) for easier comparison with the measurements and calculations from Raekers et al. [56]. To obtain experimental estimates for the sizes of the band gaps without the performance of new calculations the theoretical highest occupied and lowest unoccupied state were extrapolated by lines in the spectra of SmScO3 , GdScO3 and DyScO3 . The slopes of the XAS and XES spectra were extrapolated to the abscissa in order to receive the band gaps of the four further samples. The measurements show some states near the band gap which result from defects, these resulting states which come from the sample and not the system were ignored. By using the extrapolated lines in combination with the measurements we got the band gaps listed in table 2.3. For the determination of the band gaps states occurring in the band gap, caused by crystal defects, were neglected. The values for the band gaps obtained here are in a good agreement with band gaps which were determined by different methods, like ellipsometry measurements [73, 74], ultraviolet absorption results [90] and a combination of internal photoemission and photoconductivity measurements [91]. This technique is a good means to determine 46 2.4 XAS and XES/RIXS of R, Sc and O Figure 2.33: O K-edge XAS and O 2p → 1s XES of GdScO3 Figure 2.34: O K-edge XAS and O 2p → 1s XES of TbScO3 PrScO3 NdScO3 SmScO3 EuScO3 GdScO3 TbScO3 DyScO3 this work 5.7 eV 5.6 eV 5.6 eV 5.7 eV 5.8 eV 6.1 eV 5.9 eV Cicerrella[90] 5.7 eV 5.5 eV 5.4 eV 5.2 eV 5.6 eV 5.3 eV 5.5-6.0 eV 6.5 eV Lim et al. [73] Afanasev et al. [91] 5.6 eV Lucovsky et al. [74] 5.8 eV 5.7 eV Table 2.3: Band gaps of rare-earth scandates (in eV) as found in the present work (the upper line) in comparison with previously reported values. Since we applied the identical equivalent experimental conditions, the relative error bars are ± 0.1-0.2 eV; the absolut error could be larger. 47 2 RScO3 Figure 2.35: O K-edge XAS and O 2p → 1s XES of DyScO3 the band gap, especially when optical measurements are not possible for various reasons. The size of the band gaps changes in dependence of the involved rare-earth ions and is formed by rare-earth 4f , 5d and scandium 3d states hybridized with oxygen 2p. But surprisingly there is no linear dependence on the size of the rare-earth ions. To find out which structural parameter could be responsible for this unexpected nonlinear dependence, the band gaps together with various structural parameters were plotted in figure 2.36. The here discussed structural parameters were taken from Liferovich et al. [5]. The first checked value is the tolerance factor, the so called Goldschmidt factor. The Goldschmidt factor is a measure for the degree of distortion of a crystal with perovskite structure (ABO3 ). The ion radius of A, the rare-earth ion radius, is proportional to the Goldschmidt factor. For decreasing the ion radius the Goldschmidt factor becomes also decreases and thus is not suitable for describing the band gap behavior of the rare-earth scandates. The next test was to check the dependence between the size of the band gaps and the bonding angles between the scandium and the oxygen ions distances (Sc-O1, Sc-O2), respectively. But also there no correlation was found. The best accordance was found between the Sc-O mean bond length and the band gaps. The larger the Sc-O mean bond length the smaller the band gap. To explain this behavior, the electronic structure of the systems in question is reduced to that of 48 2.4 XAS and XES/RIXS of R, Sc and O conventional, ionic perovskite-type oxides: the formal valencies are R3+ Sc3+ and O2− , with the Sc 3d and R 5d states forming the conduction band, and O 2p states the valence band. At closer inspection it can be seen that the bonding is, typical for many perovskites, partially covalent. That leads to a small admixture of O 2p in the conduction band and an additional state of Sc 3d in the valence band. This could be evidenced by first-principles calculations (see figure 3 in Raekers et al. [56]), and by the present experiments. In the following two aspects will be discussed. First the Sc 3d states are not partially occupied, so that no Jahn-Teller effect comes about to induce the distortion of the ScO6 octahedral. Secondly, the splitting between the R 4f valence band states and unoccupied R 4f states admixed into the conduction band is much larger than the band gap and do not contaminate the edges of the latter. The band gap is, therefore, primarily influenced by the strength of the O 2p - Sc 3d interaction, which moves apart the barycenters of the valence band and the conduction bands, from which, further on, the (half-) width of each of these bands must be subtracted. As it seems, the band widths are not markedly dependent on structure parameters of individual compounds, whereas the band gaps (revealing, as it seems, the strength of O 2p - Sc 3d interaction) inversely follow a non-trivial variation of the mean Sc-O distance, as R changes - see figure 2.36 d). A remarkable observation is that the variations of bond length of ±0.4% give rise to amplified band gap variations of ±4.2%! Obviously, the real trend is much more complex, as the deformation of ScO6 octahedra is accompanied by non-negligible variation of Sc-O-Sc bond lengths. However, the latter do not fall onto any noticeable trend in the electronic structure. Also, no obvious correlation exists with the size of the R ion. 2.4.4 Conclusion The previous section contains a complete set of x-ray absorption an x-ray emission measurements for RScO3 (R = Pr, Nd, Sm, Eu, Gd, Tb, Dy). The O K-edge (XAS) and O 2p → 1s (XES) measurements were combined to determine the band gaps of the crystals. A determination of the band gap by this combination is possible, due to the delocalized and hybridized O 2p states. Finally a dependence between the band gap and the Sc-O mean distance was discovered. 49 2 RScO3 Figure 2.36: a) Tolerance factor (Goldschmidt factor) for RScO3 , b) Sc−O−Sc bond angles of RScO3 , c) Distances between Sc and the two oxygen spectra, d) Sc−O mean distance and experimental band gaps. All structural parameters have been extracted from Liferovich et al. [5]. 50 3 LuFe2O4 In this chapter the magnetic ground state configuration of the magneto electric, ferroelectric compound LuFe2 O4 is determined by means of (high field) x-ray magnetic circular dichroism (XMCD). Experimental data are compared with multiplet calculations, which are performed with the TT multiplet program [44] taking into account charge transfer and the crystal field. 3.1 Introduction Multiferroics are materials which exhibit more than one primary ferroic order parameter simultaneously. There are three basic primary ferroic order parameters, ferromagnetism, ferroelectricity and ferroelasticity. The way they can affect each other are shown in a multiferroic triangle in figure 3.1. Ferroelectricity P E Ferroelasticity Ferromagnetism P ε M ε σ N M S H Figure 3.1: Multiferroic triangle One class of multiferroics are multiferroic transition metal oxides, they have gained enormous attention during the last few years [66, 92–95]. Beside the group of per- 51 3 LuFe2 O4 ovskites and related compounds [93, 96, 97] the charge frustrated, layered compound LuFe2 O4 has attracted vast of interest due to its fascinating ferroelectric and magneto electric properties [6, 98, 99]. Especially the use of magneto electric coupling and multiferroics in spintronics has led to these huge interest in ferro electric magnets. The spinel LuFe2 O4 is a very promising candidate for such applications because of its giant room temperature magneto dielectric response [6], which suggests a strong coupling between spin moment and electric dipole [92]. The resulting giant magneto capacitance is due to charge ordering of iron ions [100]. In ferroelectric crystals a spontaneous polarization is arising from the arrangement of electric dipoles. First principle calculations [101, 102] and electron density analysis [103] of ferroelectric materials have revealed that the covalent bond between the anions and cations, or the hybridization of electrons on both ions, plays a key role in establishing the dipolar arrangement. However, for LuFe2 O4 an alternative model for electronic ferroelectricity was hypothesized by Portengen et al. [104]. They sad that the electric dipole depends on electron correlation, rather than on the covalence, that has been confirmed by Ikeda et al. [105]. A complex two dimensional ferrimagnetism plays an important role for the multiferroic properties of LuFe2 O4 . Below 250 K a long range ferrimagnetic order sets in [106]. The fact that the ferroelectricity is caused by correlated electrons from the iron ions leads to unusual properties and unique capabilities of LuFe2 O4 . A large response of the dielectric constant by applying small magnetic fields has been found, opening a route for future devices [6]. Phase transitions from the charge ordered phase have been very recently associated with a non linear current voltage behavior, and an electric field induced phase transition, which might be of interest for potential electric pulse induced resistive switching applications [107, 108]. The large magneto electric coupling has been attributed to an intricate interplay of charge and spin degrees of freedom with the crystal lattice and external electrical and magnetic fields, to some extent on a short range order [109–113]. However, there is still some confusion about √ the √ nature of spin-charge coupling in LuFe2 O4 . In particular a model proposing a 3 x 3 charge ordered (CO) ground state [100, 114] is challenged by simulations implying that the electrical polarization in LuFe2 O4 is due to spin-charge coupling and a spin frustrated magnetic ground state in a chain CO state [115, 116].On the other hand the first model finds a ferromagnetic spin ground state where Fe2+ and 1/3 of Fe3+ make up majority spin, and 2/3 of the Fe3+ make up minority spin, which is confirmed by XMCD experiments performed at around 200K and fields up to 6T [7, 117]. However, the discussion about the magnetic properties and the nature of the magnetoelectric coupling in LuFe2 O4 goes on. E. g., Phan et al. found a complex magnetic phase diagram with not only a ferrimagnetic transition at 240K but also additional magnetic transitions at 225K and 170K, and 55K [118]. Furthermore, it was not possible to fully saturate the LuFe2 O4 crystal at low temperatures in the above mentioned XMCD experiments, since one needs high fields up to 16 Tesla in order to saturate LuFe2 O4 , which then shows widely open hysteresis loops [111]. Therefore, we want to go beyond 52 3.2 Basic Properties of LuFe2 O4 the research done so far and perform a systematic temperature dependent study of the magnetic ground state of LuFe2 O4 with the possibility to perform experiments at the Fe K edge and Lu L2,3 edges under high fields up to 18 T. 3.2 Basic Properties of LuFe2O4 The investigated single crystal LuFe2 O4 has a two-dimensional layered rhombohedral (R3m) structure, with the lattice constants of a = 3.439Å and c = 25.258Å. It is composed of the alternate stacking of the hexagonal FeO2.5 layer (W-layer) and the hexagonal LuO1.5 (U-layer) along the c-axis [119]. The W layers comprise two triangular nets of Fe ions, the resulting electric polarization is induced via a frustrated charge ordering of Fe2+ and Fe3+ ions on the resulting honeycomb lattice below 330K [105, 120, 121]. The color of the crystal is between black and purple. In figure 3.2 on the left side the layered arrangement of lutetium, iron and oxygen is shown. On the right side the iron double layers are shown with a triangular interconnectivity. The Fe-Fe distances within a layer are 3.44 Å and are longer than the Fe-Fe distance between the layers which have a distance of 3.156Å. [6] 3.3 Preparation of LuFe2O4 The investigated LuFe2 O4 single crystal was made by D. Prabhakaran in cooperation with the group of S. Blundell at the University of Oxford, Oxford, UK. The crystal was grown by the conventional Czochralski technique. 3.4 XMCD of Iron L2,3-edges in LuFe2O4 The here presented measurements were made at beamline 4.0.2 at the advanced light source (ALS) in Berkeley, USA. The XMCD measurements were performed at 150K at 6T. The sample was cleaved at an ambient pressure of around 5x10−5 mbar and a temperature of around 80K (so called nitrogen precooling) before being transferred into the helium cryostat with a pressure around 5x10−8 mbar. The c-axis of the LuFe2 O4 single crystal was aligned parallel to the external applied magnetic field [7]. This alignment is common for all presented XMCD measurements in this work. In figure 3.3 the Fe L2,3 -edges spectra, recorded with left and right circular polarized x-rays and measured in TEY (black), are presented in the upper part of the graph. The belonging dichroic signal is plotted in green (a). Two multiplet calculations considering different possible spin orderings are plotted in the lower part of the graph for comparison (b and c in orange). The Fe2+ and Fe3+ peak of the L3 -edge are located at 708 eV and 709.5 eV, respectively. The best agreement between multiplet calculations and experiment is achieved with 50% antiferromagnetic Fe3+ and 50% magnetic mixed 53 3 LuFe2 O4 Figure 3.2: Crystal structure of LuFe2 O4 with Lu (large dark-gray spheres), Fe (small black spheres) and O (large white spheres). Taken from Subramanian et al. [6]. valent bulk, resulting in overall Fe3+ contribution of 75%. The Fe L2 -edge peaks are located at 721 eV and 723 eV with a shoulder at 720 eV. The experimental dichroic signal at the L2 -edge is very small, but the dichroism in the calculated L2 -edge can be found also in the experiment. The L3 -edge shows a clear dichroism at the Fe2+ peak and a smaller dichroism at the Fe3+ peak which is inverted. In figure 3.3, graph (b) is calculated with the following spin ordering: Fe2+ (↑)+1/3*Fe3+ (↑)+2/3*Fe3+ (↓) and (c) with: 2/3*Fe2+ (↑)+1/3*Fe2+ (o)+2/3*Fe3+ (↑)+1/3*Fe3+ (↓). The agreement at the dichroism at the Fe3+ peak of the Fe L3 -edge with the calculated dichroism signal in (b) is a clear indication of a majority spin at the Fe2+ sites, while 1/3 of the Fe3+ spin is in majority and 2/3 is in minority. This configuration gives a perfect agreement with the experiment besides the shoulder at 707 eV which is over estimated by the calculation. Such a configuration was found before in LuFe2 O4 by Mössbauer and neutron diffraction [122, 123]. The calculation of the magnetic moment of LuFe2 O4 54 3.5 XAS and XMCD of Iron K-edge in LuFe2 O4 Figure 3.3: LuFe2 O4 Fe L2,3 -edges XMCD performed at 150K (black), the belonging dichroic signal is green (a). It is compared to multiplet calculations considering different possible spin orderings (b and c). The data are taken from Kuepper et al. [7]. with the sumrules applied on the XMCD measurement gives a result of 2.33µB /f.u. by using the Teramura coefficient (presented in section 1.1.5). This is in a good agreement with the results presented by Iida et al. [124]. A more detailed analysis of the XMCD on iron L2,3 -edges can be found in Kuepper et al. [7]. 3.5 XAS and XMCD of Iron K-edge in LuFe2O4 To investigate the magnetic ground state of the magnetoelectric and ferroelectric compound LuFe2 O4 some high field x-ray magnetic circular dichroism (XMCD) measurements were made. For a complete magnetic saturation a magnetic field of 15T at a temperature of 10K is needed. These kind of measurements should be made at the high field endstation at the ESRF on beamline ID12, which can create magnetic fields up to 18T. But after problems with the power supply, the second endstation with magnetic fields up to 6T was used. So it was not possible to get into magnetic saturation, but the here presented measurements were comparable to the XMCD measurements on the Fe L2,3 -edges, performed at the SXF-endstation at the ALS (see 1.3.2.2) and presented in the previous section 3.4. Figure 3.5 shows iron K-edge spectra of LuFe2 O4 at different temperatures, aligned parallel to the crystal c-axis. The measurements were made at an external magnetic field of 6T. The dichroic signal is plotted for 125K, 55 3 LuFe2 O4 Figure 3.4: XMCD measurement (dark green) and XAS measurement (light green) of the Fe K-edge on LuFe2 O4 performed at 125K and 6T. 150K, 175K, 200K, 225K. 250K, 275K and 300K. The intensity of the signal increases at lower temperatures, so in this case the maximal dichroic signal is visible at 6T and 125K. Exemplarily the XMCD spectra for 6T at 125K and the corresponding XAS spectrum are plotted in figure 3.4. The XMCD shows a rather sharp feature between 7112 eV and 7114 eV. This area is called the pre-edge and stems from 1s → 3d transitions, it is followed by a small double peak with opposite sign located between 7115 eV and 7122 eV. From 7122 eV up to 7133 eV a rather broad feature appears with a positive sign. In figure 3.6 the pre-edge of the dichroic signal is enhanced and plotted in green. Now a small minimum is visible, followed by the main peak. In addition a multiplet-calculation (black) is plotted. The multiplet calculation results from a mixture of Fe3+ and Fe2+ calculations. It was considered that Fe3+ is five times weighted and Fe2+ four time weighted, due to the different number of d-electrons. The spin ordering of the iron ions in LuFe2 O4 was already determined by XMCD measurements and multiplet-calculations of the Fe L2,3 -edges [7]. The experiment allowed the following calculated spin ordering: Fe2+ (↑)+1/3*Fe3+ (↑)+2/3*Fe3+ (↓). Also this spin ordering was considered. The multiplet calculation is in a very good agreement with the experiment. The first minimum and maximum are very good mapped by the multiplet calculation. The second minimum and maximum over swing and the third minimum is 2 eV shifted to smaller photon energies. It is not surprisingly that the calculations are after the second minimum in a worse agreement with the experiment because of the delocalized 1s → 2p dipole transitions at higher photon energies, which 56 3.5 XAS and XMCD of Iron K-edge in LuFe2 O4 Figure 3.5: XMCD measurements of the Fe K-edge on LuFe2 O4 at different temperatures. Figure 3.6: XMCD Pre-edge measurements (green) at the Fe K-edge of LuFe2 O4 compared to multipletcalculation (black). will not be reproduced by multiplet-calculations. The main-edge has to be calculated by band structure calculations. These calculations are planed in the near future. 57 3 LuFe2 O4 Fe2+ 1s2 3d6 initial Fe2+ 1s1 3d7 final Fe3+ 1s2 3d5 initial Fe3+ 1s1 3d6 final F23d3d 10.966 11.680 9.634 10.189 F43d3d 6.815 7.258 6.028 6.370 Slater integrals G32p3d 0.0584 0.0524 Spin-orbit coupling LS3d 0.052 0.0672 0.058 0.075 Table 3.1: Slater integrals (in eV) used for the Fe2+ and Fe3+ charge-transfer multiplet simulations of the Fe K-edge XAS. 58 3.6 XAS and XMCD of Lutetium L2,3 -edges in LuFe2 O4 3.6 XAS and XMCD of Lutetium L2,3-edges in LuFe2O4 Figure 3.7: XMCD (orange) and XAS (green) measurements on the Lu L2,3 edges of LuFe2 O4 . The temperature was 150K and the external applied magnetic field had 9T. The k was parallel to the c-axis of the crystal. The lutetium L2,3 -edges was measured at the ID 12 beamline of the ESRF in Grenoble. The reason for this measurement was to elucidate if also the Lu ions carry a magnetic moment and in which orientation this moment is orientated with respect to the magnetization of the iron ions. In figure 3.7 the XAS (green) and the XMCD signal (orange) of the Lu L2,3 -edges are plotted. The measurements were made at 150K and 9T, so they are in the same temperature range and the same magnetic field range as the previous shown Fe L2,3 -edges XMCD measurements and the Fe K-edge XMCD measurements. The Lu L3 -edge and the Lu L2 -edge have a quite large distance, so the energy axis (abscissa) is splitted. On the left side from 9220 eV up to 9300 eV the Lu L3 -edge is shown. The XAS spectra is dominated by a sharp edge resulting in a main peak at 9250 eV. This main peak is followed by a small shoulder/peak at 9260 eV and two further peaks at 9270 eV and 9290 eV. The right part of the axis belongs to the Lu L2 -edge and has an energy range from 10330 eV up to 10410 eV. The Lu L2 -edge signal starts with higher intensities, due to an underground caused by the Lu L3 -edge. From this background a sharp edge is rising at 10350 eV also resulting in the main peak at 10355 eV. The following smaller peaks resemble the peak structure of the Lu L3 -edge. There is again a small shoulder like peak close to the main peak at 10365 eV and two bigger peaks at 10380 eV 10395 eV. In the same graph the XMCD signal is shown in orange. The measurements with different orientations of light and magnetic field 59 3 LuFe2 O4 are not shown, due to the very small differences in the absorption spectra of different polarized light. The measurements were taken with all four possible combinations of polarization of the light and magnetic field. The XMCD signal at the Lu L3 -edge is dominated by a double peak with opposite sign, starting in negative direction. A wavelike structure with four positive and four negative signals followed. Again the Lu L2 -edge is very similar to the Lu L3 -edge apart from the first double peak with opposite direction, in this case the double peak starts with a positive signal. So it can be concluded that the Lu ions carry a small magnetic moment, due to the clear dichroic signal, although the size of the effect is very small. The orbital moment and the spin moment for the Lu L2,3 -edges can be calculated by using the XMCD sum rules which are explained in section 1.1.5. Therefore the integrals over the dichroicRsignal at the L3 and L2 -edge, respectively, are needed. The following values are used: L3 (µ+ − µ− )dω R R = -0.001415, L2 (µ+ − µ− )dω = 0.018649 and L3 +L2 (µ+ + µ− )dω = 32.562. These values result to an orbital moment of approximately morb = 0.011µB /f.u. and a spin moment in range of mspin = −0.018µB /f.u.. These results are vulnerable to mistakes, due to the small intensity of the used signal. But although it is difficult to extract the magnetic moment quantitatively exact from the sum rules, the overall shape of the XMCD signal allows to conclude that the moments of the Lu ions are aligned in a similar way as those of the iron ions. 3.7 SQUID and XMCD Hysteresis SQUID and XMCD are two methods to get information about the magnetization of a material. Both methods were applied to the single crystal LuFe2 O4 and compared. The SQUID measurement were performed at the University of Ulm, Germany by Dr. Kuepper. The XMCD measurements on the iron K-edge as mentioned in section 3.5 at the ID12 beamline of the ESRF. Both presented measurements are shown in figure 3.8. They were made at around 150K. The magnetic field was changed from -6T to 6T and vice versa. As result there is in both cases a hysteresis loop visible. The hysteresis are in both cases normalized and the shape is absolutely equal. The XMCD hysteresis is a result of the changing hight of the XMCD signal of the iron K-edge. There are no sumrules available for this method so the analysis is absolutely qualitatively. The perfect fit with the SQUID measurement is a proof that the magnetic behavior is really good reproduced by XMCD K-edge measurements on iron. The SQUID measurements yield a magnetic moment of 1.96µB per formula unit. 3.8 Conclusion LuFe2 O4 was investigated by XAS/XMCD at the iron L-edges, iron K-edge and lutetium L-edges, respectively. Supplemental SQUID measurements were made. The 60 3.8 Conclusion Figure 3.8: SQUID measurements (blue) compared to XMCD measurements (green), both measurements are recorded at 150K. iron pre-edge of the iron K-edge measurements were simulated by multiplet-calculations, which reproduce the localised quadrupole transitions. The changing in the intensity of the XMCD signal at 150K at the iron K-edge in dependents from the strength of the external magnetic field is in agreement with SQUID measurements at the same temperature. So it can be ascertained, that XMCD at the iron K-edge is suitable to get information about the element specific magnetization of a material. Additionally the pre-edge was simulated by multiplet calculations. The XMCD measurements on the lutetium L2,3 -edges show that the ions carry a moment, even though it is quite small. The sum rules were used but the results were discussed on their plausibility. 61 62 4 Iron-Based Magnetic Polyoxometalates In the present chapter an experimental and theoretical study of three giant Keplerate VI VI structural-type molecules of the type {(M)M5 }12 FeIII 30 (M=Mo , W ) is presented. Besides x-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) measurements, DFT calculations and multiplet calculations are used to interpret the experimental results. Furthermore, a detailed study of the Fe3+ to Fe2+ photoreduction process, which is induced under soft x-ray radiation in these molecules, is presented. In the last section x-ray magnetic circular dichroism (XMCD) and SQUID magnetometry measurements of W72 Fe30 sulfate are presented. The molecules were synthesized by the group of Prof. Achim Müller from the University of Bielefeld, Germany. The DFT calculations were performed by Prof. Andrei Postnikov from the Paul Verlaine University in Metz, France. Parts of this chapter are already submitted for published by Kuepper, Derks et al. [125]. 4.1 Introduction Polyoxometalates are an interesting class of inorganic compounds. One part of this class are the giant Keplerate structural-type molecules. Three of these molecules will be presented in the following chapter: Namely Mo72 Fe30 acetate, W72 Fe30 sulfate and Mo72 Fe30 sulfate. The formulas and equal nomenclatures used in the following chapter are shown here: i)[M o72 F e30 O252 (CH3 COO)10 M o2 O7 (H2 O)H2 M o2 O8 (H2 O)3 (H2 O)91 ]•ca.100H2 O ≡ 1 ≡ 1a • ca.100H2 O ≡ M o72 F e30 acetate ii)N a6 (N H4 )20 [F eIII (H2 O)6 ]2 [(W IV )W5IV O21 (SO4 )12 F e(H2 O)30 (SO4 )13 (H2 O)34 ] ≡ 2 ≡ N a6 (N H4 )20 [F eIII (H2 O)6 ]2 • 2a • ca.100H2 O ≡ W72 F e30 sulf ate iii)N a9 K3 [K20 ⊂ M oV70I F eIII 3 0O252 (SO4 )24 (H2 O)75 ] • ca.140H2 O ≡ 3 ≡ N a9 K3 3a • ca.140H2 O ≡ M o72 F e30 sulf ate 63 4 Iron-Based Magnetic Polyoxometalates Further information about previous experimental and theoretical studies even in the field of magnetic behaviour of Mo72 Fe30 acetate can be found in several articles [126–131]. Information about W72 Fe30 sulfate are reported by Todea et al. [132] as well as information about Mo72 Fe30 sulfate [133]. 4.2 Core Level XPS of Iron, Oxygen, Molybdenum and Wolfram In this section the results of the x-ray photoelectron spectroscopic (XPS) studies of the three molecules Mo72 Fe30 acetate, W72 Fe30 sulfate and Mo72 Fe30 sulfate are presented. A special focus will be set on the different ligands according to their influence on the iron, wolfram, molybdenum and oxygen ions during the x-ray measurements. 4.2.1 Specific Experimental Details The following photoelectron spectra were recorded at the local PHI 5600ci MultiTechnique XPS-System. The total energy resolution of the used monocromated xray source is around 0.3-0.4 eV. The resolution of the electron energy analyser is around 80 meV. The monochromatized Al Kα source was used with 250 W power supply. To keep a stable charging of the samples the measured core level lines were performed under the use of a low energy electron flood gun. The pressure was around 1*10−9 mbar. The investigated molecules are poly-crystalline powders and were fixed with carbon tape on a sample holder. The measurements were performed at room temperature. 4.2.2 Iron Core Levels The formal valence state of the iron ions in Mo72 Fe30 acetate, W72 Fe30 sulfate and Mo72 Fe30 sulfate is Fe3+ [126, 132, 133]. From earlier measurements on molecules it is known that x-rays can change the valence state of ions in molecules. Hence profile measurements were taken to get separated spectra over a long timescale. In the left panel of figure 4.1 the first and the last Fe 2p spectrum of W72 Fe30 sulfate of a 17 hours lasting measurement is shown. It is obvious that the spectra changed during the measurement. The first spectrum plotted in red at the bottom of the left panel of figure 4.1 looks close to the Fe3+ reference, LiFeO2 , which is plotted in black at the bottom of the figure. The Fe 2p3/2 main peak is located at 711 eV and the characteristic Fe2+ satellite around 715 eV is not visible. Nevertheless there is to mention that the characteristic Fe3+ satellite around 719 eV is not visible as well. The Fe 2p1/2 peak is at 725 eV and so the spin orbit splitting is 14 eV. The final spectrum is shown in red at the top of the left panel of figure 4.1. It looks closer to the reference for Fe2+ , FeO, which is plotted in black at the top of the graph. The 64 4.2 Core Level XPS of Iron, Oxygen, Molybdenum and Wolfram Figure 4.1: XPS measurements of Fe 2p (left panel) and Fe 3s (right panel) of W72 Fe30 sulfate (red) in comparison with Fe2+ to Fe3+ reference compounds (black). main Fe 2p3/2 peak now is located at 709.7 eV with a shoulder at 709 eV. In addition the two satellites at 715 eV and 728.5 eV are typical for Fe2+ . Both spectra seem to reflect a mixed Fe2+ , Fe3+ valence state during the measurement. In the right panel of figure 4.1 the Fe 3s peak is measured, as shown before for Fe 2p, in the beginning and after 17 hours. The first measurement is plotted in red at the bottom of the graph and the last measurement also in red on the top of the graph. The Fe3+ and Fe2+ references are plotted in black, as before, LiFeO2 (Fe3+ ) at the bottom and FeO (Fe2+ ) at the top of the graph. In this case the shape differs less than at the Fe 2p spectra. A clear interpretation of the valence state is not possible. Next the two Fe 2p spectra of Mo72 Fe30 sulfate and Mo72 Fe30 acetate are discussed. In the left panel of figure 4.2 the first and the last spectrum of a 19 hours lasting measurement of Mo72 Fe30 sulfate is shown in red and in the right panel of figure 4.2 the first and the last measurement of a 4 hours lasting measurement of Mo72 Fe30 acetate is shown also in red. The references for the iron valence states 2+ and 3+ are plotted in black. For the first measurements in both cases the Fe 2p3/2 peaks are around 711 eV and the Fe 2p1/2 peaks around 725 eV. That leads to a spin-orbit splitting of 14 eV. At the last measurements which are plotted in red on the top of the panels a chemical shift to lower binding energies and a shoulder around 709 eV becomes obvious. So a reduction from Fe3+ to Fe2+ can be assumed. On this first view the spectra look similar to each other and to the Fe 2p spectrum of W72 Fe30 sulfate, but we have to take into account, that the measurements of the 65 4 Iron-Based Magnetic Polyoxometalates Figure 4.2: XPS measurements of Fe 2p of Mo72 Fe30 sulfate (left panel, red) and Mo72 Fe30 acetate (right panel, red) in comparison with Fe2+ to Fe3+ reference compounds (black). molecules containing sulfate took nearly 4 to 5 times longer. So it was shown that the changing of the ligand acetate to sulfate slows the reduction process down. But there is no quantitative analysis possible by these XPS measurements. This will be done in section 4.3.2, by using x-ray absorption spectra of iron in all three molecules. 4.2.3 Oxygen Core Levels The oxygen core levels of W72 Fe30 sulfate (a), Mo72 Fe30 sulfate (b) and Mo72 Fe30 acetate (c) can be seen in figure 4.3. The spectra at the bottom are always the first measurements and the spectra at the top of the graph refer the last measurements. At the beginning the O 1s peak of W72 Fe30 sulfate is located at 530.2 eV, for Mo72 Fe30 sulfate it is 530.4 eV and for Mo72 Fe30 acetate the O 1s is located at 530.6 eV. After 17 hours radiation the O 1s peak of W72 Fe30 sulfate is still at the same position. The O 1s peak of Mo72 Fe30 sulfate is shifted after 19 hours radiation marginal to 530.4 eV. A similar result has been found for Mo 3d and will be discussed within the chapter 4.2.4. Mo72 Fe30 acetate has a small shift to 530.2 eV after 4 hours of x-ray radiation. So it can be assumed that the oxygen and its electronic configuration is influenced by the x-ray radiation. This result is used in the next section 4.3.3 by discussing the O K-edge spectrum of the molecules. There seem to be no radiation caused effects. 66 4.2 Core Level XPS of Iron, Oxygen, Molybdenum and Wolfram Figure 4.3: XPS measurements of O 1s of W72 Fe30 sulfate (a), Mo72 Fe30 sulfate (b) and Mo72 Fe30 acetate (c). 4.2.4 Wolfram and Molybdenum Core Levels The wolfram 4f core levels of W72 Fe30 can be seen in figure 4.4. The first measurement is plotted at the bottom of the graph and the last spectrum on the top. The first spectrum consist of a W 4f7/2 peak which is located at 36 eV and a W 4f5/2 peak at 37.8 eV. Additional there are two shoulders at 34.1 eV and 35.3 eV. The shoulder around 41.8 eV is the W 5d3/2 peak. The positions of the peaks are an indicator for W6+ [134]. The spin orbit splitting is 1.8 eV. There seems to be no significant amount of lower valences, otherwise there would be a triplet instead of a doublet. The last spectrum, at the top of the graph, is taken after 17 hours. The W 4f7/2 peak is located at 35.9 eV and the W 4f5/2 peak at 37.9 eV. The spin orbit splitting is 2 eV and the W 5d3/2 peak is located at 41.9 eV. There are only marginal shifts of the peak positions. So it seems that there is a stable W6+ valence state under the permanent x-ray radiation. The molybdenum 3d core levels are plotted in figure 4.5, Mo72 Fe30 acetate on the left panel and Mo72 Fe30 sulfate on the right panel. The first taken spectra are, as usual, plotted at the bottom of the graphs and the last spectra on the top. The Mo 3d5/2 and Mo 3d3/2 peak positions of Mo72 Fe30 acetate are equal for the first and the last measurement at 232.6 eV and 235.8, respectively. For Mo72 Fe30 sulfate the Mo 3d3/2 peak is shifted a little bit from 235.6 eV to 235.5 eV and the Mo 3d3/2 peak is shifted 67 4 Iron-Based Magnetic Polyoxometalates Figure 4.4: XPS measurement of W 4f of W72 Fe30 sulfate. from 235.6 eV to 235.5 eV. In both cases the peak positions are typical for Mo6+ [46], which was expected. Mo72 Fe30 sulfate shows a small satellite around 240 eV. The O 1s peak of this molecule shows also such a peak in the same distance. As the O 1s spectrum also the Mo 3d spectrum of Mo72 Fe30 sulfate contains a small satellite located at 4.5 eV higher binding energy than the main peak (240 eV in case of Mo 3d). 4.2.5 Conclusion In conclusion it was shown that the three molecules change under x-ray radiation, but there are differences in the affected ions. There was a change in the valence state of the iron ions from 3+ to 2+. The amount of the reduction could not be denominated in numbers, nevertheless differences at the reduction rate became obvious. At the three investigated molecules it could be observed that the sulfate ligand reduces or at least slows the reduction. The second result is that there seem to be no significant changes in the valence states of oxygen, molybdenum or wolfram under x-ray irradiation. 68 4.2 Core Level XPS of Iron, Oxygen, Molybdenum and Wolfram Figure 4.5: XPS measurement of Mo 3d of Mo72 Fe30 acetate (left) and Mo72 Fe30 sulfate (right). 69 4 Iron-Based Magnetic Polyoxometalates 4.3 XAS In the last section (4.2.2) the x-ray photoelectron measurements of iron in the molecules 1 to 3 were reported. It was not possible to give a quantitative estimation of the amount of Fe2+ and Fe3+ in the sample after the samples were radiated by a certain flux of x-rays. In the following section the order of the reduction process and a basic approach about the underlying mechanism is given. The shown XAS measurements and the multiplet calculations are already submitted for publication by Kuepper, Derks et al. [125]. 4.3.1 Specific Experimental and Theoretical Details The here presented x-ray absorption measurements were performed at two different beamlines. W72 Fe30 sulfate and Mo72 Fe30 acetate were measured at the Advanced Light Source, beamline 8.0.1. The experiments were made at the x-ray fluorescence end station from the University of Tennessee at Knoxville at room temperature. For more detailed information see section 1.3.2.1. Mo72 Fe30 acetate and Mo72 Fe30 sulfate were measured at the Russian-German Beamline at Bessy II. All spectra were recorded at room temperature. Additional information can be found in section 1.3.4.1. All absorption spectra were taken in total electron yield (TEY) mode. The measured Fe L2,3 -edges x-ray absorption spectra were simulated within the charge-transfer multiplet model using the TT-multiplet program [36, 45, 135]. After the atomic energy levels of the initial (2pn 3dm ) and final (2pn−1 3dm+1 ) states were calculated and reduced to 80% of their Hartree-Fock values (see table 4.1), an octahedral crystal field was considered. Finally, we considered charge transfer by introducing 3dm+1 L states and broadened the simulated spectra, considering lifetime broadening and spectrometer resolution. The first-principles density-functional calculations, performed by the SIESTA method [136], were made by Prof. A. Postnikov. The SIESTA method uses norm-conserving pseudo potentials in combination with numerical atom-centered strictly confined basis functions. Exchange-correlation potential was taken after generalized gradient approximation (GGA) in the formulation of Perdew-Burke-Ernzerhof [137]. The molecules (neutral Mo72 Fe30 acetate and 6-charged W72 Fe30 sulfate) were placed in a cubic simulation cell having a 36 Å edge, preventing an overlap of basis functions across the cell boundary with the molecule’s spurious replicas. Basis functions were generated by the split-norm technique, the standard technique in SIESTA. Typically, the quality of the basis set was double-zeta with polarization orbitals‘(see [138]), except for the ’ Fe 3d status, which was of triple-zeta quality. 70 4.3 XAS Fe2+ 2p6 3d6 initial Fe2+ 2p5 3d7 final Fe2+ Fe2+ Fe3+ 2p6 3d7 L 2p5 3d8 L 2p6 3d5 initial initial final Fe3+ 2p5 3d6 final Fe3+ Fe3+ 2p6 3d6 L 2p5 3d7 L initial final Slater integrals F23d3d 10.966 11.779 9.762 10.623 12.043 12.818 10.966 11.779 F43d3d 6.815 7.327 6.018 6.560 7.535 8.023 6.815 7.327 F22p3d 6.793 6.143 7.446 6.793 G12p3d 5.004 4.467 5.566 5.004 G32p3d 2.844 2.538 3.166 2.844 8.200 8.202 8.199 8.200 Spin-orbit coupling LS2p LS3d 0.000 0.000 0.000 0.000 0.059 0.074 0.052 0.067 Table 4.1: Slater integrals (in eV) used for the Fe2+ and Fe3+ -charge-transfer multiplet simulations of the Fe L2,3 -edges XAS. The spin-orbit parameters were not reduced, whereas the d-d and p-d integrals were reduced to 80% of the Hartree-Fock values for the subsequent simulation of the spectra. 4.3.2 Iron L2,3 -edges There were two series of the molecules measured. The first one with the molecules 1 and 2 at the undulator based beamline 8.0.1 at the ALS and the second series of the molecules 1 and 3 at the dipole Russian-German-Beamline at the Bessy II. Due to the different kind of beamlines the two series are discussed seperately. A final conclusion is made in the end of this chapter. All measurements were compared to spectra simulated by the charge-transfer multiplet model using the TT-multiplet program [36, 45, 135]. So the following quantitative analysis stem from this simulations. The specifications for these simulations are described above in section 4.3.1. 4.3.2.1 Mo72 Fe30 acetate and W72 Fe30 sulfate at the ALS The now described Fe L2,3 -edges measurements are displayed in figure 4.6. The results for Mo72 Fe30 acetate on the left side and for W72 Fe30 sulfate on the right side. 71 4 Iron-Based Magnetic Polyoxometalates For the first measurement on molecule 1 the beamline photon flux was reduced to 12.5 % of its normal intensity and took, like all following measurements, 8 minutes. For this scan there was, compared to multiplet calculations, a fraction of 90 % Fe3+ in the sample. Due to technical reasons this is the only measurement with such a small intensity, afterwards the lowest used intensity is 25 % intensity of the maximum flux. On a fresh, second spot, there were four measurements made with 25 %, 50 %, 75 % and 100 % intensity of the photon flux. The peaks at the L3 -edge around 707 eV correspond to the Fe2+ and the peaks at 709 eV to the Fe3+ contribution. At the L2 -edge the peak for Fe2+ is located at 720 eV and for Fe3+ at 722 eV. The reduction from Fe3+ to Fe2+ is easy visible on the L3 -edge were the left peak corresponding to Fe2+ increased and the right peak decreased. In numbers, the percentage quotation of Fe3+ decreased from 80 % to 12.5 %. All values can be seen in table 4.2. For molecule 2 the reduction rate is slower. After the first cycle the Fe3+ percentage quotation is 85 % and after the fourth cycle, with full intensity, the Fe3+ fraction is still 37.5 %. To get the percentage quotation independent from the beam current the intensity and the beam current were combined to a new variable photon flux‘. The Fe3+ fraction is ’ plotted over the photon flux‘in figure 4.7. This plot makes obvious, that the reduction ’ processes have different timescales and different final values. Possible reasons for these differences will be discussed after the results of molecule 1 and molecule 3 from the Bessy II measurement were interpretated in the following section (4.3.2.2). Sample Spot Scan Intensity Fe3+ total time total flux 1 1 1 12.5% 90% 8 min 6.6 % 1 2 1 25% 80% 8 min 9.1 % 1 2 2 50% 35% 16 min 29.87 % 1 2 3 75% 30% 24 min 57.52 % 1 2 4 100% 12.5% 32 min 100 % 2 1 1 25% 85% 8 min 6.3 % 2 1 2 50% 67.5% 16 min 22.6 % 2 1 3 75% 47.5% 24 min 53.7 % 2 1 4 100% 37.5% 32 min 95.2 % Table 4.2: Fraction of Fe3+ after x-ray radiation with different intensities. 72 4.3 XAS Figure 4.6: Fe L2,3 -edges XAS series of 1 (left) and 2 (right). The experimental data are green and the black lines represent the corresponding simulated spectra, which were obtained by superimposing corresponding fractions of the simulated Fe3+ and Fe2+ spectra. Figure 4.7: Fraction of Fe3+ versus the percentage x-ray photon flux for the XAS series shown in figure 4.6. 73 4 Iron-Based Magnetic Polyoxometalates 4.3.2.2 Mo72 Fe30 acetate and Mo72 Fe30 sulfate at the Bessy The molecules Mo72 Fe30 acetate and Mo72 Fe30 sulfate were measured at the dipole Russian-German Beamline at the Bessy II. All spectra were taken with full intensity. The spectra of sample 3 are plotted in figure 4.8. Each scan took 14 minutes and it is obvious, that there is a reduction from Fe3+ to Fe2+ . To make the results comparable (to the previous measurements of molecule 1 and 2) the Fe L2,3 -edges of molecule 1 is also measured and plotted in figure 4.9. Each scan took 14 minutes. Already after three scans a Fe3+ percentage quotation of only 25 % is reached. To make the reduction more obvious, the Fe3+ percentage quotation of the three series of the two molecules are plotted over the photon flux ‘in figure 4.10 and additionally shown in ’ table 4.3. It can be seen, that the reduction from Fe3+ to Fe2+ happens faster in the molecule 1. An even much faster Fe3+ to Fe2+ photoreduction process has been observed for star shaped Fe4 single magnetic molecule [139], where the Fe3+ ions are coordinated within an octahedral environment comprising four oxygen atoms and two nitrogen ligands. This is a further indication that the reason for the photoreduction process and especially the timescale of the photoreduction process depends from the different coordination chemistry. Figure 4.8: First (left) and second (right) series of Fe L2,3 -edges of molecule 3. The green lines represent the experimental data and the black lines represent the corresponding simulated spectra that were obtained by superimposing corresponding fractions of simulated Fe2+ and Fe3+ spectra. 74 4.3 XAS Figure 4.9: Series of Fe L2,3 -edges of molecule 1 in green and simulated spectra in black. Figure 4.10: Fraction of Fe3+ versus the percentage x-ray photon flux for the XAS series shown in figure 4.9 and 4.8. 75 4 Iron-Based Magnetic Polyoxometalates Sample Spot Scan Intensity Fe3+ total time total flux 3 1 1 100% 85% 14 min 9.7 % 3 1 2 100% 65% 28 min 19.3 % 3 1 3 100% 60% 42 min 28.28 % 3 1 4 100% 55% 56 min 38.7 % 3 1 5 100% 52.5% 70 min 40.3 % 3 1 6 100% 50% 84 min 42.5 % 3 1 7 100% 50% 98 min 48 % 3 1 8 100% 47.5% 112 min 50.5 % 3 1 1 100% 75% 14 min 15.2 % 3 1 2 100% 55% 28 min 30.1 % 3 1 3 100% 52.5% 42 min 44.6 % 3 1 4 100% 47.5% 56 min 53.9 % 3 1 5 100% 45% 70 min 72.8 % 3 1 6 100% 42.5% 84 min 86.5 % 3 1 7 100% 40% 98 min 100 % 1 1 1 100% 60% 14 min 10.1 % 1 1 2 100% 37.5% 28 min 20.2 % 1 1 3 100% 25% 42 min 30.1 % Table 4.3: Fraction of Fe3+ after x-ray radiation at the Bessy 76 4.3 XAS 4.3.3 Oxygen K-edge In the following, the oxygen K-edge XAS of molecules 1 and 2 will be discussed in comparison to the projected calculated DOS. These first-principles density-functional calculations, performed by the SIESTA method [136], were made by Prof. A. Postnikov. For detailed information see section 4.3.1. On top of figure 4.11 the O K XAS of Mo72 Fe30 acetate (left) and W72 Fe30 sulfate (right) are shown. They were taken at a new spot with a low photon flux in order to get mainly information about the “original” electronic ground state. As it was already shown in the XPS section 4.2.3, the oxygen ions are less influenced by potential soft x-ray induced modifications. There is a strong hybridization between Fe 3d, Mo 4d or W 5d and the unoccupied O 2p states, so the x-ray absorption spectra of the O K-edge represents the conduction band. For comparison of the measurements and the calculated DOS, the spectra were shifted to a common energy scale, with the Fermi energy set to zero. Looking at the left side of figure 4.11, three main features stand out for Mo72 Fe30 acetate. The first one at 2 eV, the second at 4 eV and a less intense feature at 7 eV, which depends on the O 2p/Mo 4d hybridization. There is a minimum intensity at 4.5 eV in the calculated DOS, which can not be recovered in the measurement. One reason could be the missing core-hole potentials in the calculations. The spectra of W72 Fe30 sulfate can be seen in the right panel of figure 4.11. The dominating parts are the peaks at 2 eV and 3 eV as well as the broad feature from 7 to 11 eV. They are a consequence of the O 2p/W 5d hybridization. The calculated DOS for the ions are shifted toward the Fermi level. The hybridization between O 2p and Mo 5d or W 4d is much stronger, than that between O 2p and Fe 3d, but there are still small features visible at 0.5 eV and 0.75 eV. They are results from the Fe 3d t2g states. The Fe 3d eg states are overlapped by the contributions of Mo 4d and W 5d, respectively. That way they are not visible in the spectra. 4.3.4 Conclusion It was observed that the photoreduction rate strongly depends on the ligand structure surrounding the Fe ions, with negatively charged ligands leading to a dramatically reduced photoreduction rate. This opens up the possibility of tailoring such polyoxometalates for x-ray spectroscopic studies and also for potential applications in the field of x-ray induced photochemistry and catalysis. 77 4 Iron-Based Magnetic Polyoxometalates Figure 4.11: Top: O K edge XAS of 1 (left) and 3 (right). The spectra were taken at a low photon flux and a fresh spot of the corresponding sample. Bottom: Calculated unoccupied densities of states for Mo72 Fe30 acetate 78 (left) and W72 Fe30 sulfate (right). 4.4 Magnetic Measurements on W72 Fe30 sulfate 4.4 Magnetic Measurements on W72Fe30 sulfate Magnetic properties of materials become more and more important for future applications. W72 Fe30 sulfate is a molecule which shows a magnetic behavior. To get more detailed information about the nature of the magnetic properties, SQUID magnetometry and XMCD measurements were performed, compared and shown in this section. 4.4.1 Specific Experimental and Theoretical details The here presented SQUID magnetometry measurements were performed with a Quantum Design MPMS SQUID magnetometer in Ulm, Gemany (1.3.6), with a maximum field of 5.5T and a temperature range from 2K to 15K. The x-ray magnetic circular dicroism experiments were performed at the surface and interface microscopy (SIM) beamline of the Swiss Light Source. The used endstation was the 7T cryomagnetic TBT-XMCD endstation. For more detailed information about the endstation see section 1.3.3. The temperature during the measurements were around 0.7K. The W72 Fe30 sulfate powder sample was put on carbon tape before connecting the sample holder to the cryostat coldfinger. The used record mode was the total electron yield (TEY). To reduce the radiation damage process the maximum flux of approximately 1012 photons per second at a photon energy around 700 eV, was reduced to 1-2% of the maximum photon flux. 4.4.2 SQUID Superconducting Quantum Interference Device measurements, so called SQUID - measurements, give information about the integral magnetization as a function of the internal field and temperature. The measurements presented here are already submitted by Kuepper, Derks et al. [125]. W72 Fe30 sulfate was measured in a capsule with 27.1 mg powder of the sample. The molecular weight of W72 Fe30 sulfate is 25820 g/mol and the sample comprised 6.321·1017 molecules. In figure 4.12 the magnetization per molecule is plotted against the magnetic field B at 2 K, 5 K and 15 K. The largest magnetic moment per molecule is µM = 58µb at 5.5 T and 2 K. There are no changes in the magnetization by increasing the temperature to 5 K. At 15 K the magnetization is significant lower and is up to µM = 43µb at 5.5 T. A qualitatively similar magnetization curve was obtained for Mo72 Fe30 acetate [126, 127]. For molecule 1 and 2 a nearest neighbor antiferromagnetic Fe-Fe interaction has been published recently [132]. These SQUID data will be used as reference for the XMCD measurements, which are presented in the following section. 79 4 Iron-Based Magnetic Polyoxometalates Figure 4.12: SQUID measurements of W72 Fe30 sulfate from -6T up to 6T at 2 (orange),5 (green) and 15K (black) 4.4.3 XMCD at the Iron L2,3 -edges of W72 Fe30 sulfate As shown in the previous section the radiation damage is quite small for W72 Fe30 sulfate, providing that some boundary conditions are considered. So the following XMCD-spectra give information about the magnetic properties of iron in the not reduced molecule. In the first graph of figure 4.13 the Fe L2,3 -edges spectra, recorded with right and left circular polarized light, is shown. The external applied field was 6.5 T and the sample temperature was 0.7 K. The resulting XMCD signal and its integral are also plotted. The XMCD measurement and the isotropic XAS measurement, respectively, can be simulated by a combination of 85% Fe3+ and 15% Fe2+ . They are shown in the second and third graph of figure 4.13. This corresponds to the Fe L-edges measurement of W72 Fe30 sulfate on a fresh spot, which was shown before. So it can be assumed that the reduction process was reduced to a minimum and the results can be interpretated as results of a nearly unchanged sample. XMCD measurements will be interpretated by using the sum rules [19]. In this case the magnetic spin moment of W72 Fe30 sulfate is determined to µs = 51.5 µB /molecule. The orbital moment contribution is almost quenched. The here determined µs = 51.5 µB /molecule is smaller than the µM of 58 µb at 5.5 T and 2 K as measured by SQUID magnetometry measurements shown before. One reason could be, that the spin sum rules underestimate the moments for ionic systems due to core-hole Coulomb interactions [140, 141]. So the spin sum rule correction factors for Fe2+ (1/0.685) and Fe2+ (1/0.875) as derived by Teramura [140] have been used. Then the final value is 52 µB /molecule. This value 80 4.4 Magnetic Measurements on W72 Fe30 sulfate is still smaller. There are four possible reasons for this dicrepance: • Weak initial Fe3+ to Fe2+ photoreduction processes can not be entirely excluded. • The sum rules correction factors were calculated for perfectly octahedral and homogeneous crystal fields, already small differences can lead to different correction factors [141]. • At low temperatures, like in this case for 0.7 T, weak antiferromagnetic intermolecular interactions might be present. • It was shown (4.3.3) that there might be some hybridized spin states because of the element specific XMCD signal. These spin states may not be entirely included. 4.4.4 Conclusion The magnetic properties of the molecule W72 Fe30 sulfate were investigated by XMCD and SQIUD. SQUID is a very well established method to determine magnetic moments and gives reliable data. But in contrast to XMCD it is not element specific. So XMCD is a good extension. In this case the magnetic moment calculated from XMCD measurements leads to little to small values. The possible reasons for this discrepancy are presented above. The charge-transfer multiplet calculations were in good agreement with the experiment 81 4 Iron-Based Magnetic Polyoxometalates Figure 4.13: XMCD measurement of the Fe L2,3 -edges on W72 Fe30 sulfate. The experimental results are plotted on the top of the graph. The paddle in the middle shows the experimental dichroic signal (green) compared to CT multiplet simulations (black). In the panel on the bottom the experimental XAS signal (green) is compared to CT multiplet simulations (black). The measurements were made at 6.5T and 0.7K. 82 5 Conclusion The here presented work, gives an overview about three groups of materials, which have in common, that they are materials with potential for possible future applications. The presented materials are a series of rare-earth scandates, the muliferroic LuFe2 O4 and three iron-based magnetic polyoxometalates of the type {(M)M5 }FeIII 3 0 (M = MoV I ,WV I ). They were examined by several different x-ray spectroscopic techniques and multiplet calculations, respectively. The results are summarized below. • RScO3 (R=Pr, Nd, Sm, Eu, Gd, Tb and Dy) In chapter 2, a coherent picture of seven rare-earth scandates was conveyed. In the first part core-level photoelectron spectroscopy results were shown, these included the rare-earth R 3d (R=Pr, Nd, Sm, Eu, Gd, Tb and Dy), oxygen O 1s and scandium Sc 2p spectra. They provided information of the valence states of the ions in the crystal. The measurements confirmed the expected valences of 3+ for the rare-earths and scandium, respectively and the valence 2for oxygen. To get more detailed information about the occupied and unoccupied states x-ray absorption measurements (XAS) and x-ray emission measurements (XES) were made. The M4,5 -edges (XAS) and the transition from R 4f → 3d (XES) of the rare-earth were measured as well as the L2,3 -edges (XAS) and the transition from Sc 3d → 2p (XES) of the scandium. The oxygen O K-edge (XAS) and the transition from O 2p → 1s (XES) were measured and used to determine the band gap of the rare-earth scandates. This was possible due to the hybridization of the Sc 3d states and the O 2p states. So all electronic states close to the Fermi level were represented by the XAS and XES spectra of the oxygen. Hence a conclusion to the band gaps was possible. A comparison with different crystallographic characteristics, determined a correlation between the mean-distance of the scandium and oxygen ions and the size of the band gaps. The results summarized here have already been published [56–58]. • LuFe2 O4 In chapter 3, LuFe2 O4 was investigated by x-ray absorption and x-ray magnetic circular dichroism measurements at the iron L2,3 -edges, iron K-edge and lutetium L2,3 -edges, respectively. Supplemental SQUID measurements were made. The iron pre-edge of the iron K-edge measurement at a temperature of 150K was simulated by multiplet calculations, which reproduce the localized quadrupole transitions. The changing in the intensity of the XMCD signal at 150K at the 83 5 Conclusion iron K-edge in dependents from the attached magnetic field is in perfect agreement with SQUID measurements, performed at the same temperature. So it can be ascertained, that XMCD at the iron K-edge is suitable to get information about the element specific magnetization of a material. The XMCD measurements on the lutetium L2,3 -edges show that the ions carry a moment, even though it is quite small. The sum rules were used but the results have to be discussed on their reliability. • Iron-Based Magnetic Polyoxometalates The last investigated group of materials were three iron-based magnetic polyVI VI oxometalates of type (M)M5 FeIII 30 (M=Mo W ) in chapter 4. The core levels were measured by photoelectron spectroscopy (XPS). During the measurements the spectra changed due to two different reasons. On the one hand the samples were charged by the leaching of the photoelectrons. This charging effects could be partially compensated in the here presented measurements by the use of an electron flood gun. On the other hand the valence of the iron ions changed due to the x-ray radiation and so the iron 2p spectra changed during the measurement. Depending on the ligand, it was observed that the change in the spectra occurred at different time scales. A quantitative analysis of the time, was not possible due to the charging effects, which could not be reduced completely. But the qualitative analysis could be made in connection with x-ray absorption measurements (XAS). During the absorption measurements (XAS) the sample had a stable state of charge. Furthermore, in the x-ray absorption spectrum the difference between iron 3+ and iron 2+ can be clearly recognize, due to an obvious changing in the peak positions for those two valence states. By using multiplet calculations the ratio of iron 3+ to iron 2+ was determined. Depending of the ligands of the molecule, the rate of conversion from iron 3+ to iron 2+ is different. Furthermore an irradiance can be determined, in which the results of the measurements were relatively close to the undamaged material. So this low intensity was used for magnetic measurements by x-ray magnetic circular dichroism (XMCD) and the results can be interpreted as results of an almost unreduced molecule. The results of these measurements were compared with SQUID measurements and a basic agreement was found. The results of the XAS measurements, XMCD measurements, SQUID measurements and calculation, respectively are submitted for publication [125]. 84 Fazit Die hier vorgestellte Arbeit gibt einen Überblick über drei Gruppen von Materialien, die alle potenzielle Kandidaten für mögliche zukünftige Anwendungen sind. Die vorgestellten Materialien sind eine Reihe von seltenen-erden Skandaten, das muliferroische LuFe2 O4 und drei auf Eisen basierenden magnetischen Polyoxometallaten vom VI VI Typ {(M)M5 }FeIII 3 0 (M = Mo ,W ). Sie wurden mit mehreren verschiedenen rötgen spektroskopischen Techniken und Multiplett-Rechnungen untersucht. Die Ergebnisse sind nachstehend zusammengefasst. • RScO3 (R=Pr, Nd, Sm, Eu, Gd, Tb und Dy) In Kapitel 2 wurde ein zusammenhängendes Bild über sieben seltene-erd Skandate gezeichnet. Im ersten Teil wurden Ergebnisse der XPS Messungen gezeigt, dazu gehören Spektren der selten-erd Elemente R 3d (R=Pr, Nd, Sm, Eu, Gd, Tb und Dy), O 1s und Sc 2p. Sie liefern durch ihre Form und Peakpositionen, Informationen über die Valenzen der Ionen im Kristall. Die Messungen bestätigten die zu erwartenden Valenz 3+ für die seltenen-Erden und Skandium, sowie die Valenz 2- für Sauerstoff. Um detaillierte Informationen über die besetzten und unbesetzten elektronischen Zustände zu bekommen, wurden Röntgen Absorptions- (XAS) und Röntgen Emissions-Messungen (XES) gemacht. Es wurden die M4,5 -Kanten der seltenen-erd Elemente mit XAS und der Übergang von R 4f →3d mit XES, sowie die L2,3 -Kanten (XAS) und der Übergang von Sc 3d→2p (XES) von Skandium gemessen. Die Sauerstoff K-Kante (XAS) und der Übergang von O 2p→1s (XES) wurden gemessen und verwendet, um die Bandlücken der selten-erd Skandate zu bestimmen. Dies war, aufgrund der Hybridisierung der Sc 3d Zustände mit den O 2p Zuständen möglich. So können alle elektronischen Zustände in der Nähe des Fermi-Niveaus durch XAS und XES Spektren des Sauerstoffs abgebildet werden. Dies macht eine Bestimmung der Bandlücken möglich. Ein Vergleich mit den verschiedensten kristallografischen Eigenschaften zeigte eine Korrelation zwischen dem durschnittlichen Abstand der Skandium und Sauerstoff Ionen und der Größe der Bandlücken. Die hier zusammengefassten Ergebnisse sind in diesen Publikationen veröffentlicht [56– 58]. • LuFe2 O4 In Kapitel 3 wurde LuFe2 O4 an den Eisen L-Kanten, der Eisen K-Kante und den Lutetium L-Kanten mit Röntgen Absorptionsmessungen und zirkularem, 85 Fazit magnetischen Röntgendichroismus untersucht. Zusätzlich wurden SQUID Messungen gemacht. Die vordere Kante der Eisen K-Kanten Messungen wurden, bei einer Temperatur von 150K, mit Multiplett Rechnungen simuliert, diese berücksichtigen die lokalisierten Quadrupolübergänge. Die Änderung der Intensität des XMCD Signals an der Eisen-K-Kante stimmt bei 150K und unterschiedlich angelegten externen Magnetfeldern perfekt mit den SQUID-Messungen überein. So konnte gezeigt werden, dass XMCD an der Eisen-K-Kante geeignet ist, um Informationen über die elementspezifische Magnetisierung eines Materials zu erhalten. Die Messungen an den XMCD Lutetium L2,3 -Kanten zeigen, dass die Ionen ein magnetisches Moment haben, obwohl es sehr klein ist. Die Summen Regeln wurden angewendet, aber die Ergebnisse müssen, aufgrund der sehr kleinen Messsignale, auf ihre Belastbarkeit überprüft werden. • Eisenbasierte, magnetische Polyoxometallate Als letzte Materialgruppe wurden im Kapitel 4 drei eisenbasierte, magnetische VI VI Polyoxometalate des Typs (M)M5 FeIII 30 (M=Mo , W ) in Kapitel vier untersucht. Mit Photoelektronenspektroskopie (XPS) wurden die kernnahen Niveaus gemessen. Die Spektren veränderten sich während der Messung, was zwei verschiedene Ursachen hatte. Auf der einen Seite wurden die Proben durch das Herauslösen der Photoelektronen aufgeladen. Diese Aufladungseffekte konnten aber teilweise durch die Benutzung einer Elektronenkanone kompensiert werden. Auf der anderen Seite wurde deutlich, dass bei den Eisen 2p Spektren eine Veränderung eintrat, die sich durch die Änderung der Valenz des Eisens erklären lassen konnte. Je nach Ligand konnte beobachtet werden, dass die Veränderung mit unterschiedlichen Geschwindigkeiten auftrat. Eine quantitative Analyse zum zeitlichen Ablauf konnte aber aufgrund der Aufladungseffekte nicht erfolgen. Die qualitative Analyse konnte jedoch im Anschluss mit Hilfe von Röntgen Absorptionsmessungen (XAS) durchgeführt werden. Während der Absorption war ein stabiler Ladungszustand der Probe gewährleistet. Des Weiteren lässt sich in dem XAS-Spekrum deutlich der Unterschied zwischen Eisen 3+ und Eisen 2+ erkennen. Mit Hilfe von Multiplett-Rechnungen konnte das Verhältnis von Eisen 3+ zu Eisen 2+ bestimmt werden. Es konnte eine Tendenz der Geschwindigkeit der Umwandlung von Eisen 3+ zu Eisen 2+, in Abhängigkeit vom Liganden, erkannt werden. Außerdem konnte eine Bestrahlungsstärke bestimmt werden, bei der die Messergebnisse relativ nah am unbeschädigten Material liegen. Diese Erkenntnis wurde genutzt um magnetische Messungen mit Hilfe des zirkular magnetischem Röntgendichroismus (XMCD) am nahezu unveränderten Molekül zu machen. Die Ergebnisse dieser Messungen wurden mit SQUID Messungen verglichen und eine grundsätzliche Übereinstimmung wurde gefunden. 86 Danksagung Ich möchte mich zuerst für die finanzielle Unterstützung durch den Fachbereich Physik der Universität Osnabrück bedanken. Außerdem wurde ich mit einem Abschlussstipendium des “Pools Frauenförderung ” durch die Zentralen Kommission für Gleichstellung gefördert. Als nächstes gilt mein Dank Herrn apl. Prof. Prof. H.C. Dr. Dr. H.C. Manfred Neumann. Seine wissenschftliche Erfahrung und seine persönliche Betreuung waren eine große Hilfe und machten diese Arbeit erst möglich. Dann habe ich auch Dr. Karsten Küpper für die wissenschaftliche und persönliche Zusammenarbeit herzlich zu danken. Seine wissenschaftliche Hilfe und Unterstützung haben mir bei der Erstellung dieser Arbeit sehr geholfen. Seine Anträge machten die zahlreichen Synchrotronmessungen, die ein Kern dieser Arbeit sind erst möglich. Vielen Dank für die Unterstützung in den letzten Wochen und Monaten vor Abgabe dieser Arbeit! Des weiteren gilt mein Dank Prof. Dr. Andrei Postnikov. Er hat einen wichtigen Teil zu der Interpretation experimenteller Ergebnisse beigetragen. Seine Hilfsbereitschaft bei der Besprechung und dem Korrekturlesen von Papern soll hier besonders hervor gehoben werden. Ich möchte auch Prof. Dr. Frank de Groot von der Abteilung für Anorganische Chemie und Katalyse an der Universität Utrecht danken. Er hat es uns, durch das zur Verfügungstellen des TTmultiplet Programms, ermöglicht Multiplettrechugen selber durchzuführen. Bei der Erstellung von neuen Inputfiles stand er uns immer mit Rat und Tat zur Seite. Ganz wichtig war für mich die Arbeitsgruppe Elektronenspektroskopie. Sie wird häufig vergessen, trotzdem ist sie mir sehr wichtig und darum möchte ich den ehemaligen und aktuellen Mitgliedern von ganzem Herzen für die freundschaftliche Zusammenarbeit danken. Vielen Dank an: Dr. Michael Raekers, Dr. Manuel Prinz, Dr. Christian Taubitz, Dipl. Phys. Stefan Bartkowski, Dipl. Phys. Miriam Baensch, M. Sc. Olga Schuckmann, B. Sc. Andreas Meyering und M. Sc. Daniel Taubitz. Als momentan (fast) letztes Arbeitsgruppenmitglied, möchte ich mich bei meiner Kollegin und Freundin Anna Buling bedanken. Es war eine Freude mit ihr zu Arbeiten und ich hoffe, dass uns das Schicksal noch einmal in ein gemeinsames Büro verschlägt. Ein weiteres Dankeschön geht an Werner Dudas und seine magischen Hände bei der Betreuung der ESCA. Marion von Landsberg war als Arbeitsgruppensekretärin und Mensch ein Segen. Wenn an anderer Stelle häufiger die Worte “so einfach geht das nicht” fielen. hat Marion es einfach ohne viel Aufhebens möglich gemacht. Dafür 87 Danksagung meinen ganz herzlichen Dank! Ein weiterer freundschaftlicher Kontakt ist über die Jahre zu Claudia Meyer entstanden. Ihr Engagement bei der Finanzierung meiner Arbeit war großartig. Auch an dieser Stelle noch einmal herzlichen Dank. Die letzten Monate habe ich auf dem Flur der AG Wollschläger verbracht. An dieser Stelle ein herzliches Dankeschön an alle AG Mitglieder für die nette Aufnahme in ihren unterhaltsamen Kreis, exemplarisch möchte ich Dipl. Phys. Henrik Wilkens erwähnen, weil er so gerne in Danksagungen steht. Zuletzt möchte ich mich bei meinen Eltern, meiner Schwester und meinem Freund bedanken. Vielen Dank für Eure Geduld. Ich bin jetzt fertig! 88 References [1] Chastain, J.: Handbook of X-ray Photoelectron Spectroscopy. Perkin Elmer Coorporation , Eden Prairie, 1992. [2] Suzuki, Chikashi, Jun Kawai, Masao Takahashi, Aurel Mihai Vlaicu, Hirohiko Adachi, and Takeshi Mukoyama: The electronic structure of rare-earth oxides in the creation of the core hole. 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Ziemann Electronic structure nd softX-ray-induced photoreduction studies of iron-based magnetic polyoxometalates of VI VI type{(M)M5 }12 FeIII 30 (M=Mo , W ) submitted to Dalton Transactions • A. Postnikov, C. Derks, K. Kuepper and M. Neumann Electronic Structure of Rare-Earth Scandates from X-Ray Spectroscopy and First-Principles Calculations (2012) in print • C. Derks, K. Kuepper, M. Raekers, and M. Neumann Band gap variation in RScO 3 (R=Pr, Nd, Sm, Eu, Gd, Tb, and Dy): X-ray absorption and O K-edge x-ray emission spectroscopies Physikal Review B 86 Issue:15, 155124 (2012) • T.V. Kuznetsova, V.I. Grebennikov, H. Zhao, C. Derks, C. Taubitz, M. Neumann, C. Persson, M.V. Kuznetsov, I.V. Bodnar, R. W. Martin and M. V. Yakushev A photoelectron spectroscopy study of the electronic structure evolution in CuInSe2 -related compounds at changing copper content, Applied Physics Letters 101, 11, 11607 (2012) • H.H. Pieper, C. Derks, M. H. Zoellner, R. Olbrich, L. Tröger, T. Schroeder, M. Neumann and M. Reichling Morphology and nanostructure of CeO2 (111) surfaces of single crystals and Si(111) supported ceria films, Physical chemistry chemical physics : PCCP 14 Issue:44,155361-8 (2012) • A. Gryzia, H. Predatsch,A. Brechling, V. Hoeke, E. Krickemeyer, C. Derks, M. Neumann, T. Glaser and U. Heinzmann Preparation of monolayers of [Mn-III Cr-6(III)](3+) single-molecule magnets on HOPG, mica and silicon surfaces and characterization by means of non-contact AFM, Nanoscale Research Letters 6, 486 (2011) • K. Kuepper, M. Raekers, C. Taubitz, M. Prinz, C. Derks, M. Neumann, A.V. Postnikov, F.M.F. deGroot, C. Piamonteze, D. Prabakharan and S.J. Blundell Charge order, enhanced orbital moment, and absence of magentic frustration in layered multiferroic LuFe2 O4 , Physical Review B 80, 220409(R) (2009) 101 102 Erklärung Hiermit erkläre ich an Eides Statt, die vorliegende Abhandlung selbständig und ohne unerlaubte Hilfe verfasst, die benutzten Hilfsmittel vollständig angegeben und noch keinen Promotionsversuch unternommen zu haben. Christine Derks Osnabrück, 15. Dezember. 2012