* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Resolving Subsurface Magnetism at Atomic Scale by - SPring-8
Survey
Document related concepts
Energy applications of nanotechnology wikipedia , lookup
High-temperature superconductivity wikipedia , lookup
Nitrogen-vacancy center wikipedia , lookup
Hall effect wikipedia , lookup
State of matter wikipedia , lookup
Scanning SQUID microscope wikipedia , lookup
Tunable metamaterial wikipedia , lookup
Superconductivity wikipedia , lookup
Condensed matter physics wikipedia , lookup
Nanochemistry wikipedia , lookup
Geometrical frustration wikipedia , lookup
Neutron magnetic moment wikipedia , lookup
Low-energy electron diffraction wikipedia , lookup
Multiferroics wikipedia , lookup
Transcript
Materials Science : Electronic & Magnetic Properties Resolving Subsurface Magnetism at Atomic Scale by "Diffraction Spectroscopy" Future data storage densities will soon need to analyzer (DIANA) [3,4] installed at the circularly exceed one terabyte (10 12 bytes) per square inch, polarized soft X-ray beamline BL25SU for measuring requiring bits just 10 nm or less across. However, this AEDs. As illustrated in Fig. 1(a), electrons emitted is the scale at which surface magnetism appears. from the sample were energy-analyzed and their Thus, it is critical to understand unusual magnetic angular distributions were projected onto a effects from a nano scopic point of view. The fluorescence screen. The Ni LMM AED pattern magnetism of outermost atoms can be detected excited with normal incident soft X-rays (852.8 eV) individually by magnetic STM, whereas much are shown in Fig. 1(b). The fcc cluster indicates the difficulty lies in the case of subsurface atoms. relation of crystal orientation and FFPs. FFPs clearly So far, several methods have been developed for appear along the directions of atoms surrounding probing the magnetic structure of buried interfaces excited atoms in the AED pattern. The intensity at an and multilayers. Recently, Amemiya et al . have extremely low emission angle especially in the [100] succeeded in evaluating the magnetic moments of direction is mainly due to the emission from the surface and subsurface using the emission angle outermost surface atoms. The FFPs in the [101] and dependence of Auger electron probing depth [1]. [001] directions indicate the existence of Ni atoms However, no atomic-layer resolved characterization at the second and third layers, respectively. technique for subsurface magnetic structures has We obtained a XANES spectrum by monitoring been developed until now. Auger electron intensity while scanning excitation A newly developed technique, diffraction photon energy. The intensity of different FFPs spectroscopy, enables direct access to the subsurface corresponds to the signal of different atomic layers. region, which connects surface and bulk worlds [2]. Figure 2(a) shows the atomic-layer resolved Ni L3-XANES spectra of a 15 ML Ni film. The first- to We combined two existing techniques: X-ray fourth-layer spectra were extracted from [100], [101], absorption spectroscopy and Auger electron [001] and [103] FFP intensities, respectively. Note that diffraction (AED) measurement. X-ray absorption near the intensity of the so-called 6-eV satellite is diminished edge structure (XANES) and X-ray magnetic circular in the surface layer spectrum. This is due to the atomic dichroism (XMCD) measurements by Auger electron and electronic structures peculiar to the outmost yield detection are powerful analysis tools for the surface layer where atoms bond to a relatively few electronic and magnetic structures of surfaces. neighboring atoms. The L3 peaks in the spectra of the However, all the information from atoms within the surface layer shift to higher photon energy compared electron mean-free-path range is averaged into the to those in the spectra of interior layers by about 0.1 eV. obtained spectra. Forward-focusing peaks (FFPs) in AED patterns indicate the directions of atoms surrounding the excited (a) (b) Sample: hv [001] 3 atom. Taking advantage of the FFP Wedged [103] 4 as an excellent element- and siteσ+ Circularly Ni Film [101] 2 selective probe, we disentangled [112] Polarized Light [301] spectra from different atomic layers. The spin reorientation transitions θ (SRTs) of a magnetic epitaxial film [010] 1 Surface Normal from in-plane to perpendicular [100] 3rd layer Direction 4th layer direction are intriguing phenomena. 2nd layer A Ni ultrathin film on a Cu(001) 1 45° Incidence 1st layer 2 surface exhibits both SRT and Screen θ = 45° 3 reversal SRT, which are regarded as 4 a basic starting point of nanoscale magnetism. We used diffraction Fig. 1. (a) Schematic diagram of display-type analyzer. An Auger electron spectroscopy to visualize both the diffraction (AED) snapshot is taken in a short time, typically 1 sec. magnetic and electronic properties of (b) AED pattern of Ni thin film (kinetic energy: 841.0 eV) together with fcc cluster. Electron-emitting atoms are specified from forward focusing peaks. subsurface layers on the atomic scale in a nondestructive way. We used the display-type 76 Normal Incidence θ = 0° 10 15 ML Perpendicular Magnetization L2 6 eV Satellite 5 0 850 855 860 870 Photon Energy (eV) 875 surface local atomic orbital in-plane magnetization L2 0.5 (b) XMCD Intensity (arb. units) magnetic domains, which leads to more data per square inch. In conclusion, we have developed a direct method for atomic-layer resolved analysis of electronic and magnetic structures for surfaces and thin films. This technique opens a door for the study of the correlations of the atomic, electronic, and magnetic structures of nanostructures at the surfaces. L3 Orbital and Spin Moment Ratio Absorption Intensity (arb. units) (a) 0.0 [103] 4th Layer [101] 2nd Layer – 0.5 [001] 3rd Layer –1.0 [100] Surface –1.5 [100] 1st Layer (Surface) [101] 2nd Layer [001] 3rd Layer [103] 4th Layer (~Bulk) –2.0 –2.5 850 L3 855 870 Photon Energy (eV) 0.3 0.2 8 ML 15 ML interior itinerant spin perpendicular magnetization 0.1 1st 875 Fig. 2. (a) Atomic-layer-resolved Ni L-XANES and (b) X-ray magnetic circular dichroism (XMCD) spectra of perpendicularly (15 ML) magnetized Ni films. 2nd 3rd Surface Layer 4th Ni thin film 15 ML 8 ML The L3 shifts correspond to the surface core level shifts. The XMCD spectrum is the difference between the Ni L-XANES spectra excited with positive (σ+ )and negative (σ- )-helicity light. Figure 2(b) shows the atomic-layer-resolved XMCD spectra for the 15 ML perpendicularly magnetized films. We analyzed Ni magnetic structures of various film thicknesses on the atomic scale and deduced the spin (µspineff) and orbital (µ orb) magnetic moments for each atomic layer. The orbital magnetic moments are more enhanced at the surface layers compared with the spin magnetic moments in the case of 8 ML (in-plane); the variations in the moments are rather small in the case of 15 ML (perpendicular). The orbital is sensitive to the local bonding configuration, while the spin reflects the long-range magnetic ordering. At a thickness beyond SRT, the magnetic moment orientation in a perpendicular direction is caused by the spin magnetic moment in the interior layers, whereas in the region below SRT, where the spin magnetic moment is small, the orbital magnetic moment of the surface layers determines the orientation to be in-plane. SRT at 10 ML is based on such a subtle balance of magnetic moments. Knowing exactly how these magnetic moments change throughout the structure will be useful for making perpendicular magnetic recording devices. Perpendicular magnetic domains are stably packed closer together for greater density than in-plane Cu(001) substrate Fig. 3. Ratio of orbital to spin components (µ orb /µ spineff) of each atomic layer in the Ni thin film on a Cu(001) surface. Fumihiko Matsui a,*, Tomohiro Matsushita b and Hiroshi Daimon a a Graduate School of Materials Science, Nara Institute of Science and Technology b SPring-8 /JASRI * E-mail: [email protected] References [1] K. Amemiya et al.: Phys. Rev. B 71 (2005) 214420; ibid. 72 (2005) 201404(R). [2] F. Matsui, T. Matsushita, Y. Kato, F.Z. Guo, M. Hashimoto, K. Inaji and H. Daimon: Phys. Rev. Lett. 100 (2008) 207201. [3] H. Daimon: Phys. Rev. Lett. 86 (2001) 2034. [4] T. Matsushita et al.: Phys. Rev. B 75 (2007) 085419. 77