Download Optical and magneto-optical properties of UPtGe

Optical and magneto-optical properties of UPtGe
J. Schoenes, M. Marutzky, U. Barkow, S. Weber, and R. Troc1
UPtGe has an orthorhombic structure and a substantial magnetic and electric anisotropy. We have
determined the optical properties of UPtGe single crystals in dependence of the polarization and
propagation vector of the incident light relative to the crystal axes from 6 meV to 32 eV. In addition, the
polar magneto-optical Kerr effect has been measured between 1 and 4.5 eV at 12 T. At the lower energies
both measurements confirm the strong anisotropy of the thermodynamic data. At higher energies
additional band structure effects are found.
UPtGe orders antiferromagnetically at TN = 50
K [1] (see Fig. 1). It has an incommensurable
[2,3] cycloic spin structure with a propagation
vector along the a axis which is observed
rarely in 5f systems. At high fields above 20 T
metamagnetic transitions have been reported.
Fig. 1: Temperature dependence of the magnetization measured on three single crystals along the
different crystal directions.
The crystal structure has been determined to
be of the noncentrosymmetric EuAuGe type
with two uranium sites with different magnetic
moments [3]. The electrical and magnetic
properties show a small anisotropy in the ac
plane, while the electric and magnetic behavior
along the b axis is distinct from the ac plane.
Fig. 1 shows the temperature dependence of
the normalized magnetization along the three
crystallographic directions for three single
crystals cut perpendicular to the main axes.
One clearly recognizes the particular behavior
when the magnetic field is aligned parallel to
the b axis. This singularity is also manifest in
the dielectric function which was determined
by optical ellipsometry. Fig.2 displays the real
(ε1) and imaginary (ε2) part of the dielectric
function between 1 and 10 eV for different
orientations of the electric field of the electromagnetic radiation. We recognize most easily
in the dispersive part (ε1) of the complex
dielectric function that UPtGe is more metallic
for E parallel to b. The same conclusion can
be drawn by inspecting the optical
conductivity derived from a Kramers-Kronig
transformation of reflectivity data. Fig. 3
shows the so-obtained real part of the optical
conductivity for E parallel to either a or b. The
horizontal lines on the left indicate the
electrical dc conductivity values. One sees that,
while the latter are still somewhat higher than
the conductivities at the lowest optical
frequencies, the anisotropy is the same.
Besides this conduction electron effect Fig.2
evidences a strong anisotropy in the interband
part of the spectra. Thus, for E parallel to b a
large structure appears in the infrared near 300
cm-1 while for E parallel to a a much smaller
structure is found near 400 cm-1. Also for E
parallel to b a broad peak occurs around 5000
cm-1 in contrast to a narrower peak at 7000
cm-1 for E parallel to a. Measurements [5]
between 15 and 32 eV performed at the VUVellipsometer in Berlin show in addition
maxima at 19 and 20 eV for E parallel to a and
E parallel to b, respectively.
As an example for the polar Kerr effect
measurements, Fig. 4 displays the Kerr
rotation spectra θba and Kerr ellipticity spectra
ηba for E parallel to b and the magnetic field B
of 12 T parallel to c at 51 K. We find a
maximum signal of 0.15 deg in agreement with
the expectation for a hard uranium based
antiferro-magnet [6]. The narrow shape and
Fig. 2: Dielectric function with polarization vectors
essentially parallel to the a, b, and c-axis. The
measurements from 1 to 4 eV have been performed
with the home ellipsometer and between 4 and 10
eV at the VUV-ellipsometer at BESSY II.
Additionally, the dielectric function calculated by
Kramers-Kronig transformation of the reflectivity
data is shown.
Fig. 3: Real part of the optical conductivity,
calculated from the reflectivity spectra by KKR.
the size of the magneto-optical spectra at
energies where the diagonal conductivity is
rather small indicates that the structures near 4
eV involve spin-polarized 5f states. The small
U-Pt and U-Ge separation is likely to favor a
strong hybridization of 5d(Pt) and 4p(Ge)
states with 5f(U) states near the Fermi energy
EF. The anisotropic hybridization is made
responsible for the anisotropic transport and
optical properties in the infrared. Empty 5f
states above EF can serve as final state for
interband transitions from occupied states (like
Pt 5d states) 4 eV below EF.
Fig. 4: Kerr rotation θba and Kerr ellipticity ηba for
E parallel to b and the magnetic field B of 12 T
parallel to c at 51 K.
1
W. Trzebiatowski Institute of Low Temperature and
Structure research, Polish Academy of Sciences,
Wroclaw, Poland
[1] R. Troc and V. H. Tran, J. Magn. Magn. Mater. 73,
389 (1988).
[2] R. A. Robinson, A. C. Lawson, J. W. Lynn, and K. H.
J. Buschow, Phys. Rev. B 47, 6138 (1993).
[3] D. Mannix, S. Coad, G. H. Lander, J. Rebizant, P. J.
Brown, J. A. Paixao, S. Langridge, S. Kawamata, and Y.
Yamaguchi, Phys. Rev. B 62, 3801 (2000).
[4] R. Troc, J. Stepien-Damm, C. Sulkowski, and A. M.
Strydom, Phys. Rev. B 69, 094422 (2004).
[5] M. Marutzky, U. Barkow, S. Weber, J. Schoenes, and
R. Troc, Phys. Rev. B 74, 205115 (2006).
[6] J. Schoenes, "Magneto-Optical Properties of Metals,
Alloys and Compounds" in "Materials Science and
Technology", Eds.: R.W. Cahn, P. Haasen and E.J.
Kramer, Vol. 3A, VCH Weinheim , Germany 1992.