Photon Factory Activity Report 2018 #36 (2019)

BL-12C/2017G061

Temperature-dependent EXAFS study of ferro-orbital ordered FeV2O4
Jun Okabayashi1*, Shigeki Miyasaka,2 and Setsuko Tajima2
1Research Center for Spectrochemistry, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033
2 Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, JapanThe orbital degree of freedom has attracted
interest in the condensed-matter physics because ofadding of new functional functionalities. Couplings
between orbital and spin degrees of freedom intransition metal (TM) oxides exhibit a wide variety ofinteresting physical phenomena studied in stronglycorrelated electron systems. The orbital degeneracy oft2g or eg orbitals, split by the crystal field in TMoxides, gives rise to the orbital ordering phenomenain perovskite-type Fe or V oxides accompanied by
Jahn-Teller distortion. Spinel-type FeV2O4 is acandidate to study orbital ordering since orbital
magnetic moments of Fe2+ (d6) are strongly affectedby the Jahn-Teller distortion in the Fe sites, whichbrings the ferro orbital ordering in V sites through thespin-orbit interaction. An unresolved issue related toorbital ordering of spinel-type vanadium oxides is therelationship between the orbital magnetic moments invanadium sites and orbital ordering [1]. In case ofFeV2O4, the V orbital states consist of complex wave
functions of degenerated yz and zx states, which
expect the ferro-orbital ordering with the large orbital
moments of almost 1 µB. On the other hand, in thecase of MnV2O4, real wave functions in the V sites
which are described as dyz and dzx are orderedalternatively, resulting in an antiferro orbital orderingwith quenching orbital moments [2]. Therefore, theinvestigation of orbital moments in V sites enables todiscuss the types of orbital ordering.
In this study, we aim to investigate theelement-specific electronic and structural propertiesof FeV2O4 by temperature-dependent XAFS andEXAFS across the orbital-ordered temperature. We
examined the relationship between orbital magneticmoments and orbital ordering. We found that thefinite orbital magnetic moments in V sites contributeto the orbital ordering type through the mixing of realand complex wave functions accompanied by thedistortion of Fe sites.
Spinel-type FeV2O4 was synthesized by asolid-state reaction method in the polycrystalline
form. Then, single crystals were grown from thepolycrystalline samples using the floating-zone
method. The magnetic ordering temperature of
single-crystalline bulk FeV2O4 was estimated to be
110 K under applying 500 Oe. Fe K-edge XAFS andV K-edge XAFS were performed at BL-12C, Photon
Factory, High-Energy Accelerator Organization(KEK), Japan. Temperature-dependent spectra weremeasured by the transmission geometry. Photon
energies for XAFS measurements were calibratedusing Fe and V foil as references. Transmitted x-rays
were detected in an ionization chamber filled with amixture of N2 and Ar gases. The sample used for
XAFS measurements was diluted with BN powder
formed into pellets. The sample was cooled by theconventional helium cryostat method from room
temperature to 15 K.
Figure 1 shows the Fe and V K-edge XAFS atroom temperature and 20 K. The K-edge XAFS
probes the bulk information compared with theL-edge one. Spectral line shapes indicate that Fe2+with the additional components of Fe3+ and single V3+states. Small pre-edge structures originating from 1s
to 3d transitions appear for both absorption edges,
which reflects the environments generated by ligand
fields around Fe ions. Furthermore, spectral line
shapes almost remain unchanged depending on
temperature. This suggests that the electronic states
are not modulated through the spin and orbital
ordering transitions. Therefore, we can confirm thatthe phase transitions depending on temperature do notaffect directly the changes in valence electronic states.
On the other hand, EXAFS profile clearly exhibits thepeak shift of first nearest neighbor site. The distancebecomes large at low temperature, which is theopposite tendency with a thermal vibration effect. Itprobes the trigonal distortion of atomic sites acrossthe orbital ordering [3].
VanadiumÅFig. 1. Temperature-dependent XAFS spectra ofFeV2O4 for (a) V and (b) Fe K-edges. (c) EXAFS ofV site.

References[1] Y. Nii et al., Phys. Rev. B 86, 125142 (2012).
[2] J. Okabayashi et al., J. Phys. Soc. Jpn. 84, 104703 (2015).
[3] J. Okabayashi et al., Jpn. J. Appl. Phys. 57 0902BD (2018).
(Selected Topics in Applied Physics)*jun@chem.s.u-tokyo.ac.jp