Photon Factory Activity Report 2014 #32 (2015) B

BL-16A/2014G159, 2013S2-004

X-ray magnetic circular dichroism of the spinel-type oxide CoV2O4
Yosuke Nonaka1, *, Goro Shibata1, Rui Koborinai2, Keisuke Ishigami1, Shoya Sakamoto, 1
Arata Tanaka3, Tsuneharu Koide4, Takuro Katsufuji2, 5, Atsushi Fujimori1
1
Department of Physics, University of Tokyo, Tokyo 113-0033 Japan2
Department of Physics, Waseda University, Tokyo 169-8555, Japan3
Department of Quantum Matter, ADSM, Hiroshima University, Higashi-Hiroshima 739-8530, Japan
4
Photon Factory, Tsukuba 305-0801, Japan5Kagami Memorial Laboratory for Material Science and Technology, Waseda University, Tokyo 1690051, Japan

2 Experiment
Bulk single crystals of CoV 2O4 were grown by thefloating zone method. In order to avoid the formation ofthe V2O3 impurity phase, the compositions of the feed rod
was not stoichiometric (Co : V = 1 : 2) but containing 50%
excess Co relative to the stoichiometric one (Co : V = 1.5 :2). The Co:V ratio of the synthesized single crystals wasestimated to be 1.21:1.79 by induction-coupled plasmaanalysis.
The XAS and XMCD measurements at the Co and V L2,3edges were performed at the BL-16A of KEK-PF. The
spectra were taken in the total electron yield mode at 29 K
under a magnetic field (H) of 1.0 T. The x-ray incident
angle was fixed to keep the optical axis parallel to the cubic[111] direction, and the magnetic field was applied along
the [001], [110], and [111] directions (in the cubic notation)

Fig. 1: Geometric arrangement in our XAS and XMCD
experiment. The optical axis is parallel to the cubic [111]direction.

CoV2O4 Co L2,3 edge
H = 1T
H // <111>
T = 29K

Intensity [arb. unit](a)H // <110>
H // <001>

XAS+H //<111>
H //<110> / cos(35.3deg)
H //<001> / cos(54.7deg)

XMCD-770775(b)Intensity [arb. unit]

1 Introduction
The spinel-type oxides are suitable compounds for theprecise discussion of the relationship between the orbitalstates and the crystal structures because of the absence ofGdFeO3-type distortion usually present in the perovskitetype oxides. Among them, spinel-type vanadates AV2O4
are studied as model systems to study novel phases
emerging from the orbital degrees of freedom and the2geometrical spin frustration of the V3+ [𝑡2𝑔] ions.
MnV2O4 shows a large negative magnetostriction(contraction along the magnetic field direction) induced byanti-ferro orbital order of the V3+ ions accompanied by astructural phase transition at a ferrimagnetic transition
temperature TN = 57K [1]. On the other hand, FeV2O4
shows a positive magnetostriction (elongation along themagnetic field direction). This material does not show theorbital order of the V3+ ions, and the positivemagnetostriction is caused by the elongation of FeO4
3tetrahedra with the orbital active Fe2+ [ 𝑒𝑔3 𝑡2𝑔] ion [2].
CoV2O4 has no structural phase transition down to 4K.
However, magnetostriction and its temperature-dependent
sign reversal have been reported [3]. Thus, detailed studiesof the spin and orbital states are required to identify thespin and orbital states of the V and Co ions in CoV2O4.
In this study, we have performed x-ray absorption
spectroscopy (XAS) and x-ray circular magnetic dichroism(XMCD) spectroscopy measurements on CoV2O4 bulk
single crystals.

780 785 790 795
Photon Energy [eV]800

CoV2O4 V L2,3 edge
H = 1TT = 29K

H // <111>
H // <110>
H // <001>

XAS
XMCD+

H // <111>

510

515

x4

H // <110> / cos(35.3 deg)
H // <001> / cos(54.7 deg)

520
525
530
Photon Energy [eV]535Fig. 2: XAS and XMCD spectra at the (a) Co L2,3 and(b) V L2,3 edges under the magnetic field of 1.0 T along the[001], [110], and [111] directions.

Intensity (arb. units)Photon Factory Activity Report 2014 #32 (2015) B(a) CoV2O4 Co L2,3 edge (Td sym., Co2+ )charge transferenergy ΔCo2+V3+

5.6
5.0

crystal field
Slater-Koster
splitting 10Dq parameter pdσ
-0.7
0.9

0.8
2.5

using ‘vector magnet’ XMCD apparatus [4]. In order toobtain clean surfaces, the samples were fractured in an
ultra-high vacuum.
3 Results and Discussion
Figures 2 (a) and (b) show the XAS and XMCD spectraat the Co and V L2,3 edges. Since the x rays and the appliedmagnetic field were not parallel except for the case ofH//[111], the XMCD spectra in Fig. 2 have been divided
by cos, where  is the angle between the x rays and theapplied field. The signs of these XMCD signals show thatthe magnetic moments of Co are aligned parallel to themagnetic field direction and that those of V are antiparallel to it. This orientation of the spins are opposite tothat expected from the spin moments of the in ionicelectronic configurations shown in Fig. 3 (a).
We have simulated the obtained XAS and XMCD
spectra using the cluster model calculation. Figures 4 (a)and (b) show experimental spectra with magnetic fieldalong the [001] direction and best-fit calculated spectra.
Parameters used in the calculation are shown in Table 1.
In these figures, there are differences in the relative
intensities of the XMCD signals to XAS between theresults of experiment and calculation. The small difference
for Co-L2,3 XMCD shown in Fig. 4 (a) is interpreted as aresult of the substitution of the excess Co to the octahedral
site (in the form of Co3+, low spin state). On the other hand,
the large difference for V L2,3 XMCD shown in Fig. 4 (b)implies spin fluctuations and/or canting of the V spins.

Intensity (arb. units)

Table 1: Parameters used in the cluster model
calculations (in units of eV).

Expt: H= 1T
Calc: -BH mol = 0.01 eV

Expt
Calc
Expt
Calc

XMCD
-10

Fig. 3: Schematic pictures of electronic configurations
and spin orientations. (a) Electronic configurations of theCo2+ ion at the tetrahedral site and the V3+ ion at theoctahedral site. (b) Spin orientations expected from theionic model (left) and those revealed by experiment(right).

T = 29 K
H // [001]

XAS-50510Relative Energy [eV]1520(b) CoV2O4 V L2,3 edge (Oh sym., V3+ )T = 29 K
H // [001]

Expt:
H= 1T
Calc: - BH mol = 0.01 eV

XAS
Expt
Calc

XMCD
-10

-5x40510
Relative Energy [eV]

Expt
Calc

15Fig. 4: Experimental and calculated XAS and XMCD
spectra of CoV2O4 at the Co (a) and V L2,3 (b) edges.

References[1] T. Suzuki et al., Phys. Rev. Lett. 98, 127203 (2007).
[2] T. Katsufuji et al., J. Phys. Soc. Jpn. 77, 053708 (2008).
[3] R. Koborinai et al., arXiv: 1505.04864v1[4] M. Furuse et al., IEEE Trans. Appl. Supercond. 23,4100704 (2013).
* nonaka@wyvern.phys.s.u-tokyo.ac.jp