J. Phys. Soc. Jpn. 77, 034713 (2008) [16 Pages]
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Magnetic, Optical, and Magnetooptical Properties of Spinel-Type ACr2X4 (A=Mn, Fe, Co, Cu, Zn, Cd; X=O, S, Se)

Kenya Ohgushi1, Yoichi Okimoto2, Takeshi Ogasawara2, Shigeki Miyasaka1, and Yoshinori Tokura1,2,3
+ Affiliations
1Department of Applied Physics, University of Tokyo, Tokyo 113-86562Correlated Electron Research Center (CERC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-85623Multiferroics Project, ERATO, Japan Science and Technology Agency (JST), c/o Department of Applied Physics, University of Tokyo, Tokyo 113-8656
Received November 14, 2007; Accepted December 28, 2007; Published March 10, 2008

A comprehensive study of magnetic, optical, and magnetooptical properties was carried out for single crystals of the spinel-type A Cr 2 X 4 ( A =Mn, Fe, Co, Cu, Zn, and Cd; X =O, S, and Se). The optical reflectivity measurements for 0.1–30 eV revealed a wide variation in electronic structures on a large energy scale between oxides ( X =O) and chalcogenides ( X =S and Se). For A =Fe and Co, we observed the intra-atomic d – d transitions of A 2+ ions with a tetrahedral coordination, and successfully deduced the crystal field splitting Δ E , the Racah parameter B , and the spin–orbit coupling constant ζ by analysis based on the ligand field theory. A comparison of these optical parameters between oxides and chalcogenides indicated the strong covalency effect in the chalcogenides. In A =Cu, the insulator–metal transition between X =O and Se was clearly demonstrated by optical conductivity spectra. Magnetic properties were discussed in relation to electronic structures. A compound with a small optical gap is typically a ferrimagnet with antiparallel arrangements of A 2+ and Cr 3+ spins, whereas a compound with a large optical gap undergoes first-order phase transition into spiral spin ordering at a low temperature. We found that the magnetic anisotropy constants K 1 for A Cr 2 S 4 ( A =Mn, Fe, and Co) are approximately scaled by the inverse of the intra-atomic d – d transition energies of A 2+ ions in agreement with the second-order perturbation theory for single-ion anisotropy. The magnetooptical spectra in a wide energy range (0.2–4.5 eV) were measured for chalcogenides focusing on the d – d transition resonance. We observed gigantic magnetooptical signals up to 4.1° in the energy range of 4 A 2 → 4 T 2 and 4 A 2 → 4 T 1 transitions of Co 2+ ions for CoCr 2 S 4 , and analyzed them in the framework of the ligand field theory. We propose that the strong covalency of the ligand sulfur, as well as the local breakdown of inversion symmetry, in the tetrahedral site plays a crucial role in the enhancement of magnetooptical responses.

©2008 The Physical Society of Japan
https://doi.org/10.1143/JPSJ.77.034713
KEYWORDS: optical spectroscopy, chromite, spinel chalcogenide, ferrimagnetism, magnetooptical Kerr effectd–dtransition

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