Magnetic, optical, and magnetooptical properties of spinel-type ACr ₂X₄ (A = Mn, Fe, Co, Cu, Zn, Cd; X = O, S, Se)

A comprehensive study of magnetic, optical, and magnetooptical properties was carried out for single crystals of the spinel-type ACr₂X 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 A²⁺ ions with a tetrahedral coordination, and successfully deduced the crystal field splitting ΔE, 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 A²⁺ and Cr³⁺ 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 K₁ for ACr₂S₄ (A = Mn, Fe, and Co) are approximately scaled by the inverse of the intra-atomic d-d transition energies of A²⁺ 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 ⁴A₂ → ⁴T₂ and ⁴A₂ → ⁴T₁ transitions of Co²⁺ ions for CoCr ₂S₄, 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.