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 t...

Magnetic, Optical, and Magnetooptical Properties of Spinel-Type ACr2X4 (A=Mn, Fe, Co, Cu, Zn, Cd; X=O, S, Se)

Some recent work on the magnetooptical properties of magnetic insulators and semiconductors is reviewed. Emphasis is placed on understanding the atomic origin of the Faraday and Kerr effects in terms of molecular-orbital energy levels. The classes of materials discussed are chromium trihalides, orthoferrites, garnets, ferrites, chromium spinels, and divalent europium compounds.

Faraday and kerr effects in magnetic compounds

The optical absorption spectra of HgCr 2 Se 4 single crystals in the intrinsic transition region were measured as a function of magnetic field at a number of fixed temperatures. The absorption edge, 0.80 eV, at room temperature shifted to 0.27 eV at liquid helium temperature. The temperature dependence of the absorption edge was nonlinear and remarkable around the Curie temperature. The absorption edge also shifts to the lower energy side with the increasing magnetic field. The field dependence of the edge shift in weaker magnetic field was larger at the Curie temperature than it in the higher temperature region. The temperature and magnetic field dependence of the absorption edge was compared to a calculated spin correlation function in spinels with the nearest neighbour exchange interaction between localized Cr +3 ions. Other kind of exchange interaction would be required to explain the temperature dependence of the absorption edge.

Magnetoabsorption in Single-Crystal HgCr 2 Se 4

Magnetically ordered multinary semiconductors

Optical transitions and band structure model for cadmium chromium chalcogenides

We report on the preparation and optical properties of single-crystal films of Cd${\mathrm{Cr}}_{2}$${\mathrm{S}}_{4}$. The present data show no large blue shift associated with magnetic ordering near the Curie temperature contrary to that previously observed in bulk Cd${\mathrm{Cr}}_{2}$${\mathrm{S}}_{4}$ data. A large red shift correlated to the magnetic ordering is observed for a peak in the highly absorbing region. The weakness and narrowness of this absorption is inconsistent with that expected for band-to-band transitions resulting in unbound electrons and holes. We suggest an exciton model to explain this absorption.

L. Ekstrom

Papers

Optical Properties of Single-Crystal Films of Cd Cr 2 S 4