MAGNETIC-PROPERTIES OF CHROMIUM CHALCOGENIDE SPINELS - CD1-XAGXCR2SE4AND HG1-XAGXCR2SE4

The effect of Cr ions of different valences (Cr2+, Cr3+, and Cr4+) on the magnetic anisotropy and ferromagnetic resonance (FMR) linewidth in ferromagnetic semiconductors, with specific application to the chromi um chalcogenide spinels CdCr2Se4 and HgCr2Se4 (''as grown and Ag doped ''), is studied theoretically and experimentally. We generalize variou s previous calculations and show that the line shape of the FMR depend s on the conductivity, the frequency, and the sample dimension. Crysta l-field theory is used to calculate the Cr ions low-lying energy level s and their effect on the magnetic anisotropy and FMR relaxation. in p articular, we find that the orbital ground state of the Cr2+ ion is a doublet and causes a symmetry breaking of the anisotropy from cubic, a t high temperatures, to noncubic at low temperatures. We report FMR da ta of single crystals of CdCr2Se4 with 0.1 mole % Ag doped, where this effect is seen. Moreover, several relaxation mechanisms an identified which allow the estimates of exchange fields and microscopic relaxati on times of Cr ions in these crystals.