CRYSTALLOGRAPHIC AND MAGNETIC-PROPERTIES OF COXFE1-XCR2S4

The mixed series CoxFe1-xCr2S4 has been studied by x-ray, Mossbauer sp ectroscopy, and superconducting quantum-interference device (SQUID) ma gnetometry. The crystal structure is found to be a cubic spinel, and t he lattice constant a0 decreases linearly with increasing cobalt conce ntration. Mossbauer spectra of CoxFe1-xCr2S4 have been taken at variou s temperatures ranging from 4.2 to 300 K. The iron ions are ferrous an d occupy the tetrahedral sites. The Curie temperature increases linear ly with cobalt concentration, suggesting that the superexchange intera ction for the Co-S-Cr link is stronger than that for the Fe-S-Cr link. Magnetic hyperfine and quadrupole interactions in Co0.9Fe0.1Cr2S4 at 4.2 K have been studied, yielding the following results: H(hf) = 80.8 kOe, 1/2e2qQ(1 + 1/3eta2)1/2 = 2.65 mm/s, theta = 15-degrees, phi = 75 -degrees, and eta = 1.O. It is notable that, as the temperature decrea ses below the Curie temperature, quadrupole splitting increases with d ecreasing temperature, suggesting the presence of an electric field gr adient and accompanying relaxation effects. Magnetic susceptibility me asurements by SQUID magnetometry show that superexchange interactions between Fe2+ ions are ferrimagnetic.