CRYSTALLOGRAPHIC AND MAGNETIC-PROPERTIES OF THE SPINEL PHASE FOR NIXFE1-XCR2S4

Ferrimagnetic NiFe1-xCr2S4 is found to crystallize with a pure spinel structure in the composition range 0 less-than-or-equal-to x less-than-or-equal-to 0.4. Fe-57 Mossbauer spectra of NixFe1-xCr2S4 have been taken at various temperatures ranging from 13 K to room temperature. Analysis of Mossbauer spectra and x-ray crystallographic data indicate that Ni ions occupy the tetrahedral site, and the lattice constant is in good agreement with Vegard's law. The isomer shifts indicate that the valence states of the Fe ions have a ferrous character. The Neel temperature increases linearly with Ni concentration, suggesting that the superexchange interaction for Ni-S-Cr link is stronger than that for Fe-S-Cr link. The anomalous reduction of the magnetic hyperfine field below 70 K could be explained in terms of the cancellation effect between mutually opposite orbital current field H(L) and the Fermi contact field H(D). It is notable that, as the temperature decreases below the Neel temperature, both quadrupole shift and asymmetrical line broadening appear and increase with decreasing temperature, suggesting the presence of an electric-field gradient and accompanying relaxation effects.