Influence of cation substitution on the magnetic properties of the FeCr2S4 ferrimagnet

The influence of cation substitution on the magnetic properties of single and polycrystals of FeCr2S4 spinel has been studied. The tetrahedral A-site substitution of the Fe by Cu in Fe1−xCuxCr2S4 was found to increase significantly the value of temperature Tm of the spin-glass like magnetization anomaly, whereas the octahedral B-site substitution of the Cr by In decreases Tm. This effect is suggested to result from a structural transformation influenced by variation of internal (chemical) pressure due to lattice contraction (Cu) or expansion (In). The observed reduced values of the Curie temperature for Cu-substituted single crystals compared to that of the polycrystalline samples are attributed to presence of Cl ions in samples detected by electron-probe microanalysis. The observed reduced value of saturation magnetization in the polycrystals compared to the single crystals is ascribed to the effect of surface anisotropy. Based on the experimental data the superexchange is concluded to be the dominant exchange interaction for 0≤x≤0.5 in Fe1−xCuxCr2S4, whereas the indirect exchange through the charge carriers is considered of minor importance.