Structural, magnetic and transport properties of the spinel ferrites Ga_xFe_1−xNiCrO₄

Abstract

The authors report here on X-ray diffraction, magnetic hysteresis, resistivity and thermoelectric power measurements carried out at different temperatures for the system Ge_xFe_1−xNiCrO₄(x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0), which crystallises with a spinel structure. On the basis of the site-preference energy values of the individual cations (as given by Miller, and Smit and Wijn) the cation distribution is expected to be given by (Ga_x³⁺Fe_1−x³⁺) (Ni²⁺Cr³⁺)-O₄, the Ga³⁺ and Fe³⁺ ions being on the A sites and the Ni²⁺ and Cr³⁺ ions on the B sites, thus providing a selective A-site magnetic dilution with increasing x. The saturation magnetisation measurements indicate a canting of the B-site moments with an average canting angle that increases with x. Whereas the lattice is found to contract with x, the activation energy deduced from the resistivity plots and the Seebeck coefficient is found to increase with x. For a given x the resistivity decreases with temperature (T) and for a given T it increases with x. The Seebeck coefficient is found to be independent of the temperature. These results can be explained by assuming a small-polaron hopping mechanism to be responsible for the conductivity and thermoelectric effect observed in this system.