Synchrotron and Magnetic Study of Chromium-Substituted Nickel Ferrites Prepared by Using Sol-Gel Route

Structural and magnetic properties of NiCr_xFe_2-xO₄(x = 0.7 and 0.9) spinel ferrites prepared by sol-gel route have been investigated using synchrotron X-ray powder diffraction (SXPD) and magnetization measurements. The gel products after annealing at 1000 °C for 2 h are found to crystallize in cubic phase. Rietveld refinements of X-ray diffraction and SXPD patterns revealed that the compounds are almost perfect inverse spinel with only few percents of nickel atoms located at the tetrahedral sites. Transmission electron microscopy (TEM) images showed that the samples are composed of nanoparticles with mean diameter in agreement with crystallite size determined by the Rietveld analysis. The spontaneous magnetization M_s as a function of temperature was determined via magnetization curves measured using vibrating sample magnetometer in a temperature range from 88 K to above Curie temperature in applied fields up to 10 kOe. The sample x = 0.9 shows a cancellation of sublattice magnetizations at 400 K. The temperature dependence of saturation magnetization Ms of the samples can be described in terms of molecular-field model for collinear spins with the derived cation distributions.