Crystal structure, local structure and magnetic properties of NiCr2-xFexO4 (x = 0.3–0.6) spinel

Abstract

The crystal and local structures of NiCr2-xFexO4 (x = 0.3 to 0.6) spinels synthesized by a co-precipitation method are examined using x-ray diffraction (XRD) in combination with x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) techniques. Rietveld refinement of XRD patterns illustrates the stabilization of high-temperature cubic phase (space group Fd3‾m) at room temperature accompanied by a distortion of octahedron within the spinel structure. XANES analysis reveals that while Fe and Cr cations have the average oxidation state of +3 with their local environment as tetrahedral (A) and octahedral (B) sites, respectively, Ni cations have the average oxidation state of +2 and they are distributed between the A and B sites. The modeling of EXAFS spectra further quantitatively demonstrates that with increasing x, the occupancy of Ni cations in the A site decreases from almost 70 to 40%. The paramagnetic to long-range ferrimagnetic transition at Curie temperature, TC is found to increase from 253 K for x = 0.3 to above 350 K for x = 0.6 without showing any spin-spiral ordering temperature, TS. The dominance of A-B exchange interaction over B–B exchange interaction contributes to such behaviors which are further confirmed from the variation in bond lengths and bond angles obtained from the structural analysis. Showing an unusually high coercivity, HC of 36 kOe for x = 0.3 compared to other x, an increase in saturation magnetization, MS, and decrease in coercivity, HC with increasing x, have been understood through Néel's two sublattice model proposed for ferrimagnetic structure.