Role of inhomogeneous cation distribution in magnetic enhancement of nanosized Ni0.35Zn0.65Fe2O4: A structural, magnetic, and hyperfine study

Abstract

In this paper, we report the structural, microstructural, and magnetic properties of nanosized (particle size ranging from 20 to 30 nm) Ni_0.35Zn_0.65Fe₂O₄ (MA4) system synthesized via mechanochemical route followed by annealing. The Rietveld refinement is used for the first time to precisely resolve the crystal structure of a ferrite system at nanoscale. MA4 is a cubic spinel of $Fd\overline{3}m$ symmetry. According to XRD and HRTEM studies, it is a well crystalline sample which possesses large microstrain. In spite of its nanometric size, MA4 has displayed some notably distinct magnetic properties like, enhancement of magnetization (64 emu g⁻¹ at 15 K), magnetic order, magnetic ordering temperature, coercivity (1000 Oe at 15 K), magnetic anisotropy energy, and reduction of superparamagnetic relaxation compared with its counterparts synthesized by chemical route. It exhibits clear hysteresis loop (H_C = 50 Oe) at 300 K and ferrimagnetic ordering below the blocking temperature (∼250 K). These improvements in magnetic properties of the system are likely to be very helpful for its technological applications. Again, particles in the sample possess a ferrimagnetically aligned core (with small canting) surrounded by a magnetically disordered shell with canted spin structure. The magnetically disordered surface region of MA4 has an equilibrium cation distribution, whereas the ferrimagnetic core region possesses a nonequilibrium cation distribution. Moreover, the infield Mössbauer spectroscopic study reveals that the nearest neighbor ion configuration about [B] site Fe³⁺ ions is not identical. Thus, there is local chemical inhomogeneity in the sample. The cation redistribution, chemical inhomogeneity, lattice strain are identified as the causes for magnetic enhancement in MA4.