Magnetic characterization of Co 1−xNi xFe 2O 4 (0⩽ x⩽1) nanoparticles prepared by co-precipitation route

Abstract

Magnetic nanoparticles of Ni-doped cobalt ferrite [Co 1− x Ni x Fe 2O 4 (0⩽ x⩽1)] synthesized by co-precipitation route have been studied as a function of doping concentration ( x) and particle size. The size of the particles as determined by X-ray diffractometer (XRD) and transmission electron microscope (TEM) analyses was found in the range 12–48 nm. The coercivity ( H C) and saturation magnetization ( M S) showed a decreasing behavior with increasing Ni concentration. M S of all the samples annealed at 600 °C lies in the range 65.8–13.7 emu/gm. Field-cooled (FC) studies of the samples showed horizontal shift (exchange bias) and vertical shift in the magnetization loop. Strong decrease in exchange bias ( H b) and vertical shift ( δM) was found for low Ni concentrations while negligible decrease was found at higher concentrations. The presence of exchange bias in the low Ni-concentration region has been explained with reference to the interface spins interaction between a surface region (with structural and spin disorder) and a ferrimagnetic core region. M( T) graphs of the samples showed a decreasing trend of blocking temperature ( T b) with increasing Ni concentration. The decrease of T b with increasing Ni concentration has been attributed to the lower anisotropy energy of Ni +2 ions as compared to Co +2 that increases the probability of the jump across the anisotropy barrier which in turn decreases the blocking temperature of the system.