Magnetic characterization of Co1−xNixFe2O4 (0⩽x⩽1) nanoparticles prepared by co-precipitation route

Magnetic nanoparticles of Ni-doped cobalt ferrite [Co1−xNixFe2O4 (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 (HC) and saturation magnetization (MS) showed a decreasing behavior with increasing Ni concentration. MS 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 (Hb) 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 (Tb) with increasing Ni concentration. The decrease of Tb 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.