Competing magnetic interactions in nickel ferrite nanoparticle clusters: Role of magnetic interactions

The magnetic behavior of nickel ferrite nanoparticles of different sizes was studied by annealing nickel ferrite powders at temperatures ranging from 300 to 900°C. Transmission electron microscopy studies show that the average particle sizes change from ∼8 to ∼120 nm with increasing annealing temperatures. The x-ray diffraction patterns of the annealed samples reveal that a single phase is retained. Hysteresis measurements performed up to a field of 10 kOe show a tendency toward saturation. The saturation magnetization is found to increase with annealing temperature (particle size) with the magnetization tending toward the bulk value for powders annealed at 900°C. Zero field cooled-field cooled measurements performed at 0.5 kOe indicate the presence of a superparamagnetic phase up to an annealing temperature of 700°C with blocking temperatures in the range of 150-330 K. Numerical simulations are carried out using an interacting model of an array of single domain magnetic particles to explain the change in the magnetic behavior of the samples with annealing temperature and to estimate the anisotropy of the system. Our studies indicate that the observed magnetic behavior can be explained by the changes in the anisotropy of the system and the dominance of the short range interparticle exchange interactions over the long range dipolar interactions with increasing particle sizes. This change in the interaction profile is further confirmed by the Henkel plots for the particles annealed at different temperatures.