Magnetic phase transitions in ferrite nanoparticles characterized by electron spin resonance

Ferrite magnetic nanoparticles in the composition Zn0.7Ni0.3Fe2O4 were synthesized by the polyol method, with an average size of 8 nm. Electron spin resonance (ESR) measurements were carried out at a frequency of 9.45 GHz in the 100-500K temperature range. Obtained results exhibited a characteristic ESR signal in terms of resonance field, H-res, linewidth, Delta H, and peak ratio, R, for each magnetic phase. At low temperatures, the ferrimagnetic phase showed low H-res, broad Delta H, and asymmetric R. At high temperatures, these parameters exhibited opposite values: high H-res, small Delta H, and R similar to 1. For intermediate temperatures, a different phase was observed, which was identified as a superparamagnetic phase by means of zero-field cooling-field cooling and hysteresis loops measurements. The observed differences were explained in terms of the internal fields and especially due to the cubic anisotropy in the ordered phase. (C) 2015 AIP Publishing LLC.