Electron spin resonance study of NiFe2O4 nanosolids compacted under high pressure

In this article, NiFe2O4 nanosolids are prepared by compacting nanoparticles, synthesized by the coprecipitation method, under different pressures. The variations of structure and interface states of the NiFe2O4 nanosolids with compacting pressures are studied by x-ray diffraction and electronic spin resonance (ESR). It is found that the crystal structure of the nanosolids has not changed under pressure up to 6.0 GPa, but the linewidth and g-factor values of their ESR spectra increased significantly with increasing pressure up to 4.5 GPa and then decreased slightly with further increase of pressure. These variations are discussed in terms of the changes of interparticle magnetic dipole interaction and superexchange interaction in NiFe2O4 nanosolids under different pressures. The experimental results suggest that 4.5 GPa is the optimum forming pressure, under which the NiFe2O4 nanoparticles can be compacted into a dense solid with their structure and nanoproperties still preserved.