Interparticle interactions in composites of nanoparticles of ferrimagnetic (γ-Fe2O3) and antiferromagnetic (CoO,NiO) materials

The magnetic properties of mixtures of ferrimagnetic γ-Fe₂O₃ (maghemite) and antiferromagnetic NiO or CoO nanoparticles have been studied by use of ⁵⁷Fe M\"ossbauer spectroscopy, neutron powder diffraction and magnetization measurements. The studies showed that the interaction with antiferromagnetic particles has a significant influence on the magnetic properties of the γ-Fe₂O₃ nanoparticles. It was found that mixing the γ-Fe₂O₃ nanoparticles with NiO nanoparticles resulted in a faster superparamagnetic relaxation and a reduced coercivity compared to a sample consisting solely of γ-Fe₂O₃ nanoparticles. On the contrary, mixing of γ-Fe₂O₃ nanoparticles with CoO nanoparticles resulted in a suppression of the relaxation and an increase in coercivity. These results suggest that the properties of the ferrimagnetic γ-Fe₂O₃ nanoparticles are influenced by the anisotropy of their neighboring antiferromagnetic particles. The dominating type of magnetic interaction between the particles in the composites seems to be exchange interaction between surface atoms of neighboring particles.