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

Abstract

The magnetic properties of mixtures of ferrimagnetic $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ (maghemite) and antiferromagnetic $\mathrm{NiO}$ or $\mathrm{CoO}$ nanoparticles have been studied by use of $^{57}\mathrm{Fe}$ 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 $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ nanoparticles. It was found that mixing the $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ nanoparticles with $\mathrm{NiO}$ nanoparticles resulted in a faster superparamagnetic relaxation and a reduced coercivity compared to a sample consisting solely of $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ nanoparticles. On the contrary, mixing of $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ nanoparticles with $\mathrm{CoO}$ nanoparticles resulted in a suppression of the relaxation and an increase in coercivity. These results suggest that the properties of the ferrimagnetic $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ 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.