Magnetic Interactions between Nanoparticles of Different Materials

Abstract

Samples consisting of nanoparticles of α-Fe₂O₃ and mixtures of α-Fe₂O₃ and NiO or CoO have been studied by Mössbauer spectroscopy in the temperature range 20–320 K. Nanoparticles of α-Fe₂O₃, which normally do not have a Morin transition above 5 K, exhibited a Morin transition when mixed with NiO nanoparticles. Pure hematite particles, coated with oleic acid, showed a typical superparamagnetic behaviour at higher temperatures, whereas a sample of uncoated particles was less affected by relaxation, indicating a strong influence of inter-particle interactions. Admixture of NiO nanoparticles resulted in a smaller influence of inter-particle interactions on the spectra of uncoated particles. Admixture of CoO nanoparticles did not induce a Morin transition in α-Fe₂O₃ particles, but unlike NiO particles, CoO nanoparticles significantly slowed down the superparamagnetic relaxation.