Surface effect in the magnetic order of antiferromagnetic nanoparticles

The magnetic properties of NiO (3 nm) and alpha-Fe2O3 (hematite) (5 nm) nanoparticles homogeneously dispersed in a polymer matrix have been investigated. Both systems show a core-shell magnetic structure, giving rise to surface and interface effects responsible for the observed rapid increase of remanence and coercivity at low temperature. Moreover, shifted hysteresis loops were observed after field cooling (exchange bias) for alpha-Fe2O3 nanoparticles, due to the interface exchange coupling between the disordered surface magnetic structure, with multiple spin configurations, and the core "antiferromagnetically" ordered structure.