Unconventional exchange bias in oxide-coated manganese nanoparticles

Abstract

We report unconventional exchange bias in oxide-coated manganese nanoparticles, in which the Curie temperature of Mn3O4 shell is lower than the Néel temperature of the antiferromagnetic core. The coercivity (873kA∕m) of the nanoparticles, which is more than four times greater than that of bulk Mn3O4, has been enhanced significantly. A considerable enhancement in Curie temperature compared to the bulk was also observed for Mn3O4 in nanoscale. An exchange bias field as large as 400kA∕m was observed due to the strong interfacial exchange coupling. A simple phenomenological model is given to understand these phenomena in this ferri/antiferromagnetic system.