Monodispersed MnO nanoparticles with epitaxial Mn3O4 shells

We report the microstructural and magnetic properties of monodispersed nanoparticles ( NPs) of antiferromagnetic MnO ( T-N = 118 K), with epitaxial ferrimagnetic Mn3O4 ( T-C = 43 K) shells. Above T-C, an unusually large magnetization is present, produced by the uncompensated spins ( UCSs) on the surface of the MnO particles. These spins impart a net anisotropy to the MnO particles that is approximately three orders of magnitude larger than the bulk value. As a result, an anomalously high blocking temperature is exhibited by the MnO particles, and finite coercivity and exchange bias are present above T-C. When field cooled below T-C, a strong exchange bias was established in the Mn3O4 shells as a result of high net anisotropy of the MnO particles. A large coercivity was also observed. Models of several aspects of the behaviour of this unusual system emphasized the essential role of the UCSs on the surfaces of the MnO NPs.