Monodispersed MnO nanoparticles with epitaxial Mn_(3)O_(4) shells

摘要

We report the microstructural and magnetic properties of monodispersed nanoparticles (NPs) of antiferromagnetic MnO (T_(N) velence 118 K), with epitaxial ferrimagnetic Mn_(3)O_(4) (T_(C) velence 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 Mn_(3)O_(4) 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.