We report the microstructural and magnetic properties of monodispersed nanoparticles (NPs) of antiferromagnetic MnO (TN = 118 K), with epitaxial ferrimagnetic Mn3O4 (TC = 43 K) shells. Above TC, 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 TC. When field cooled below TC, 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.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Acoustics and Ultrasonics
- Surfaces, Coatings and Films