Antiferromagnetic MnO nanoparticles with ferrimagnetic Mn3 O4 shells: Doubly inverted core-shell system

We report the magnetic and microstructural properties of antiferromagnetic MnO nanoparticles with shells of ferrimagnetic Mn3 O4, which is opposite the usual arrangement of antiferromagnetically coated ferromagnetic nanoparticles. In addition, the antiferromagnetic MnO cores order at much higher temperature (TN =118 K) than the ferrimagnetic Mn3 O4 shells (TC =43 K) -another reversal of the usual situation. The single crystal MnO cores, with rocksalt structure, are crystallographically aligned with the tetragonal spinel structure of the Mn3 O4 shells. Particles field cooled in 50 kOe have large coercive force and exchange bias below TC, e.g., 5800 and 2950 Oe, respectively, at 5 K. The spontaneous magnetization at TC (Mn3 O4) is ∼20% of its value at 5 K, and remains finite for more than 20 K above TC (Mn3 O4). Hysteresis with exchange bias is present in this anomalous region. The MnO cores with their uncompensated spins are responsible for the behavior above TC (Mn3 O4). The MnO cores have a blocking temperature of 95 K, and the hysteresis and exchange bias above TC (Mn3 O4) results from the switching of the MnO spin lattices by their uncompensated spins. Analysis of the thermoremanent magnetization and field cooling and/or zero field cooling in 50 kOe, and the dependence of exchange bias on the temperature at which the cooling field was applied support this model.