Antiferromagnetic MnO nanoparticles with ferrimagneticMn3O4shells: Doubly inverted core-shell system

Abstract

We report the magnetic and microstructural properties of antiferromagnetic MnO nanoparticles with shells of ferrimagnetic ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$, which is opposite the usual arrangement of antiferromagnetically coated ferromagnetic nanoparticles. In addition, the antiferromagnetic MnO cores order at much higher temperature $({T}_{N}=118\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ than the ferrimagnetic ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ shells $({T}_{C}=43\phantom{\rule{0.3em}{0ex}}\mathrm{K})$---another reversal of the usual situation. The single crystal MnO cores, with rocksalt structure, are crystallographically aligned with the tetragonal spinel structure of the ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ shells. Particles field cooled in $50\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ have large coercive force and exchange bias below ${T}_{C}$, e.g., 5800 and $2950\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}$, respectively, at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The spontaneous magnetization at ${T}_{C}({\mathrm{Mn}}_{3}{\mathrm{O}}_{4})$ is $\ensuremath{\sim}20%$ of its value at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and remains finite for more than $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ above ${T}_{C}({\mathrm{Mn}}_{3}{\mathrm{O}}_{4})$. Hysteresis with exchange bias is present in this anomalous region. The MnO cores with their uncompensated spins are responsible for the behavior above ${T}_{C}({\mathrm{Mn}}_{3}{\mathrm{O}}_{4})$. The MnO cores have a blocking temperature of $95\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and the hysteresis and exchange bias above ${T}_{C}({\mathrm{Mn}}_{3}{\mathrm{O}}_{4})$ 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 $50\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$, and the dependence of exchange bias on the temperature at which the cooling field was applied support this model.