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Antiferromagnetic MnO nanoparticles with ferrimagnetic Mn 3 O 4 shells: Doubly inverted core-shell system

2008; American Physical Society; Volume: 77; Issue: 2 Linguagem: Inglês

10.1103/physrevb.77.024403

1550-235X

A. E. Berkowitz, G. F. Rodriguez, Jung‐Il Hong, Kyongmo An, Taeghwan Hyeon, N. Agarwal, David J. Smith, Eric E. Fullerton,

ZnO doping and properties

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.