Magnetic Proximity Effect Features in Antiferromagnetic/Ferrimagnetic Core-Shell Nanoparticles

A study of “inverted” core-shell, $\mathrm{MnO}/\gamma \mathrm{\text{−}}{\mathrm{Mn}}_2{\mathrm{O}}_3$, nanoparticles is presented. Crystal and magnetic structures and characteristic sizes have been determined by neutron diffraction for the antiferromagnetic core (MnO) and the ferrimagnetic shell ($\gamma \mathrm{\text{−}}{\mathrm{Mn}}_2{\mathrm{O}}_3$). Remarkably, while the MnO core is found to have a $T_N$ not far from its bulk value, the magnetic order of the $\gamma \mathrm{\text{−}}{\mathrm{Mn}}_2{\mathrm{O}}_3$ shell is stable far above $T_C$, exhibiting two characteristic temperatures, at $T\sim 40\mathrm{K}$ [$T_C(\gamma \mathrm{\text{−}}{\mathrm{Mn}}_2{\mathrm{O}}_3)$] and at $T\sim 120\mathrm{K}$ [$\sim T_N(\mathrm{MnO})$]. Magnetization measurements are consistent with these results. The stabilization of the shell moment up to $T_N$ of the core can be tentatively attributed to core-shell exchange interactions, hinting at a possible magnetic proximity effect.

Figure 1

Temperature dependence of the magnetic moment of (a) ${\mathrm{Mn}}^{2+}$ in MnO (the solid lines correspond to a power law fit) and (b) at the tetrahedral sites (○) and at the octahedral sites (●) of the $\gamma \mathrm{\text{−}}{\mathrm{Mn}}_2{\mathrm{O}}_3$ for sample $S$. Arrows mark the magnetic transition temperatures.

Figure 2

Temperature dependence of the intensity of the magnetic peak at 15.4° (●) for sample $S$. There are contributions from the MnO core: reflections $(1/2,1/2,1/2)$, $(1/2,-1/2,-1/2)$, and from the $\gamma \mathrm{\text{−}}{\mathrm{Mn}}_2{\mathrm{O}}_3$ shell: reflections $(1,0,1)$ and $(-1,0,1)$. The contribution from the MnO calculated from the profile analysis is shown by ▴. Shown in the inset is the observed magnetic peak at $T=250\mathrm{K}$.

Figure 3

Temperature dependence of the difference between the FC and ZFC magnetizations, $M_{\mathrm{FC}}-M_{\mathrm{ZFC}}$, for samples $S$ (●) and $L$ (○). Shown in the inset is an enlargement of the high temperature region. The error bars are smaller than the size of the symbol, and the solid lines are guides for the eye.