Interface Double-Exchange Ferromagnetism in the Mn-Zn-O System: New Class of Biphase Magnetism

In this Letter, we experimentally show that the room temperature ferromagnetism in the Mn-Zn-O system recently observed is associated with the coexistence of ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ via a double-exchange mechanism. The presence of the ZnO around ${\mathrm{MnO}}_2$ modifies the kinetics of ${\mathrm{MnO}}_2\to {\mathrm{Mn}}_2{\mathrm{O}}_3$ reduction and favors the coexistence of both Mn oxidation states. The ferromagnetic phase is associated with the interface formed at the Zn diffusion front into Mn oxide, corroborated by preparing thin film multilayers that exhibit saturation magnetization 2 orders of magnitude higher than bulk samples.

Figure 1

(a) XRD pattern corresponding to the $10\%{\mathrm{MnO}}_2–90\%\mathrm{ZnO}$ sample. Peaks can be indexed and labeled on the basis of the mixture of ZnO, ${\mathrm{Mn}}_2{\mathrm{O}}_3$, ${\mathrm{MnO}}_2$, and ${\mathrm{Zn}}_x{\mathrm{Mn}}_{3-x}{\mathrm{O}}_4$. (b) HREM image of a crystal containing both Zn and Mn cations along the $[\overline{1}31]$ zone axis. Periodicities are in agreement with a spinel cell. Inset shows the corresponding FFT.

Figure 2

Evolution of (a) ${\mathrm{M}}_{\mathrm{s}}$ and (b) XRD patterns with the annealing temperature from the $10\%{\mathrm{MnO}}_2–90\%\mathrm{ZnO}$ sample. Dashed lines, circles, and asterisk represent the main peaks from ${\mathrm{Zn}}_x{\mathrm{Mn}}_{3-x}{\mathrm{O}}_4$, ${\mathrm{Mn}}_2{\mathrm{O}}_3$, and ${\mathrm{MnO}}_2$, respectively.

Figure 3

(a) TGA from pure ${\mathrm{MnO}}_2$ and the component of ${\mathrm{MnO}}_2$ in $2\%{\mathrm{MnO}}_2–98\%\mathrm{ZnO}$. The latter was obtained by subtracting the data for pure ZnO from the experimental results. (b) Raman spectra from $2\%{\mathrm{MnO}}_2–98\%\mathrm{ZnO}$, $10\%{\mathrm{MnO}}_2–90\%\mathrm{ZnO}$, ${\mathrm{MnO}}_2$, ${\mathrm{Mn}}_2{\mathrm{O}}_3$, and $\mathrm{Zn}{\mathrm{Mn}}_2{\mathrm{O}}_4$.

Figure 4

(a) Scheme of the different phases at the Mn oxide grains after annealing at 500 °C and the oxidizing states of Mn. The graph indicates the concentration profile of Zn inside the grain. (b) Hysteresis loops from a 40 ${\mathrm{MnO}}_2/\mathrm{ZnO}$ multilayer prepared by laser ablation at 5 K (triangles) and 300 K (circles). Inset shows a detail of the loops at low $H$. The paramagnetic-diamagnetic component has been subtracted to clearly show the FM phase.