Ferromagnetic properties of the Mn-doped nanograined ZnO films

Dense nanograined pure and Mn-doped ${\text{Zn}}_{1-\text{x}}{\text{Mn}}_{\text{x}}\text{O}$ polycrystals with x ranging between 0.1–34 at. % were synthesized by the wet chemistry method from butanoate precursors. Pure and Mn-doped ZnO possesses ferromagnetic properties only if the ratio of grain boundary (GB) area to grain volume $s_{\text{GB}}$ exceeds a certain threshold value $s_{\text{th}}$ . The polycrystals in this work satisfy these conditions and, therefore, reveal ferromagnetic properties. The observed dependence of saturation magnetization on the Mn concentration shows an unexpected nonmonotonous behavior. The increase in saturation magnetization at low Mn concentration is explained by the injection of divalent ${\text{Mn}}^{2+}$ ions and charge carriers into pure ZnO. The decrease in saturation magnetization between 0.1 and 5 at. % Mn can be explained by the increase in the portion of ${\text{Mn}}^{3+}$ and ${\text{Mn}}^{4+}$ ions. The second increase in saturation magnetization above 5 at. % Mn is explained by the formation of multilayer Mn segregation layer in ZnO GBs. The shape of the dependence of saturation magnetization on Mn concentration is different for the Mn-doped nanograined ZnO manufactured by different methods. It is most probably controlled by the topology of GB network (ferromagnetic GB foam) in the ZnO polycrystals.