Grain boundaries as the controlling factor for the ferromagnetic behaviour of Co-doped ZnO

The influence of the grain boundary (GB) specific area s_GB on the appearance of ferromagnetism in Co-doped ZnO has been analysed based on a review of numerous research contributions from the literature on the origin of the ferromagnetic behaviour of Co-doped ZnO. An empirical correlation has been found that the value of the specific grain boundary area s_GB is the main factor controlling such behaviour. The Co-doped ZnO becomes ferromagnetic only if it contains enough GBs, i.e., if s_GB is higher than a certain threshold value s_th = 1.5 × 10⁶ m ²/m³. It corresponds to the effective grain size of about 1 μm assuming a full dense material and equiaxial grains. The magnetic properties of Co-doped (0 to 42 at. %) ZnO dense nanograined thin films have been investigated. The films were deposited using the wet chemistry "liquid ceramics" method. The samples demonstrate ferromagnetic behaviour with J_s up to 0.12 emu/g and coercivity H_c ∼ 0.01 T. Saturation magnetization non-monotonically depends on the Co concentration. The dependence on Co content can be explained by the changes in the structure of a ferromagnetic "grain boundary foam" responsible for the magnetic properties of pure and doped ZnO.