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)=1.5x10(6)m(2)/m(3). It corresponds to the effective grain size of about 1 mu m 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 0.12emu/g and coercivity H-c approximate to 0.01T. 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.