The influence of the grain boundary specific area sGB on the appearance of ferromagnetism in Co-doped ZnO was 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 was found that the value of the specific grain boundary area sGB was the main factor controlling such behaviour. The Co-doped ZnO becomes ferromagnetic only if it contained enough grain boundaries, i.e., if sGB was higher than a certain threshold value sth = 1.5 x 106 /m. It corresponded to an effective grain size of about 1µm assuming a fully dense material and equiaxial grains. The magnetic properties of Co-doped (0 to 42at%) ZnO dense nanograined thin films were investigated. The films were deposited using the wet chemistry “liquid ceramics” method. The samples demonstrated ferromagnetic behaviour with Js up to 0.12 emu/g and coercivity Hc  ≈ 0.01T. Saturation magnetization non-monotonically depends on the Co concentration. The dependence on Co content could be explained by the changes in the structure of a ferromagnetic so-called grain boundary foam responsible for the magnetic properties of pure and doped ZnO.

Grain Boundaries as the Controlling Factor for the Ferromagnetic Behaviour of Co-Doped ZnO. Straumal, B.B., Mazilkin, A.A., Protasova, S.G., Straumal, P.B., Myatiev, A.A., Schütz, G., Goering, E.J., Tietze, T., Baretzky, B.: Philosophical Magazine, 2013, 93[10-12], 1371-83