Grain boundaries as a source of ferromagnetism and increased solubility of Ni in nanograined ZnO

The dense nanograined ZnO films with various Ni content (between 0 and 40 at.%) were synthesized by the liquid ceramics method. The films with 0, 5, and 10 at.% Ni contain only ZnO-based solid solution with wurzite structure. The peaks of the second phase NiO become visible in the X-rays diffraction patterns above 10 at.% Ni. Using recently published papers, we constructed the dependence of Ni solubility in ZnO on the grain size. The overall Ni solubility drastically increases with decreasing grain size. The influence of the grain boundary specific area s_GB on the appearance of ferromagnetism in Ni-doped ZnO has been analysed. The review of numerous papers devoted to the ferromagnetic behaviour of Ni-doped ZnO is given. It has been shown that the main factor controlling such behaviour is the specific grain boundary area s_GB. The Ni-doped ZnO is ferromagnetic only if the amount of GBs is high enough and s_GB is above a certain crytical value s_th. = 6 m²/m³. It corresponds to the effective grain size of about 4m assuming that a material is fully dense and grains are equiaxial.