Influence of texture on the ferromagnetic properties of nanograined ZnO films

The pure ZnO thin films were deposited by the wet chemistry ('liquid ceramics') method from the butanoate precursors on the single-crystalline (102) sapphire substrates. The films annealed in air (550 degrees C, 24 h) after butanoate pyrolysis have pronounced texture, and they reveal the ferromagnetic behaviour. Argon annealed films (650 degrees C, 30 min) exhibit randomly oriented grains, where the ferromagnetism of these non-textured films is almost equal to that of bare substrate. In both cases the films consist of dense equiaxial nanograins with size similar to 20 nm. We observed that grain boundaries (GBs) and related vacancies are the intrinsic origin for RT ferromagnetism in polycrystals [Straumal et al., Phys. Rev. B 79, 205206 (2009)]. Present results demonstrate that not only the specific area of GBs in nanograined ZnO alone determines the ferromagnetic behaviour of ZnO. The GB character distribution (i.e. GB misorientation and orientation) is different in the textured and non-textured films. Most probably, the GBs with different character possess also different magnetic properties. The role of GBs, free surfaces and interfaces in ferromagnetic behaviour of GaN is discussed. In particular, their presence permits to increase the Mn solubility in GaN without precipitation of secondary ferromagnetic phases. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim