Room temperature ferromagnetism in nanostructured ZnO-Al system

The origin of the room-temperature ferromagnetism (RTF) in ZnO-based dilute magnetic semiconductors remains controversial. We experimentally demonstrate here that it is possible to induce RTF in ball milled (ZnO)(1-x)/Al-x without any ferromagnetic dopant. Our work shows that RTF in (ZnO)(1-x)/Al-x (x = 0-0.5) can be realized simply by milling a mixture of high purity ZnO and Al fine powders for 8 h. The spontaneous magnetization of the milled powders is found to increase by annealing under a reduced pressure. The magnetization value depends highly on both the ratio of Al to ZnO and the annealing temperature. X-ray photoelectron spectroscopy results have revealed that the Zn2+ ions in (ZnO)(1-x)/Al-x are partially reduced into a lower ionic state. As there are no magnetic impurities present in our samples, the origin of ferromagnetism is most likely to be due to the charge transfer between Zn and Al at the interfaces of the ball milled nanograins. Our results reinforce the significant role played by the alterations of the electronic structures in the RTF of ZnO. (C) 2009 American Institute of Physics. [doi:10.1063/1.3254224]