Crystalline Ni nanoparticles as the origin of ferromagnetism in Ni   implanted ZnO crystals
  We report the structural and magnetic properties of ZnO single crystals implanted at 623 K with up to 10 at. % of Ni. As revealed by X-ray diffraction, crystalline fcc-Ni nanoparticles were formed inside ZnO. The magnetic behavior (magnetization with field reversal and with different temperature protocol) of all samples is well explained by a magnetic Ni-nanoparticle system. Although the formation of Ni:ZnO based diluted magnetic semiconductor cannot be ruled out, the major contribution to the magnetic properties stems from crystalline nanoparticles synthesized under these implantation conditions. 
Shengqiang Zhou, K. Potzger, Gufei Zhang, F. Eichhorn, W. Skorupa, M.   Helm and J. Fassbender