  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.
