  In the last decade, transition-metal-doped ZnO has been intensively
investigated as a route to room-temperature diluted magnetic semiconductors
(DMSs). However, the origin for the reported ferromagnetism in ZnO-based DMS
remains questionable. Possible options are diluted magnetic semiconductors,
spinodal decomposition, or secondary phases. In order to clarify this question,
we have performed a thorough characterization of the structural and magnetic
properties of Co- and Ni-implanted ZnO single crystals. Our measurements reveal
that Co or Ni nanocrystals (NCs) are the major contribution of the measured
ferromagnetism. Already in the as-implanted samples, Co or Ni NCs have formed
and they exhibit superparamagnetic properties. The Co or Ni NCs are
crystallographically oriented with respect to the ZnO matrix. Their magnetic
properties, e.g., the anisotropy and the superparamagnetic blocking
temperature, can be tuned by annealing. We discuss the magnetic anisotropy of
Ni NCs embedded in ZnO concerning the strain anisotropy.
