  Recently theoretical works predict that some semiconductors (e.g. ZnO) doped
with magnetic ions are diluted magnetic semiconductors (DMS). In DMS magnetic
ions substitute cation sites of the host semiconductor and are coupled by free
carriers resulting in ferromagnetism. One of the main obstacles in creating DMS
materials is the formation of secondary phases because of the solid-solubility
limit of magnetic ions in semiconductor host. In our study transition metal
ions were implanted into ZnO single crystals with the peak concentrations of
0.5-10 at.%. We established a correlation between structural and magnetic
properties. By synchrotron radiation X-ray diffraction (XRD) secondary phases
(Fe, Ni, Co and ferrite nanocrystals) were observed and have been identified as
the source for ferromagnetism. Due to their different crystallographic
orientation with respect to the host crystal these nanocrystals in some cases
are very difficult to be detected by a simple Bragg-Brentano scan. This results
in the pitfall of using XRD to exclude secondary phase formation in DMS
materials. For comparison, the solubility of Co diluted in ZnO films ranges
between 10 and 40 at.% using different growth conditions pulsed laser
deposition. Such diluted, Co-doped ZnO films show paramagnetic behaviour.
However, only the magnetoresistance of Co-doped ZnO films reveals possible s-d
exchange interaction as compared to Co-implanted ZnO single crystals.
