  In the past decade, room temperature ferromagnetism was often observed in
transition metal doped semiconductors, which were claimed as diluted magnetic
semiconductors (DMS). Nowadays intensive activities are devoted to clarify
wether the observed ferromagnetism stems from carrier mediated magnetic
impurities, ferromagnetic precipitates, or spinodal decomposition. In this
paper, we have correlated the structural and magnetic properties of transition
metal doped ZnO, TiO2, and Si, prepared by ion implantation. Crystalline
precipitates, i.e., transition metal (Co, Ni) and Mn-silicide nanocrystals, are
responsible for the magnetism. Additionally due to their orientation nature
with respect to the host, these nanocrystals in some cases are not detectable
by conventional x-ray diffraction (XRD). This nature results in the pitfall of
using XRD to exclude magnetic precipitates in DMS materials.
