The truth about ferromagnetic ZnO

The combination of magnetic and semiconducting properties in oxides is currently one of most popular fields in materials research. Besides the expected gain of knowledge about basic physics, such materials have a large application potential in spin electronics. We present a summary of our results on transition metal doping of ZnO single crystals and thin films by means of ion implantation. We found that none of the samples investigated represents a diluted magnetic semiconductor as predicted by theory [1]. Nevertheless, transition metal ions can be dispersed within the ZnO matrix residing on different sites within the lattice depending on initial preparation conditions. The observed ferromagnetism mainly originates from secondary phase formation (metals or inverted spinels). We discuss the potential of those granular structures in spin-electronics. Moreover, we highlight the suppression of secondary phase formation by means of deliberately lowering the crystalline quality prior to the doping. In that case, purely defect induced ferromagnetic properties are observed. The effect of spin doping of such a defect induced ferromagnet is discussed.
[1] K. Sato and H. Katayama-Yoshida, Physica E 10, 251 (2001).