Magnetism in transition-metal-substituted semiconducting oxides

The nascent field of spintronics requires materials that exhibit room-temperature ferromagnetism while retaining their semiconducting properties. A clever strategy for creating these materials is by substituting magnetic transition metals onto the cation sites of commonly used binary semiconductors in order to incorporate magnetic property (e.g. Mn-substituted GaAs). A materials system of great interest is transition-metal substituted ZnO, which was predicted by Dietl and co-workers [1] to be a room temperature ferromagnet with manganese substitution and hole doping. Despite a great deal of effort in preparing thin films of transition-metal substituted ZnO, there has not been reproducible evidence of ferromagnetism in this system; Curie temperatures and magnetic moments vary significantly between studies. In our work, bulk transition metal substituted ZnO samples were prepared and characterized in order to accurately determine their properties. We find no evidence for ferromagnetic behavior in either the Zn1-xCo xO nor the Zn1-xMn xO systems; rather the dominant nearest-neighbor interactions are antiferromagnetic. A similar behavior is observed in nanoparticulate analogs of the bulk materials. Along with substituted ZnO, the 'end-member' of the Zn1-xCox O system, novel wurzite CoO was also prepared and studied. Finally, solid solutions of a dilute ferrimagnetic semiconductor system, iron-substituted ZnGa2O4, were prepared and found to possess long-range magnetic ordering ferromagnetic hysteresis at low temperatures. Optical spectroscopy indicates that the iron substitution does not greatly alter the position of the band edge, hence maintaining the semiconducting properties. Such promising results suggest that dilute ferrimagnetic semiconductors, which do not require conduction electrons to induce magnetism, are worthy of further investigation.