Ferromagnetism in doped thin-film oxide and nitride semiconductors and dielectrics

The principal goal in the field of ferromagnetic semiconductors for devices is the synthesis, characterization and utilization of semiconductors that exhibit substantial carrier spin polarization at and above room temperature. Such materials are of critical importance in the emerging field of semiconductor spintronics. The interaction leading to carrier spin polarization, exchange coupling between the dopant spins and the valence or conduction band, is known to be sufficiently weak in conventional semiconductors such as GaAs, Si and Ge, that magnetic ordering above cryogenic temperatures is extraordinarily difficult to achieve, as experience has shown. Since the provocative theoretical predictions of Curie points above ambient in p-Mn:ZnO and p-Mn:GaN [T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019], and the observation of room-temperature ferromagnetism in Co: TiO2 anatase [Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S.-Y. Koshihara, H. Koinuma, Science 291 (2001) 854], there has been a flurry of work in oxides and nitrides doped with transition metals with unpaired d electrons. It has even been claimed that room-temperature ferromagnetism can be obtained in certain d0 transition metals oxides without a dopant. In this Report, the field of magnetically doped oxides and nitrides is critically reviewed and assessed from a materials science perspective. The focus is on films prepared not only by conventional vacuum deposition methods, but also by spin coating colloidal nanoparticles in air. It is shown that despite their apparent simplicity, dilute magnetic systems are deceptively complex. High dopant mobilities and the existence of multiple structural configurations that are nearly degenerate lead to a range of embedded magnetic nanostructures within the host lattice. The resulting macroscopic magnetic properties are diverse and critically dependent on growth and processing conditions.