ABSTRACT
Reports of Curie temperatures well above room temperature for wide-bandgap oxides doped with a few percent of transition-metal cations have triggered intense interest in these materials as potential magnetic semiconductors. The origin of the magnetism is debated; in some systems, the ferromagnetism can be attributed to nanoparticles of a ferromagnetic secondary phase, but in others, properties are found that are incompatible with any secondary phase, and an intrinsic origin related to structural defects is implicated. The magnetic interactions in these materials are qualitatively different from those in magnetically concentrated compounds, and the standard “m-J paradigm” of localized magnetism is unable to explain their behavior. In this chapter, we first summarize the established view of magnetism in the oxides, before reviewing data on a selection of the most widely studied materials; then we consider the physical models that have been advanced to explain the magnetism, and raise issues that still have to be addressed. As noted in Chapter 9, Volume 2, some of the concepts also apply to magnetically doped wide-gap III–V compounds.
