Magnetic oxide semiconductors: On the high-temperature ferromagnetism in tio₂- and ZnO-based compounds

Magnetic oxide semiconductor, that is, oxide semiconductor doped with dilute transition metal, has been extensively studied since the high-temperature ferromagnetism is expected to be useful for semiconductor spintronics. After discovery of high-temperature ferromagnetism in (Ti,Co)O2, various magnetic oxide semiconductors were reported to be ferromagnetic. So far, the origin of the ferromagnetism has been of long debate. However, a recent study on the electric field control of the ferromagnetism at room temperature in (Ti,Co)O2 indicates a principal role of electron carriers to mediate the ferromagnetism [Y. Yamada et al., Science 332, 1065 (2011)]. This chapter is dedicated to review fundamental properties for understanding of the ferromagnetism in the context of the carrier-mediated mechanism, although the detailed mechanism is still not clear. This chapter is organized as follows. In Section 3.1, we introduce the magnetic oxide semiconductors briefly. In Section 3.2, we review magnetic properties of both rutile and anatase (Ti,Co)O2 because various experimental results of (Ti,Co)O2 are available, in addition to recent electric-field-effect experiment on anatase (Ti,Co)O2. In Section 3.3, we examine magnetic properties of transition-metal-doped ZnO from the aspect of carrier-mediated ferromagnetism. In Section 3.4, we discuss the effect of carriers on the ferromagnetism in transition-metal-doped TiO2 and ZnO and the origin of high-temperature ferromagnetism. In Section 3.5, we summarize this chapter.