INTRODUCTION Ferromagnetic diluted magnetic semiconductors (DMS’s) are key materials for future “spin electronics” or “spintronics”, in which one manipulates both the spin and charge de- grees of freedom of electrons, because interaction between the spins of host sp -band electrons and the magnetic moments of doped magnetic ions enables us to control the spin degrees of freedom of electrons [1, 2]. In fact, the ferromagnetism of the III-V DMS’s Ga$_{1}$$_{−}$$_{x}$Mn$_{x}$As and In$_{1}$$_{−}$$_{x}$Mn$_{x}$As arises from carrier-mediated interaction between Mn spins [3], and new functional devices utilizing both magnetism and electrical conductivity have been fabricated based on these materials, e.g., spin-dependent circular light-emitting diodes [4] and field- effect transistors controlling ferromagnetism [5]. New control methods of magnetization have also been demonstrated, such as electrical manipulation of magnetization reversal [6] and current-induced domain-wall switching [7]. For practical applications of such devices, ferromagnetic DMS’s having Curie temperatures ( T$_{C}$ ’s) above room temperature are nec- essary. Recently, oxide-based DMS’s have attracted much attention first motivated by the theoretical predictions of high T$_{C}$ ’s in wide-gap semiconductors [8, 9] followed by various reports of ferromagnetism at room temperature [10]. Ever since the discovery of ferromagnetism in Zn$_{1}$$_{−}$$_{x}$Co$_{x}$O thin films [11], ZnO-based DMS’s have been investigated intensively because the host material ZnO has potential for optical applications due to the wide band gap of 3 . 4 eV and the large exciton energy of 60 meV. In particular, Zn$_{1}$$_{−}$$_{x}$Co$_{x}$O is one of the most investigated DMS’s, and there have been many reports about the magnetic properties of Zn$_{1}$$_{−}$$_{x}$Co$_{x}$O. Schwartz and Gamelin [12] have reported reversible switching of room-temperature ferromagnetism in Zn$_{1}$$_{−}$$_{x}$Co$_{x}$O by intro- ducing and removing interstitial Zn defects. Reproducibly, exposure of Zn$_{1}$$_{−}$$_{x}$Co$_{x}$O thin film to Zn vapor creates interstitial Zn defects and induces room-temperature ferromagnetism, and annealing the thin film in air quenches the ferromagnetism due to elimination of the in- terstitial Zn by oxidation. Neal et al . [13] have observed magnetic circular dichroism (MCD) signals with open hysteresis loop at the ZnO band edge of Co-doped ZnO thin films at room temperature, implying a splitting of the conduction band of ZnO through interaction with the magnetic moments of the Co ions. These observations rule out Co-cluster segregations or secondary phase formation as the origin of the ferromagnetism. In addition, there are some reports about the observation of anomalous Hall effects in Zn$_{1}$$_{−}$$_{x}$Co$_{x}$O [14, 15], provid- 3