  Since the discovery of room temperature ferromagnetism in (Ti,Co)O2, the
mechanism has been under discussion for a decade. Particularly, the central
concern has been whether or not the ferromagnetic exchange interaction is
mediated by charge carriers like (Ga,Mn)As. Recent two studies on the control
of ferromagnetism in anatase (Ti,Co)O2 at room temperature via electric field
effect [Y. Yamada et al., Science 332, 1065 (2011)] and chemical doping [Y.
Yamada et al., Appl. Phys. Lett. 99, 242502 (2011)] indicate a principal role
of electrons in the carrier-mediated exchange interaction. In this article, the
authors review fundamental properties of anatase (Ti,Co)O2 and discuss the
carrier mediated ferromagnetism.
