Yuji Matsumoto,¹ Makoto Murakami,¹ Tomoji Shono,¹ Tetsuya Hasegawa,¹ Tomoteru Fukumura,² Masashi Kawasaki,² Parhat Ahmet,³ Toyohiro Chikyow,³⁴ Shin-ya Koshihara,⁵ Hideomi Koinuma^1367*

Dilute magnetic semiconductors and wide gap oxide semiconductors are appealing materials for magnetooptical devices. From a combinatorial screening approach looking at the solid solubility of transition metals in titanium dioxides and of their magnetic properties, we report on the observation of transparent ferromagnetism in cobalt-doped anatase thin films with the concentration of cobalt between 0 and 8%. Magnetic microscopy images reveal a magnetic domain structure in the films, indicating the existence of ferromagnetic long-range ordering. The materials remain ferromagnetic above room temperature with a magnetic moment of 0.32 Bohr magnetons per cobalt atom. The film is conductive and exhibits a positive magnetoresistance of 60% at 2 kelvin.

¹ Ceramics Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan.
² Department of Innovative and Engineered Materials, Tokyo Institute of Technology, Yokohama 226-8502, Japan.
³ National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
⁴ National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan.
⁵ Department of Materials Science, Tokyo Institute of Technology, 2-12-1 Okayama, Meguro-ku, Tokyo, Japan, and Kanagawa Academy of Science and Technology, KSP East Building 301, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa, 213-0012, Japan.
⁶ Frontier Collaborative Research Center, Tokyo Institute of Technology, Yokohama 226-8503, Japan.
⁷ CREST-Japan Science and Technology, Waseda University, 55th Building, 3-4-1 Ohkubo, Shinjyuku-ku 169-8555, Japan.
^*   To whom correspondence should be addressed. E-mail: koinuma{at}oxide.rlem.titech.ac.jp