A high- T C heavy rare earth monoxide semiconductor TbO with a more than half-filled 4f orbital.

Terbium monoxide, TbO, with an unusual valence state of Tb 2+ ([Xe]4f 8 5d 1 ) has been known to exist as a gas phase. In this study, we report a solid phase rock-salt structured TbO in the form of an epitaxial thin film. This TbO is a narrow gap ferromagnetic semiconductor with a bandgap of ∼0.1 eV and high Curie temperature of 231 K, and exhibited negative magnetoresistance of -14% at 9 T and 2 K. Its magnetization steeply increased with the appearance of a wasp-waisted hysteresis curve significantly below the Curie temperature at about 20 K. These results suggested that both 4f and 5d magnetic sublattices were formed at a single magnetic Tb 2+ site in TbO.