Spin-transfer torque in disordered weak ferromagnets

We study theoretically the spin transfer effect on a domain wall in disordered weak ferromagnets. We have identified the adiabatic condition for the disordered case as \lambda \gg \lambda_{\rm D}\equiv \sqrt{{\hbar D}/{\spol}}, where $D$ and \spol are the diffusion constant and the spin splitting energy due to the $s$-$d$ type exchange interaction, respectively, and found out that perfect spin-transfer effect occurs even in weak ferromagnets as long as this condition is satisfied. The effective $\beta$ term arising from the force turns out to govern the wall dynamics, and therefore, the wall motion can be as efficient as in strong ferromagnets even if \spol is small

Carrier transport properties of the Group-IV ferromagnetic semiconductor Ge1-xFex with and without boron doping

We have investigated the transport and magnetic properties of group-IV ferromagnetic semiconductor Ge1-xFex films (x = 1.0 and 2.3 %) with and without boron doping grown by molecular beam epitaxy (MBE). In order to accurately measure the transport properties of 100-nm-thick Ge1-xFex films, (001)-oriented silicon-on-insulator (SOI) wafers with an ultra-thin Si body layer (~5 nm) were used as substrates. Owing to the low Fe content, the hole concentration and mobility in the Ge1-xFex films were exactly estimated by Hall measurements because the anomalous Hall effect in these films was found to be negligibly small. By boron doping, we increased the hole concentration in Ge1-xFex from ~1018 cm-3 to ~1020 cm-3 (x = 1.0%) and to ~1019 cm-3 (x = 2.3%), but no correlation was observed between the hole concentration and magnetic properties. This result presents a contrast to the hole-induced ferromagnetism in III-V ferromagnetic semiconductors

IV族強磁性半導体Ge(1-x)Fexにおけるキャリア濃度の制御、磁性、バンド描像

学位の種別: 課程博士審査委員会委員 : （主査）東京大学教授 田中 雅明, 東京大学教授 高木 信一, 東京大学教授 平川 一彦, 東京大学教授 田畑 仁, 東京大学准教授 大矢 忍, 東京大学准教授 ファムナムハイUniversity of Tokyo(東京大学