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Electron hydrodynamics is currently known to emerge only when electron-electron interaction dominates over the momentum-nonconserving scatterings of electrons, where the electron transport is described by a hydrodynamic equation. Here we show that electron transport in electron systems with the spin Hall effect is also given by the hydrodynamic equation, whose kinetic viscosity is determined by the spin diffusion length and the transport lifetime. The electric current vorticity is proportional to the spin accumulation due to the spin Hall effect in two-dimensional systems. We demonstrate by solving the hydrodynamic equation in a two-dimensional system with a cavity, combined with micromagnetic simulation for an attached chiral magnetic insulator, that the spin accumulated near the boundary of the cavity creates a magnetic skyrmion. Our findings and demonstration shed light on a novel aspect of electron hydrodynamics and spin transport., Comment: 6 pages, 4 figures

The quantum field theoretical approach with the Kubo formula has successfully captured spin torques, such as spin-transfer torques and spin-orbit torques, for continuum systems. We examine the field theoretical approach to current-induced spin-transfer torques in a magnetic junction system. We first give a brief overview of the field theoretical approach to spin torques. Then, we consider a five-layers system consisting of three nonmagnetic metal layers separated by two ferromagnetic metal layers and apply an electric field perpendicular to the layers. We demonstrate that the Slonczewski-type spin-transfer torque, or shortly the Slonczewski torque, on the magnetizations in ferromagnetic layers is obtained by evaluating nonequilibrium electron spin density, based on the linear response theory with the Green function method. The obtained coefficient of the Slonczewski torque has a quantum oscillation at absolute zero temperature, which has not been mentioned before. A field-like torque accompanied by the Slonczewski torque is also evaluated., 11pages, 5 figures, any comments are welcome

We theoretically investigate a manipulation method of nonequilibrium spin accumulation in the paramagnetic normal metal of a spin pumping system, by using the spin precession motion combined with the spin diffusion transport. We demonstrate based on the Bloch-Torrey equation that the direction of the nonequilibrium spin accumulation is changed by applying an additional external magnetic field, and consequently, the inverse spin Hall voltage in an adjacent paramagnetic heavy metal changes its sign. We find that the spin relaxation time and the spin diffusion length are simultaneously determined by changing the magnitude of the external magnetic field and the thickness of the normal metal in a commonly-used spin pumping system.

(主査) 教授 福井 学, 教授 森川 正章, 助教 小島 久弥, 参与 渡辺 幸一（(株)ヤクルト本社中央研究所）, 環境科学院

(主査) 教授 福井 学, 教授 森川 正章, 助教 小島 久弥, 参与 渡辺 幸一（(株)ヤクルト本社中央研究所）, 環境科学院