The flow of electron spins, the so-called “spin current”, is a key concept in the recent progress in spintronics [1,2].  In a ferromagnet, the spin current interacts with magnetization by the exchange interaction and induces the motion of the magnetization due to the angular momentum conservation, the so-called spin transfer torque.  Its inverse effect is called the spin-motive force which is the electric voltage generated by the magnetization dynamics due to the energy conservation between electrical current and magnetization [3].  The spin motive-force is derived by introducing spin Berry phase in the Faraday’s law of electro-magnetism. 

Spin current is induced and manipulated by heat [4] and mechanical motion [5,6] as well.  The interconversion between various energies through spin current , the so-called “power spintronics”,is discussed.

[1] “Concepts in Spin-Electronics” ed. S.Maekawa (Oxford Univeristy Press, 2006),
[2] “Spin Current” eds. S.Maekawa, et al. (Oxford University Press, 2012 and 2017),
[3] S.E.Barnes and S.Maekawa,: Phys. Rev. Lett. 98, 246601 (2007),
[4] K.Uchida et al: IEEE Proc. 104, 1946 (2016),
[5] R.Takahashi et al.: Nature Phys. 12, 52 (2016),
[6] R.Takahashi, et al.: to be published (2018).