Heat and spin

Maekawa, Sadamichi; Adachi, Hiroto

When metals and semiconductors are placed in a temperature gradient, the electric voltage is generated. This mechanism to convert heat into electric energy, the so-called Seebeck effect, has attracted much attention as the mechanism for utilizing wasted heat energy. The Seebeck effect is due to the entropy carried by the electric current so that it may be enhanced by the internal degrees of freedom of electrons, i.e., spin and orbital. Ferromagnetic insulators are good conductors of spin current, i.e., the flow of electron spins. When they are placed in a temperature gradient, generated is the spin voltage, i.e., spin accumulation. Once the spin voltage is converted into the electric voltage by spin Hall effect in attached metal films such as Pt, the electric voltage is obtained from heat. This is called the spin Seebeck effect. Here, we discuss the Seebeck effect and spin Seebeck effect based on the fluctuation-dissipation theorem and introduce a variety of the devices.