Magnons, Spin Current and Spin Seebeck Effect

When metals and semiconductors are placed in a temperature gradient, the electric voltage is generated. This mechanism to convert heat into electricity, the so-called Seebeck effect, has attracted much attention recently as the mechanism for utilizing wasted heat energy. [1]. Ferromagnetic insulators are good conductors of spin current, i.e., the flow of electron spins [2]. When they are placed in a temperature gradient, generated are magnons, spin current and the spin voltage [3], i.e., spin accumulation. Once the spin voltage is converted into the electric voltage by inverse spin Hall effect in attached metal films such as Pt, the electric voltage is obtained from heat energy [4-5]. This is called the spin Seebeck effect. Here, we present the linear-response theory of spin Seebeck effect based on the fluctuation-dissipation theorem [6-8] and discuss a variety of the devices. \\[4pt] [1] S. Maekawa et al, \textit{Physics of Transition Metal Oxides} (Springer, 2004). \\[0pt] [2] S. Maekawa: Nature Materials \textbf{\textit{8}}, 777 (2009). \\[0pt] [3] \textit{Concept in Spin Electronics}, eds. S. Maekawa (Oxford University Press, 2006). \\[0pt] [4] K. Uchida et al., Nature \textbf{\textit{455}}, 778 (2008). \\[0pt] [5] K. Uchida et al., Nature Materials \textbf{\textit{9}}, 894 (2010) \\[0pt] [6] H. Adachi et al., APL \textbf{\textit{97}}, 252506 (2010) and Phys. Rev. B \textbf{\textit{83}}, 094410 (2011). \\[0pt] [7] J. Ohe et al., Phys. Rev. B (2011) \\[0pt] [8] K. Uchida et al., Appl. Phys. Lett. \textbf{\textit{97}}, 104419 (2010).