4 Reports on Experimental Results 4.11 Spin-wave propagation and transformation in a thermal gradient

The discovery of the spin Seebeck effect by Uchida et al. [1] has launched a series of investigations on the inßuence of temperature gradients on the spin system of magnetic materials. It has been shown that a thermal gradient applied between the two sides of a magnetic material gives rise to a spin voltage between the hot and the cold area. Pure spin currents can thus be established over long distances and are detected utilizing the inverse spin Hall effect. A variety of experimental and theoretical work has been carried out to gain a deeper insight into the microscopic origin of the creation of a spin voltage due to thermal gradients and to understand phenomena like the regular and longitudinal spin Seebeck effect, respectively [2, 3]. Especially intriguing is the observation that the creation of a spin voltage in a thermal gradient is not connected to the magnetic material being electrically conducting [4]. Consequently, the spin voltage in an electrical insulator must be mediated by magnetization dynamics. For the case of yttrium iron garnet (YIG) the ßow of a pure spin current is maintained by spin waves [5]. Therefore, spin waves play an important role in spin caloric processes being a means to transport heat from a hot to a cold reservoir. Considered in another way, a thermal gradient acts as a new way of creating, amplifying or manip-ulating spin waves. In a thermal gradient the magnetic properties like anisotropy Þelds, exchang