Spin Seebeck Effect: Amplification of Spin Waves by Thermal Torque

We observe amplification of spin-wave packets propagating along a film of yttrium iron garnet (YIG) subject to a transverse temperature gradient. The amplification is attributed to a spin-transfer thermal torque created by spin currents generated by means of the spin Seebeck effect that acts on the magnetization and opposes the relaxation. The experimental data are interpreted with a simple theoretical model using spin-wave theory that gives an amplification gain in very good agreement with measurements.