Spin thermoelectric efficiency across a normal-metal/ferromagnetic-insulator interface

We investigate the spin and heat transport across a normal-metal/ferromagnetic-insulator (NM/FI) interface based on the s–d   exchange model. Under a certain temperature gradient, the heat current carried by magnons partly flows into the metal, and is partly converted to spin power corresponding to spin current. We find that when the magnon dispersion of the FI (such as yttrium iron garnet) is quadratic, the conversion efficiency of heat current to spin power is about ηs∼0.2ηCηs∼0.2ηC with ηCηC being the Carnot efficiency. The corresponding spin thermopower is roughly Ss∼110μV/K. The efficiency and the spin thermopower can be enhanced by opening a gap via, for example, introducing a magnetic field. Effects of temperature in the presence of a gap and dimensionality are also discussed briefly.