Pure Spin Current Driven by a Thermally Induced Magnon Chemical Potential*

A major goal of spintronics is to develop devices that rely on spin current, rather than charge current. Of the few methods for generating pure spin current in magnetic insulators (MI), the spin Seebeck effect (SSE) is an attractive method, due to its simplicity. The SSE relies on a thermal gradient to generate a nonequilibrium distribution of magnons, collective spin quasiparticles, which carry spin current. Describing these magnons requires a thermally driven magnon chemical potential, never before measured in such a nonequilibrium system. Here we report the use of Brillouin light scattering for measuring a magnon chemical potential generated by a thermal gradient in the MI yttrium iron garnet (YIG): Y3Fe5O12. Boltzmann transport analysis allows for the quantification of spin currents due to the magnon temperature and chemical potential gradients. Finally, the range of energies and wavevectors of the magnons that contribute to the spin current are identified. Experimental determination of these items will facilitate advancing the theories describing coupled heat and spin transport. Furthermore, this technique allows for the determination intrinsic spin current generating ability of an MI, not possible using previous techniques.