The Onsager reciprocity applied to the spin Peltier effect (SPE) and spin Seebeck effect (SSE) for a system composed of yttrium iron garnet (YIG, orange area, left) and platinum (Pt, blue area, right). In SPE (top), a voltage V drives a current in the Pt, where the spin Hall effect (SHE) spin polarizes conduction electrons near the interface with YIG. This spin accumulation launches a spin flux in the magnons in the YIG. The magnons in turn couple to the phonons and thus give rise to a measurable heat flux and temperature difference ΔT in YIG. In SSE (bottom), a temperature difference applied to the YIG results in a phonon flow. The phonons couple to the magnons in YIG and result in a spin flux, which polarizes some conduction electrons in the Pt. Spin-polarized electrons in Pt give rise to an electric field in the Pt by the inverse spin Hall effect (ISHE), itself the Onsager reciprocal of the SHE.