Spin Seebeck effect in two-sublattice ferrimagnets in the vicinity of $T_{\rm C}$

Abstract

Spin Seebeck effect refers to the magnonic thermal spin injection from a magnet into the adjacent heavy metal. A ferrimagnetic insulator yttrium iron garnet (YIG) is the material most studied for the spin Seebeck effect. Here, to account for a convex downward temperature dependence of the spin Seebeck effect observed in YIG/Pt system near the Curie temperature $T_{\rm C}$, we develop Ginzburg-Landau theory of the spin Seebeck effect in two-sublattice ferrimagnets. We find that only when we take into account the "N\'{e}el coupling", i.e., interfacial exchange coupling between the N\'{e}el order parameter of YIG and spin accumulation of Pt, the convex downward temperature dependence is explained. The present result sheds light on the importance of the N\'{e}el coupling in ferrimagnetic spintronics.