Spin Seebeck effect in two-sublattice ferrimagnets in the vicinity of TC

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 TC, we develop Ginzburg-Landau theory of the spin Seebeck effect in two-sublattice ferrimagnets. We find that only when we take into account the ``Néel coupling'', i.e., interfacial exchange coupling between the Néel 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 Néel coupling in ferrimagnetic spintronics.