Mechanism of the two sign changes in the spin Seebeck effect of a compensated ferrimagnet

Spin Seebeck effect is the mechanism of thermal spin injection from a precessing ferromagnet into an attached paramagnetic metal [Uchida et al., Nature 455, 778 (2008)]. We have theoretically investigated the spin Seebeck effect in compensated ferrimagnets [Ohnuma et al., Phys. Rev. B 87, 014423 (2013)] and predicted that the sign of the spin Seebeck signal changes at the compensation temperature, which is recently confirmed by an experiment [Gepr\"{a}gs et al., arXiv:1405.4971 (2014)]. Interestingly, the experiment found another sign change at a lower temperature. Here we explain its origin by taking account of sublattice dependence of the exchange coupling at the ferrimagnet/paramagnet interface.