  We performed temperature-dependent optical pump - THz emission measurements
in Y3Fe5O12 (YIG)|Pt from 5 K to room temperature in the presence of an
externally applied magnetic field. We study the temperature dependence of the
spin Seebeck effect and observe a continuous increase as temperature is
decreased, opposite to what is observed in electrical measurements where the
spin Seebeck effect is suppressed as 0K is approached. By quantitatively
analysing the different contributions we isolate the temperature dependence of
the spin-mixing conductance and observe features that are correlated to the
bands of magnon spectrum in YIG.
