  We investigate the generation of magnonic thermal spin currents and their
mode selective spin transport across interfaces in insulating, compensated
ferrimagnet/normal metal bilayer systems. The spin Seebeck effect signal
exhibits a non-monotonic temperature dependence with two sign changes of the
detected voltage signals. Using different ferrimagnetic garnets, we demonstrate
the universality of the observed complex temperature dependence of the spin
Seebeck effect. To understand its origin, we systematically vary the interface
between the ferrimagnetic garnet and the metallic layer, and by using different
metal layers we establish that interface effects play a dominating role. They
do not only modify the magnitude of the spin Seebeck effect signal but in
particular also alter its temperature dependence. By varying the temperature,
we can select the dominating magnon mode and we analyze our results to reveal
the mode selective interface transmission probabilities for different magnon
modes and interfaces. The comparison of selected systems reveals
semi-quantitative details of the interfacial coupling depending on the
materials involved, supported by the obtained field dependence of the signal.
