  The temperature dependent longitudinal spin Seebeck effect (SSE) in heavy
metal (HM)/ Y3Fe5O12 (YIG) bilayers is investigated as a function of different
magnetic field strength, different HM detection material, and YIG thickness
ranging from nm to mm. A large enhancement of the SSE signal is observed at low
temperatures leading to a peak of the signal amplitude. We demonstrate that
this enhancement shows a clear dependence on the film thickness, being more
pronounced for thicker films and vanishing for films thinner than 600 nm. The
peak temperature depends on the applied magnetic field strength as well as on
the detection material and interface, revealing a more complex behavior beyond
the currently discussed phonon-magnon coupling mechanism that considers only
bulk effects. While the thickness dependence and magnetic field dependence can
be well explained in the framework of the magnon-driven SSE by taking into
account the frequency dependent propagation length of thermally excited magnons
in the bulk material, the temperature dependence of the SSE is significantly
influenced by the interface coupling to an adjacent detection layer. This
indicates that previously neglected interface effects play a key role and that
the spin current traversing the interface and being detected in the HM depends
differently on the magnon frequency for different HMs.
