Origin of the thickness-dependent low-temperature enhancement of spin Seebeck effect in YIG films

TL;DR

This study investigates how the spin Seebeck effect in YIG films varies with temperature, magnetic field, and film thickness, revealing complex interface and magnon propagation effects that influence the SSE enhancement at low temperatures.

Contribution

It uncovers the thickness-dependent low-temperature enhancement of the SSE and highlights the significant role of interface effects beyond bulk phonon-magnon interactions.

Findings

SSE signal peaks at low temperatures with a strong thickness dependence.

Enhancement is more pronounced in thicker YIG films and vanishes below 600 nm.

Interface coupling significantly influences the temperature dependence of the SSE.

Abstract

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.