Microscopic study of the spin Seebeck effect in YIG with resonant inelastic x-ray scattering

The SSE in insulating Y₃Fe₅O₁₂ (YIG) was discovered in 2010, defining it as a spin-wave-based transport effect generated by applying a temperature gradient [1]. Since then a multitude of transport studies have been reported indicating the temperature and thickness dependence, and the effect of magnetic field on SSE [2-3]. From a theoretical perspective, many models have been built to explain the experimental results [4-6]. However, no consensus has been reached on the mechanism behind the SSE in YIG; this is due to the lack of a suitable microscopic probe compatible with temperature gradient conditions and sensitive to the elementary excitations leading the transport.

 

Here, we present state-of-the-art resonant inelastic x-ray scattering (RIXS) results on a YIG device operating in presence of spin Seebeck effect (SSE). The study uniquely reveals a momentum (q) and energy (w) resolved picture of the excitations involved in the SSE, providing key information on the microscopic mechanism behind this transport phenomenon. By controlling the applied temperature gradient across the YIG device, our RIXS data display a clear change of the spectral weight: for increasing temperature gradient, the spectral weight is enhanced when q is parallel to the magnon current direction, while suppressed when q is antiparallel to the magnon current direction. This opposite behaviour at opposite q directions proves that our measurement is sensitive to the flowing magnon current. More specifically, we detect the shift of the magnon disturibution induced by the temperature gradient with (q,w) resolution based on the theory simulation. This result on one hand uncovers for the first time the direct observation of the magnon current, thanks to the sensitiviy of RIXS. On the other hand, the fine details of the spectral weight variation versus energy and momentum point to a crucial role of low energy acoustic spin wave mode in the SSE