  We studied the nonlocal transport behavior of both electrically and thermally
excited magnons in yttrium iron garnet (YIG) as a function of its thickness.
For electrically injected magnons, the nonlocal signals decrease monotonically
as the YIG thickness increases. For the nonlocal behavior of the thermally
generated magnons, or the nonlocal spin Seebeck effect (SSE), we observed a
sign reversal which occurs at a certain heater-detector distance, and it is
influenced by both the opacity of the YIG/heater interface and the YIG
thickness. Our nonlocal SSE results can be qualitatively explained by the
bulk-driven SSE mechanism together with the magnon diffusion model. Using a
two-dimensional finite element model (2D-FEM), we estimated the bulk spin
Seebeck coefficient of YIG at room temperature. The quantitative disagreement
between the experimental and modeled results indicates more complex processes
going on in addition to magnon diffusion and relaxation, especially close to
the contacts.
