Temperature Dependence of Spin Diffusivity in Yttrium Iron Garnet from the Time Evolution of the Intrinsic Spin-Seebeck Effect*

Recent experimental and theoretical works describe the production of spin currents within the bulk of yttrium iron garnet (YIG) concomitant with heat flow, known as the intrinsic spin Seebeck effect (SSE). We present pulsed laser measurements of the time-resolved longitudinal spin Seebeck effect (SSE) in Pt/YIG below room temperature, revealing the time-dependent diffusion of magnons produced within the bulk of YIG. Unlike the interface contribution to SSE, which evolves on sub-nanosecond timescales, the bulk magnon diffusion process occurs on time scales spanning hundreds of microseconds. The data are fit to a time-dependent model for coupled spin and heat diffusion in Pt/YIG, from which we extract the spin diffusivity of YIG. At low temperatures the spin diffusivity is orders of magnitude smaller than the thermal diffusivity. The low diffusivities and long diffusion lengths of magnons in YIG imply long magnon lifetimes, indicative of magnons with low wave vector.