Long lifetime of thermally excited magnons in bulk yttrium iron garnet

Spin currents are generated within the bulk of magnetic materials due to heat flow, an effect called intrinsic spin Seebeck. This bulk bosonic spin current consists of a diffusing thermal magnon cloud, parametrized by the magnon chemical potential (mu(m)), with a diffusion length of several microns in yttrium iron garnet (YIG). Transient optothermal measurements of the spin-Seebeck effect (SSE) as a function of temperature reveal the time evolution of mu(m) due to intrinsic SSE in YIG. The interface SSE develops at times <2 ns while the intrinsic SSE signal continues to evolve at times >500 mu s, dominating the temperature dependence of SSE in bulk YIG. Time-dependent SSE data are fit to a multitemperature model of coupled spin/heat transport using the finite-element method (FEM), where the magnon spin lifetime (tau) and magnon-phonon thermalization time (tau(mp)) are used as fit parameters. From 300 to 4 K, tau(mp) varies from 1 to 10 ns, whereas tau varies from 2 to 60 mu s with the spin lifetime peaking at 90 K. At low temperature, a reduction in tau is observed consistent with impurity relaxation reported in ferromagnetic resonance measurements. These results demonstrate that the thermal magnon cloud in YIG contains extremely low-frequency magnons (similar to 10 GHz), providing spectral insight to the microscopic scattering processes involved in magnon spin/heat diffusion.