Thermal properties of magnons in yttrium iron garnet at elevated magnetic fields

The ferrimagnetic insulator yttrium iron garnet (YIG) has become an important material in the emergent field of spin caloritronics. Despite this and the fact that this material has been studied for over half a century, the thermal properties of magnons in YIG have not been sufficiently characterized, mainly because at not very low temperatures, they are overwhelmed by the contribution of phonons. Experimental attempts to characterize the magnon specific heat and thermal conductivity in YIG make use of large magnetic fields to freeze the magnon contributions and isolate those of phonons relative to their behavior at zero field. Here we present calculations of the magnon thermal properties in YIG under elevated magnetic fields using spin-wave theory. We show that at a temperature of $10\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, a field of at least $300\phantom{\rule{0.16em}{0ex}}\mathrm{kOe}$ is necessary to decrease the magnon contributions to 10% of their zero-field values. With the results of the calculations, we reinterpret recent measurements of the magnon thermal properties in YIG at temperatures up to 20 K and a field of 70 kOe, and suggest a procedure to determine their values at room temperature with the use of a field of 300 kOe.