  We have fabricated bulk nanostructured ferrimagnetic materials with different
grain sizes by sintering ball-milled Y3Fe5O12 (YIG) nanoparticles and measured
the grain-size dependence of the thermal conductivity and spin Seebeck
thermopower. The nanostructuring reduces both thermal conductivity and
thermopower, but the reduction of the latter was found to be considerably
stronger despite the moderate difference in magnetization, which suggests that
the length scales of transport of magnons and phonons contributing to the spin
Seebeck effect are significantly larger than that of phonons carrying thermal
current. This is consistent with the measurements of high-magnetic-field
response of the spin Seebeck thermopower and low-temperature thermal
conductivity, where the quenching of magnons seen in single-crystalline YIG was
not observed in nanostructured YIG due to scattering of long-range low
frequency magnons.
