  Magnon spin currents in the ferrimagnetic garnet Tb3Fe5O12 with 4f electrons
were examined through the spin-Seebeck effect and neutron scattering
measurements. The compound shows a magnetic compensation, where the
spin-Seebeck signal reverses above and below Tcomp = 249.5(4) K. Unpolarized
neutron scattering unveils two major magnon branches with finite energy gaps,
which are well-explained in the framework of spin-wave theory. Their
temperature dependencies and the direction of the precession motion of magnetic
moments, i.e. magnon polarization, defined using polarized neutrons, explain
the reversal at Tcomp and decay of the spin-Seebeck signals at low
temperatures. We illustrate an example that momentum- and energy-resolved
microscopic information is a prerequisite to understanding the magnon spin
current.
