  Longitudinal spin Seebeck effect has been investigated for an uniaxial
antiferromagnetic insulator Cr2O3, characterized by a spin-flop transition
under magnetic field along the c-axis. We have found that temperature gradient
applied normal to Cr2O3/Pt interface induces inverse spin Hall voltage of spin
current origin in Pt, whose magnitude turns out to be always proportional to
magnetization in Cr2O3. The observed voltage shows significant enhancement for
the lower temperature region, which can be ascribed to the phonon-drag effect
on the relevant spin excitations. The above results establish that
antiferromagnetic spin waves with high frequency above 100 GHz can be an
effective carrier of spin current.
