  We report the observation of longitudinal spin Seebeck effects (LSSE) in an
all-oxide bilayer system comprising an IrO$_2$ film and an Y$_3$Fe$_5$O$_{12}$
film. Spin currents generated by a temperature gradient across the
IrO$_2$/Y$_3$Fe$_5$O$_{12}$ interface were detected as electric voltage via the
inverse spin Hall effect in the conductive IrO$_2$ layer. This electric voltage
is proportional to the magnitude of the temperature gradient and its magnetic
field dependence is well consistent with the characteristic of the LSSE. This
demonstration may lead to the realization of low-cost, stable, and transparent
spin-current-driven thermoelectric devices.
