  We present measurements of the spin Seebeck effect (SSE) by a technique that
combines alternating currents (AC) and direct currents (DC). The method is
applied to a ferrimagnetic insulator/heavy metal bilayer,
Y$_3$Fe$_5$O$_{12}$(YIG)/Pt. Typically, SSE measurements use an AC current to
produce an alternating temperature gradient and measure the voltage generated
by the inverse spin-Hall effect in the heavy metal at twice the AC frequency.
Here we show that when Joule heating is associated with AC and DC bias
currents, the SSE response occurs at the frequency of the AC current drive and
can be larger than the second harmonic SSE response. We compare the first and
second harmonic responses and show that they are consistent with the SSE. The
field dependence of the voltage response is used to characterize the
damping-like and field-like torques. This method can be used to explore
nonlinear thermoelectric effects and spin dynamics induced by temperature
gradients.
