  We investigate the Thomson-Onsager relation between the spin-dependent
Seebeck and spin-dependent Peltier effect. To maintain identical device and
measurement conditions we measure both effects in a single
Ni$_{80}$Fe$_{20}$/Cu/Ni$_{80}$Fe$_{20}$ nanopillar spin valve device subjected
to either an electrical or a thermal bias. In the low bias regime, we observe
similar spin signals as well as background responses, as required by the
Onsager reciprocity relation. However, at large biases, deviation from
reciprocity occurs due to dominant nonlinear contribution of the temperature
dependent transport coefficients. By systematic modeling of these nonlinear
thermoelectric effects and measuring higher order thermoelectric responses for
different applied biases, we identify the transition between the two regimes as
the point at which Joule heating start to dominate over Peltier heating. Our
results signify the importance of local equilibrium for the validity of this
phenomenological reciprocity relation.
