  We performed measurements of Py/Cu and Py/Ag lateral spin valves as function
of injection current direction and magnitude. Above a 'critical' current, there
is an unexpected dependence of spin injection on current direction. Positive
currents show higher polarization of spin injection than negative. This implies
that in addition to current-induced spin injection, there is a thermally
induced injection from a spin-dependent Seebeck effect. A temperature gradient
in the Py electrode, caused by Joule heating, is responsible for injecting
excess spins into the non-magnetic channel. This effect has important
consequences for understanding high-current spin-based devices, such as spin
transfer torque devices.
