Dependence of transverse magnetothermoelectric effects on inhomogeneous magnetic fields

Transverse magnetothermoelectric effects are studied in permalloy thin films grown on MgO substrates. We find that small parasitic magnetic fields below 1 Oe can produce artifacts of the order of 1% of the amplitude of the anisotropic magnetothermopower which is also detected in the experiments. The measured artifacts reveal a new source of uncertainties for the detection of the transverse spin Seebeck effect in conductive ferromagnets. Taking these results into account we conclude that the contribution of the transverse spin Seebeck effect in our permalloy-Pt devices to the detected voltages is below the noise level of 20 nV.