Longitudinal spin Seebeck effect in permalloy separated from the anomalous Nernst effect: Theory and experiment

The longitudinal spin Seebeck effect (LSSE) consists in the generation of a spin current parallel to a temperature gradient in a magnetic material. The LSSE has only been measured unequivocally in magnetic insulators because in metallic films it is contaminated by the anomalous Nernst effect (ANE). Here we report theoretical and experimental studies of the LSSE in the metallic ferromagnet N81Fe19 (permalloy-Py) separated from the ANE. We have used trilayer samples of Py/NiO/NM (NM is a normal metal, Pt or Ta) under a temperature gradient perpendicular to the plane to generate a spin current in Py that is transported across the NiO layer and reaches the NM layer, where it is converted into a charge current by the inverse spin Hall effect. The LSSE is detected by a voltage signal in the NM layer while the ANE is measured by the voltage induced in the Py layer. The separation of the two effects is made possible because the antiferromagnetic insulator NiO layer transports spin current while providing electrical insulation between the Py and NM layers. The measured spin Seebeck coefficient for Py has a value similar to the one for the ferrimagnetic insulator yttrium iron garnet, with the same sign, and is in good agreement with the value calculated with a thermoelectric spin drift-diffusion model.