Spin-polarized Seebeck effect at the nanoscale measured with scanning tunneling thermovoltage microscopy*

Spin caloritronic effects have gained recent interest due to, e.g., potential low-power applications of magnetic tunnel junctions in which a temperature gradient-driven tunneling of electrons gives rise to a spin-dependent thermovoltage due to the Seebeck effect. Here, we report spatial mapping of the spin-resolved thermovoltage in a tunneling junction formed by ferromagnetic Co islands on a Cu(111) substrate and the magnetic tip of a scanning tunneling microscope. Our measurements reveal variations of the thermovoltage as function of spin polarization, island size, and stacking orientation of the islands with respect to the substrate. This method allows to reveal nanoscopic heterogeneities of both, the magnetic structures and the spin-dependent thermoelectric power.