Enhancement of the anomalous Nernst effect in ferromagnetic thin films

The anomalous Nernst effect (ANE) is one of the most important mechanisms to explore the anomalous Hall heat current in ferromagnets. In this work, we studied the ANE in various ferromagnetic materials with in-plane anisotropy. Surprisingly, we show that the thickness dependence of the ANE on the magnitude and sign is nontrivial, even in conventional ferromagnetic metals (FMs), including Fe, Co, Ni, and Py (Ni80Fe20). While the sign of the ANE of Fe is opposite to that of Co, Ni, and Py in thicker films, it can even be reversed via decreasing thickness. Most importantly, the anomalous Nernst angles theta(ANE) for these FMs show a unified behavior. They can be significantly enhanced by up to one order of magnitude in ultrathin films. By systematically studying the thickness dependence of the electrical and thermal transport properties, we show that the enhanced ANE of FMs is dominated by spin-orbit coupling through the intrinsic and side-jump mechanisms.