Enhancement of the anomalous Nernst effect in ferromagnetic thin films*

The anomalous Nernst effect (ANE), generating spin thermoelectric signal through spin-orbit coupling, is an important mechanism to explore the interaction between charge, heat, and spin. Recently, intense researches show that a large ANE signal can be observed in the noncollinear antiferromagnet, thermopile, and multilayer structure. It is commonly thought that the ANE is proportional to magnetization. However, in this work, we find that the magnitude and sign of the ANE exhibit nontrivial thickness dependent behaviors, even in conventional ferromagnets. [1] We study the thickness dependent ANE in Fe, Co, Ni, and Py with in-plane anisotropy and vertical temperature gradient. The sign of the ANE of Fe is opposite to that of Co, Ni, and Py in thicker films, and it can be reversed via decreasing thickness. Most importantly, the anomalous Nernst angles for these FMs 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 in thin-film.