Unconventional Thickness Dependence of Anomalous Nernst Effect
Tsao-Chi Chuang1*, Po-Lung Su1, Ssu-Yen Huang1
1Department of Physics, National Taiwan University, Taipei, Taiwan
* presenting author:Tsao-Chi Chuang, email:q6712084@hotmail.com
The anomalous Nernst effect (ANE), the conversion of the thermal energy into the spin-dependent electric signal, is one of the most important mechanisms to study the coupling between charge, spin, and heat for the potential applications of spin caloritronics devices. Therefore, the thin film devices based on the ANE, including the multilayer structure and ferromagnetic thermopile, have been extensively studied. Although, the electric field driven by ANE could be strongly influenced by the finite size effect and surface scattering in the thin films, this crucial issue has never been addressed. In this work, we systematically study the thickness dependence of the ANE in several ferromagnetic metals, including permalloy (Py), iron (Fe), cobalt (Co), and nickel (Ni), at room temperature. We show that the ANE of Py and Co decreases with decreasing thickness due to finite size effect, while that of Ni and Fe exhibit significant enhancement in the thin-film region. The ANE voltages have the nonmonotonic thickness dependent behavior for the Ni case and can even change the sign for the Fe case. We suggest that the strength of the spin-orbit coupling could be enhanced in ultra-thin films of Fe and Ni, resulting in this unconventional thickness dependent behavior. The thickness dependence of the ANE angle and the ANE coefficient are also discussed.

Keywords: anomalous Nernst effect, thickness dependence, ferromagnetic metal