Quantitative analysis of the longitudinal spin Seebeck Effect

Abstract

Spin Seebeck effect (SSE) is defined as the excitation of a spin current in a magnetic ordered material by its subjection to a thermal gradient. This spin current has been proved to be carried by magnetic excitations -magnons- rather than mobile carriers, and it is usually detected by its conversion into an electrical current by means of the Inverse Spin Hall effect in an attached metallic and non-magnetic material. Thus, SSE is a complex phenomenon whose output is the result of the interplay between several parameters characterizing magnetic, electrical, magnonic, and thermal properties of the involved materials and interfaces, making its quantitative analysis far from straightforward. Here we report a series of experiments and calculations intended to enable this quantitative analysis of longitudinal SSE (LSSE) in epitaxial thin films of ferrimagnetic insulating maghemite (γ-Fe2O3). The use of an insulating ferrimagnet overcomes theoretical and experimental complications, and brings some advantages for practical applications We study the temperature dependence of the stationary LSSE, and correlate it with the thermal conductivity of both the magnetic thin-film and the substrate, measured by the 3ω method. The influence of the magnetic layer thickness is also determined and considered in the analysis. The results are fitted to theory, granting us the access to the value of different parameters influencing the spin transport through the samples and the output ISHE voltage. The knowledge of these quantities is of great relevance because it brings to light the contribution of the underlaying mechanisms to the studied effect and points out the direction in which materials and devices should be engineered in order to improve the LSSE signal.