Quantitative measurement of figure of merit for transverse thermoelectric conversion in Fe/Pt metallic multilayers

Abstract

This study presents a measurement method for determining the figure of merit for transverse thermoelectric conversion ($z_{\mathrm{T}}T$) in thin film forms. Leveraging the proposed methodology, we comprehensively investigate the transverse thermoelectric coefficient ($S_{\mathrm{T}}$), in-plane electrical conductivity (${\sigma }_{yy}$), and out-of-plane thermal conductivity (${\kappa }_{xx}$) in epitaxial and polycrystalline Fe/Pt metallic multilayers. The ${\kappa }_{xx}$ values of multilayers with a number of stacking repetitions ($N$) of 200 are lower than those of FePt alloy films, indicating that the multilayer structure effectively contributes to the suppression of ${\kappa }_{xx}$. $z_{\mathrm{T}}T$ is found to increase with increasing $N$, which remarkably reflects the $N$-dependent enhancement of the $S_{\mathrm{T}}$ values. Notably, $S_{\mathrm{T}}$ and ${\sigma }_{yy}$ are significantly larger in the epitaxial multilayers than those in the polycrystalline counterparts, whereas negligible differences in ${\kappa }_{xx}$ are observed between the epitaxial and polycrystalline multilayers. This discrepancy in ${\sigma }_{yy}$ and ${\kappa }_{xx}$ with respect to crystal growth is due to the different degree of anisotropy in electron transport between epitaxial and polycrystalline multilayers, and epitaxial growth can lead to an enhancement of $z_{\mathrm{T}}T$ in the multilayers. This study is the first demonstration in the evaluation of $z_{\mathrm{T}}T$ in thin film forms, and our proposed measurement technique reveals the transverse thermoelectric properties inherent to multilayers.

28 pages, 11 figures