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

This study presents a measurement method for determining the figure of merit for transverse thermoelectric conversion (z_TT) in thin film forms. Leveraging the proposed methodology, we comprehensively investigate the transverse thermoelectric coefficient (S_T), in-plane electrical conductivity (σ_yy), and out-of-plane thermal conductivity (κ_xx) in epitaxial and polycrystalline Fe/Pt metallic multilayers. The κ_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 κ_xx. z_TT is found to increase with increasing N, which remarkably reflects the N-dependent enhancement of the S_T values. Notably, S_T and σ_yy are significantly larger in the epitaxial multilayers than those in the polycrystalline counterparts, whereas negligible differences in κ_xx are observed between the epitaxial and polycrystalline multilayers. This discrepancy in σ_yy and κ_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_TT in the multilayers. This study is the first demonstration in the evaluation of z_TT in thin film forms, and our proposed measurement technique reveals the transverse thermoelectric properties inherent to multilayers.