Anomalous Nernst effect in Co thin films under laser irradiation

Abstract

The anomalous Nernst effect (ANE) generates electromotive forces transverse to temperature gradients and has attracted much attention for potential applications into alternative thermoelectric power generators. ANE efficiency is generally characterized by uniform temperature gradients in a steady state prepared by heaters. However, although focusing laser beams on a magnetic film can form much larger temperature gradients, the laser-irradiation method has not been sufficiently considered for quantifying the ANE coefficient due to the difficulty in estimating the localized in-homogeneous temperature gradients. In this study, we present a quantitative study of ANE in Ru(5 nm)/Co($t_{\mathrm{Co}}$) ($t_{\mathrm{Co}}$ = 3, 5, 7, 10, 20, 40, and 60 nm) bilayers on sapphire (0001) substrates by combining a laser irradiation approach with finite-element analysis of temperature gradients under laser excitation. We find that the estimated ANE coefficients are consistent with previously reported values and one independently characterized using a heater. Our results also reveal the advantages of the laser irradiation method over the conventional method using heaters. Intensity-modulated laser beams can create ac temperature gradients as large as approximately 10$^3$ K/mm at a frequency of tens of kilohertz in a micrometer-scale region.