Nonlinear Seebeck effect in Ni$_{81}$Fe$_{19}$|Pt at room temperature

TL;DR

This study reports the observation of a nonlinear Seebeck effect in NiFe|Pt bilayers at room temperature, demonstrating a quadratic dependence on temperature gradient and a unique size scaling law, supported by a phenomenological model.

Contribution

It introduces the first experimental observation of the nonlinear Seebeck effect at room temperature and develops a model explaining its dependence on spin-current interactions.

Findings

Nonlinear Seebeck voltage increases with the square of the temperature gradient.

The nonlinear effect scales inversely with sample length along the temperature gradient.

The phenomenological model accurately reproduces experimental observations.

Abstract

The nonlinear Seebeck effect, nonlinear conversion of a temperature gradient into an electric current, was observed at room temperature. Based on a second-harmonic lock-in method combined with an a.c. temperature gradient, $\nabla T$, we measured a nonlinear Seebeck voltage in NiFe|Pt bilayers at 300 K, the amplitude of which increases in proportion to $(\nabla T)^2$. We also observed that the nonlinear Seebeck voltage increases as the sample length along the $\nabla T$ direction decreases, showing a characteristic scaling law distinct from the conventional linear Seebeck effect. We developed a phenomenological model for the nonlinear Seebeck effect incorporating the spin-current induced modulation of the Seebeck coefficient, which well reproduces the experimental results.