The spontaneous Nernst coefficient of ferromagnets from the interplay of electron scattering and Berry curvature

TL;DR

This paper derives the spontaneous Nernst coefficient in ferromagnets using Boltzmann transport, revealing its dependence on scattering and Berry curvature, and proposes ways to enhance it via band structure engineering.

Contribution

It introduces a theoretical framework linking the spontaneous Nernst effect to Berry curvature and scattering, and provides practical guidelines for material optimization.

Findings

Spontaneous Nernst coefficient inversely proportional to scattering time.

Connection between Nernst coefficient sign and orbital angular momentum.

Model predictions align with experimental data on ferromagnetic metals.

Abstract

We employ the Boltzmann transport approach to derive the spontaneous Nernst coefficient for ferromagnetic metals, explicitly treating the transverse current density due to Berry curvature as a Fermi surface property. We find that the spontaneous Nernst coefficient is proportional to the inverse of the scattering time constant, implying that efficient spontaneous Nernst materials should exhibit relatively strong scattering, a stark contrast to ordinary Nernst materials. Furthermore, we establish a direct connection between the strength and sign of the spontaneous Nernst coefficient and the itinerant contribution to orbital angular momentum density arising from the Bloch bands. Finally we construct a rigid two-bands model to evaluate the thermoelectric coefficients by which we find a good agreement with the signs and orders of magnitude of the experimental coefficients of magnetic 3d transition metal ferromagnets. We finally propose some practical recipes for maximizing the spontaneous Nernst effect through electronic band structure tailoring.