Anomalous Hall and Nernst effects in ferrimagnetic Mn$_4$N films: possible interpretation and prospect for enhancement

TL;DR

This study investigates the anomalous Hall and Nernst effects in Mn4N films, revealing magnetic structure complexities and proposing pathways to enhance thermoelectric performance for heat flux sensors.

Contribution

It provides a combined experimental and theoretical analysis of ANE and anomalous Hall effects in Mn4N, highlighting the impact of magnetic structure and suggesting methods to improve thermoelectric efficiency.

Findings

Negative anomalous Hall conductivity explained by coexisting magnetic structures.

Measured transverse thermoelectric power of +0.5 μV/K at 300 K.

Proposed enhancement of ANE by achieving a single magnetic structure with high uniaxial anisotropy.

Abstract

Ferrimagnetic Mn$_4$N is a promising material for heat flux sensors based on the anomalous Nernst effect (ANE) because of its sizable uniaxial magnetic anisotropy ($K_{\rm u}$) and low saturation magnetization ($M_{\rm s}$). We experimentally and theoretically investigated the ANE and anomalous Hall effect in sputter-deposited Mn$_4$N films. It was revealed that the observed negative anomalous Hall conductivity ($\sigma_{xy}$) could be explained by two different coexisting magnetic structures, that is, a dominant magnetic structure with high $K_{\rm u}$ contaminated by another structure with negligible $K_{\rm u}$ owing to an imperfect degree of order of nitrogen. The observed transverse thermoelectric power ($S_{\rm ANE}$) of $+0.5\, \mu{\rm V/K}$ at $300\, {\rm K}$ gave a transverse thermoelectric coefficient ($\alpha_{xy}$) of $+0.34\, {\rm A/(m \cdot K)}$, which was smaller than the value predicted from first-principles calculation. The interpretation for $\alpha_{xy}$ based on the first-principles calculations led us to conclude that the realization of single magnetic structure with high $K_{\rm u}$ and optimal adjustment of the Fermi level are promising approaches to enhance $S_{\rm ANE}$ in Mn$_4$N through the sign reversal of $\sigma_{xy}$ and the enlargement of $\alpha_{xy}$ up to a theoretical value of $1.77\, {\rm A/(m \cdot K)}$.