Observation of the anomalous Nernst effect in altermagnetic candidate Mn5Si3

Abstract

The anomalous Nernst effect generates transverse voltage to the applied thermal gradient in magnetically ordered systems. The effect was previously considered excluded in compensated magnetic materials with collinear ordering. However, in the recently identified class of compensated magnetic materials, dubbed altermagnets, time-reversal symmetry breaking in the electronic band structure makes the presence of the anomalous Nernst effect possible despite the collinear spin arrangement. In this work, we investigate epitaxial Mn5Si3 thin films known to be an altermagnetic candidate. We show that the material manifests a sizable anomalous Nernst coefficient despite the small net magnetization of the films. The measured magnitudes of the anomalous Nernst coefficient reach a scale of microVolts per Kelvin. We support our magneto-thermoelectric measurements by density-functional theory calculations of the material's spin-split electronic structure, which allows for the finite Berry curvature in the reciprocal space. Furthermore, we present our calculations of the intrinsic Berry-curvature Nernst conductivity, which agree with our experimental observations.

Figures (4)

• Figure 1: The anomalous Nernst effect in $\rm Mn_5 Si_3$. a Schematic illustration of the experiment: A longitudinal temperature gradient $\nabla T$ induces a spontaneous transverse voltage $V_{y}$. b A cross-section of a typical sample, i.e. substrate / MnSi buffer layer / $\rm Mn_5 Si_3$ layer, captured by scanning transmission electron microscopy, including crystal structure models in the left panel. c Transverse Nernst signal $S_{yx}$ as a function of applied magnetic field for a sample temperature of 216 K. The figure also includes the field dependence of the anomalous Hall conductivity for comparison. dAb initio calculations of the band structure of $\rm Mn_5 Si_3$ (the left panel) including the calculated anomalous Nernst conductivities at 58 K and 216 K (the right panel).
• Figure 2: Temperature dependence of the anomalous Nernst effect in $\rm Mn_5 Si_3$. a Detailed image of the experimental setup: The longitudinal temperature gradient in the sample is induced by heating up the sample with a platinum resistive heater. The colour map of the sample temperature for a particular setting was calculated by a finite-element simulation. b Magnetic-field dependence of the anomalous Nernst signal $S_{yx}$ for four average sample temperatures. Each figure shows the anomalous Hall conductivity measured at the same temperature for comparison. c Dependence of the detected out-of-plane magnetization on the applied magnetic field for different sample temperatures. The main figure features the spontaneous magnetization of our $\rm Mn_5Si_3$ layers after subtracting a saturating contribution from the substrate, whereas the raw data are given in the inset. In the presented data, the dominating diamagnetic signal of the silicon substrate has been subtracted.
• Figure 3: Decomposition of the anomalous Nernst signal. a The anomalous Nernst signal measured at 121 K decomposed into its spontaneous and sigmoid contributions. The anomalous Hall signal is also plotted for comparison. b Temperature dependence of the coercive field of the spontaneous ANE contribution together with the respective dependence for the AHE.
• Figure 4: Comparison of the anomalous Nernst effect measured in epitaxial $\rm Mn_5 Si_3$ and in other materials pan2022giantsakai2020ironikhlas2017guin2019anomalousasaba2021colossalhe2020magnonwuttke2019berrybeckert2023anomalousli2023largescheffler2023anomalousxu2023largereichlova2018largeyou2022anomalousxu2019large. The ANE magnitude $S_{xy}$ is captured as a function of materials' magnetization. The symbol size linearly decreases with the highest atomic number of materials' formula. Antiferromagnetic materials are indicated by full circles, while ferromagnets are denoted by hollow circles and the altermagnet by a crossed circle. Materials showing the spontaneous Nernst effect are labelled in bold font.