Large anomalous Nernst effect at room temperature in a chiral antiferromagnet

Abstract

A temperature gradient in a ferromagnetic conductor can generate a transverse voltage drop perpendicular to both the magnetization and heat current. This anomalous Nernst e ect has been considered to be proportional to the magnetization 1-7 , and thus observed only in ferromagnets. Theoretically, however, the anomalous Nernst e ect provides a measure of the Berry curvature at the Fermi energy 8,9 , and so may be seen in magnets with no net magnetization. Here, we report the observation of a large anomalous Nernst e ect in the chiral antiferromagnet Mn 3 Sn (ref. 10). Despite a very small magnetization ∼0.002 µ B per Mn, the transverse Seebeck coe cient at zero magnetic field is ∼0.35 µV K −1 at room temperature and reaches ∼0.6 µV K −1 at 200 K, which is comparable to the maximum value known for a ferromagnetic metal. Our first-principles calculations reveal that this arises from a significantly enhanced Berry curvature associated with Weyl points near the Fermi energy 11 . As this e ect is geometrically convenient for thermoelectric power generation-it enables a lateral configuration of modules to cover a heat source 6 -these observations suggest that a new class of thermoelectric materials could be developed that exploit topological magnets to fabricate e cient, densely integrated thermopiles.Current intensive studies on thermally induced electron transport in ferromagnetic materials have opened various avenues for research on thermoelectricity and its application [12][13][14][15] . This trend has also triggered renewed interest in the anomalous Nernst effect (ANE) in ferromagnetic metals [3][4][5][6][7]15 , which is the spontaneous transverse voltage drop induced by heat current and is known to be proportional to magnetization (Fig. 1a). On the other hand, the recent Berry phase formulation of the transport properties has led to the discovery that a large anomalous Hall effect (AHE) may arise not only in ferromagnets, but in antiferromagnets and spin liquids, in which the magnetization is vanishingly small 10, [16][17][18][19][20][21][22] . As the first case in antiferromagnets, Mn 3 Sn has been experimentally found to exhibit a large AHE 10 . While the AHE is obtained by an integration of the Berry curvature for all of the occupied bands, the ANE is determined by the Berry curvature at E F (refs 8,9). Thus, the observation of a large AHE does not guarantee the observation of a large ANE. Furthermore, the ANE measurement should be highly useful to clarify the Berry curvature spectra near E F and to verify the possibility of the Weyl metal recently proposed for Mn 3 Sn (ref. 11).Mn 3 Sn has a hexagonal crystal structure with a space group of P6 3 /mmc (ref. 23). Mn atoms form a breathing type of kagome lattice in the ab-plane (Fig. 1b), and the Mn triangles constituting the kagome lattice are stacked on top along the c axis forming a tube of face-sharing octahedra. On cooling below the Néel temperature of 430 K, Mn magnetic moments of ∼3µ B lying in the ab-plane form a coplanar, chiral magnetic structure chara...

Noncollinear antiferromagnetic Mn3Sn films

“…Mn 3 Sn and Mn 3 Ge were theoretically predicted to show large AHE driven by nonvanishing Berry curvature [13], while band-structure calculations reveal that both compounds demonstrate a Weyl semimetal state with several Weyl points around the Fermi level [22,23]. In accordance with these theoretical calculations, single crystals of Mn 3 Sn and Mn 3 Ge were found to exhibit large AHE [24][25][26] and large anomalous Nernst effect [14,15], reaching the same order of magnitude as in ferromagnetic materials. These studies concern bulk single crystal, but to extend to spintronic devices thin films of these materials are required.…”

“…Among many different antiferromagnetic [7][8][9] or artificial antiferromagnetic materials [10,11], the noncollinear chiral antiferromagnets have attracted much interest, due to their remarkable structural, magnetic, and electrotransport properties. The trianglular spin structure of these compounds gives rise to a large anomalous Hall effect (AHE) [12,13], thermoelectric effect [14][15][16], magneto-optical Kerr effect [17,18], and spin Hall effect (SHE) [19]. Inspired by experimental work in Mn 3 Ir [19], ab initio calculations confirmed large anisotropic anomalous Hall current and spin Hall current in these materials [20], while predicting that charge current is also spin polarized [21].…”

Anomalous Nernst effect in Ir22Mn78/Co20Fe60B20/MgO layers with perpendicular magnetic anisotropy

“…21 A typical setup, utilizing an out-of-plane temperature gradient and an in-plane magnetic field, has been used to investigate the ANE in previous works. [22][23][24][25][26] The trouble with this configuration is that, in addition to generating an ANE voltage, the out-of-plane temperature gradient can also generate a spin current through the so-called longitudinal spin Seebeck effect (LSSE), 22,[27][28][29][30][31][32][33] which flows directly from the ferromagnetic (FM) into the adjacent non-magnetic metal (NM) and generates a voltage because of the inverse spin Hall effect (ISHE). In order to distinguish the spin Seebeck effect (SSE) and ANE, extensive efforts [34][35][36][37] have been made to compare the voltage in different temperature gradient configurations.…”

Large Anomalous Nernst Effect in a van der Waals Ferromagnet Fe3GeTe2

“…In AHE and ANE, the current is driven by an electric field and temperature gradient ∇ respectively, while a voltage is measured perpendicular to both the current and the magnetization M of the material. While the AHE is dominated by the sum of the Berry curvatures for all the occupied states, ANE is determined by the Berry curvature at the Fermi level F , thus providing a different probe of the Berry curvature near F and the topological nature of materials [16][17][18][19]. Therefore, FGT provides a unique platform for studying the ANE in 2D vdW ferromagnets and their topological properties.…”