Large anomalous Nernst e ect at room temperature in a chiral antiferromagnet

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, and thus observed only in ferromagnets. Theoretically, however, the anomalous Nernst e ect provides a measure of the Berry curvature at the Fermi energy, 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 Mn3Sn (ref. 10). Despite a very small magnetization ∼0.002μB per Mn, the transverse Seebeck coe cient at zero magnetic field is ∼0.35μVK at room temperature and reaches ∼0.6μVK at 200K, 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. As this e ect is geometrically convenient for thermoelectric power generation—it enables a lateral configuration of modules to cover a heat source—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. This trend has also triggered renewed interest in the anomalous Nernst effect (ANE) in ferromagnetic metals, 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. As the first case in antiferromagnets, Mn3Sn has been experimentally found to exhibit a large AHE. 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 EF (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 EF and to verify the possibility of the Weyl metal recently proposed for Mn3Sn (ref. 11). Mn3Sn has a hexagonal crystal structure with a space group of P63/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 430K, Mn magnetic moments of ∼3μB lying in the ab-plane form a coplanar, chiral magnetic structure characterized by Q= 0 wavevector, as clarified by the previous neutron diffraction studies. The combination of geometrical frustration and Dzyaloshinskii−Moriya interaction leads to the inverse triangular spin structure with uniform vector chirality (Fig. 1b). The chiral antiferromagnetic order has orthorhombic symmetry and thus induces a very tiny magnetization ∼2mμB per Mn, which is essential to switch the non-collinear antiferromagnetic structure by using magnetic field. On further cooling below ∼50K, a clusterglass phase appears as spins cant toward [0001] (c axis). In this study, we used two as-grown single crystals that have a single phase of hexagonal Mn3Sn (ref. 29) (Methods and Supplementary Information), with slightly different compositions, that is, Mn3.06Sn0.94 for Sample 1, and Mn3.09Sn0.91 for Sample 2 (Methods). Hereafter, we focus on the coplanar magnetic phase at T >60K. We first provide clear evidence of the large anomalous Hall and Nernst effects observed in Samples 1 and 2. Figure 2a,b shows the field dependence of the Hall resistivity ρH(B) in B ‖ [011̄0] for Samples 1 and 2, respectively. Clearly, there is a sharp jump in ρH(B) with a small coercivity of <200 G. In particular for Sample 2, the size of the jump 1ρH reaches ∼9μ cm at 100K, which would be equivalent to the Hall resistivity due to an ordinary Hall effect under about a few hundred Tesla for free conduction electrons with density of order one electron per Mn atom. To make comparison with theory later, here we take the x , y and z coordinates along [21̄1̄0], [011̄0] and [0001], and estimate the Hall conductivity employing the expression that takes care of the anisotropy of longitudinal resistivity (Supplementary Fig. 1), namely, σji≈−ρji/(ρjjρii), where (i, j)= (x , y), (y , z), (z , x) (ref. 21) (Supplementary Information). Figure 2c,d shows −σzx versus B for Samples 1 and 2, respectively. The zero-field value is∼50cm at 300K for both samples and it becomes particularly enhanced for Sample 2 at low temperatures and reaches 120cm at 100K. The sign change in the Hall effect as a function of field should come from the flipping of the tiny uncompensated moment that follows the rotation of the sublattice moments. Our main experimental observation of a large ANE at room temperature is provided in Fig. 3a. The Nernst signal (transverse thermopower)−Szx of Sample 1 shows a clear rectangular hysteresis with an overall change of 1Szx ∼ 0.7 μVK as a function of the in-plane field. This is significantly large for an antiferromagnet and comparable to the values reported for ferromagnets, as we will