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 effect has been considered to be proportional to the magnetization1,2,3,4,5,6,7, and thus observed only in ferromagnets. Theoretically, however, the anomalous Nernst effect provides a measure of the Berry curvature at the Fermi energy8,9, and so may be seen in magnets with no net magnetization. Here, we report the observation of a large anomalous Nernst effect in the chiral antiferromagnet Mn3Sn (ref. 10). Despite a very small magnetization 0.002?μB per Mn, the transverse Seebeck coefficient 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 energy11. As this effect is geometrically convenient for thermoelectric power generation—it enables a lateral configuration of modules to cover a heat source6—these observations suggest that a new class of thermoelectric materials could be developed that exploit topological magnets to fabricate efficient, densely integrated thermopiles.

Main

Current intensive studies on thermally induced electron transport in ferromagnetic materials have opened various avenues for research on thermoelectricity and its application12,13,14,15. This trend has also triggered renewed interest in the anomalous Nernst effect (ANE) in ferromagnetic metals3,4,5,6,7,15, 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 small10,16,17,18,19,20,21,22. As the first case in antiferromagnets, Mn3Sn has been experimentally found to exhibit a large AHE10. 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 430?K, 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 studies24,25. The combination of geometrical frustration and Dzyaloshinskii −Moriya interaction leads to the inverse triangular spin structure with uniform vector chirality (Fig. 1b)24,25,26. The chiral antiferromagnetic order has orthorhombic symmetry and thus induces a very tiny magnetization 2?mμB per Mn, which is essential to switch the non-collinear antiferromagnetic structure by using magnetic field. On further cooling below 50?K, a cluster-glass phase appears as spins cant toward [0001] (c axis)25,27,28. 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 > 60?K.

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 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 ΔρH reaches 9?μΩ?cm at 100?K, 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 , 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 50?Ω−1cm−1 at 300?K for both samples and it becomes particularly enhanced for Sample 2 at low temperatures and reaches 120?Ω−1cm−1 at 100?K. 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 moments10,24,26.

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 ΔSzx 0.7?μV?K−1 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 discuss. While the in-plane Nernst signal exhibits hysteresis with almost no anisotropy, the out-of-plane c-axis component is zero within experimental accuracy, indicating no spontaneous effect in this direction. To further characterize the ANE, we compare it with the magnetization M by plotting in Fig. 3a both −Szx and M sharing an x axis for the in-plane field. In low fields, the hystereses in both data almost overlap on top of each other. On the other hand, in the higher field region than the coercivity of 100 G, the Nernst effect remains nearly constant, while M increases linearly with field as the sublattice moments cant toward the external field direction. These indicate negligible contributions from both the normal Nernst effect and the conventional ANE. Furthermore, the remnant Nernst signal at zero field is nearly the same as the saturated value in high field, demonstrating that the single-domain crystal has a large spontaneous Nernst signal as the first case for an antiferromagnet. Similar behaviour was found in the field cycles made for Sample 2 (Fig. 3b). A systematic change of the Nernst signal was observed with varying T for both Samples 1 and 2, (Fig. 3c, d). For Sample 1, the Nernst signal peaks at 200?K, reaching a value of 0.6?μV?K−1, comparable to the maximum value known for a ferromagnetic metal at room temperature7, while Sample 2 shows a smaller value, peaking at 250?K with 0.3?μV?K−1. A distinct mechanism from the ferromagnetic case should be at work here as the spontaneous M in Mn3Sn is nearly 1,000 times smaller than ordinary ferromagnets.

To further characterize thermoelectric properties, we measured the longitudinal Seebeck coefficient Sii as a function of T and B. No field dependence was seen in Sii for both samples in the T and B range of the measurements (Supplementary Information and Supplementary Fig. 2a, b). The Seebeck coefficient for Sample 1 positively peaks at 300?K (Fig. 3c, inset), and takes a minimum value with a negative sign around 50?K. This minimum coincides with the steep rise in the specific heat, and thus, this sign change in S(T) may be related to the effect of phonon drag that typically occurs at ΘD/5, where ΘD is the Debye T (Supplementary Information and Supplementary Fig. 3). Much reduced or no low T sign change was seen for Sample 2 (Fig. 3d, inset).

Generally, temperature gradient T in an open-circuit condition is known to produce longitudinal and transverse electric field E, which can be expressed as J = σ E + α (−T) = 0. Here J, σ and α are the current density, electrical conductivity tensor, and thermoelectric conductivity tensor, respectively. This contains the transverse electric field coming from the thermal Hall effect, which should be negligible as it is usually one order of magnitude smaller than other contributions3,4,30. Assuming this, the Nernst signal can be expressed as Sji = ρjj(αjiSiiσji), using the Seebeck coefficient Sii(= S) and the Hall conductivity σji(= σH). For this analysis, we used the T dependence of the Hall conductivity shown in Fig. 3e. As shown in Fig. 3c, d, −ραji estimated using this relation is found to be larger than −Sji. We further estimated the transverse thermoelectric conductivity −αzx versus T (Fig. 3f), which shows a systematic increase on cooling and has the maximum at T 150(100) K for Sample 1(2). At low temperatures, Sample 2 has twice smaller values than Sample 1. In ferromagnets, αji normally shows an increase on cooling, scaling linearly with M, and decreases linearly with T after M saturates3,4. In Mn3Sn, however, no correlation with M was seen in −αzx(T) (Fig. 3f).

To further demonstrate the qualitative difference between the ANE observed in Mn3Sn and in ferromagnets, we made a full logarithmic plot of the anomalous Nernst signal versus the magnetization for various ferromagnetic metals and Mn3Sn (Fig. 4 and Methods). Here, the absolute values of the Nernst signal were taken in the magnetically ordered states of each material, and were plotted using B and T as implicit parameters. Similarly to AHE16, the ANE for ferromagnets is known to be proportional to magnetization. Figure 4 confirms that the overall trend for a broad range of ferromagnetic metals is for the anomalous Nernst signal to become larger with increasing magnetization (Methods). The shaded region that covers all of the data points indicates that the anomalous Nernst signal is indeed roughly proportional to the magnetization M, that is, |Sji| = |Qs| μ0M with the anomalous Nernst coefficient |Qs| ranging between 0.05 and 1?μV?K−1T−1. Following this relation, Mn3Sn would have produced the Nernst signal of the order of 0.01 2?nV?K−1 with the observed magnetization. Strikingly, however, Sji 0.3?μV?K−1 found at room T is more than 100 times larger than what would be expected based on the above scaling relation for ferromagnets.

The significantly large anomalous Nernst and Hall effects in Mn3Sn do not follow their conventional scaling relation with M, and thus it is natural to assume that both effects arise through the same mechanism distinct from the conventional one for ferromagnets. The inverse triangular spin structure for Mn3Sn reduces the lattice symmetry from six-fold to two-fold in the plane and thus based on the symmetry argument, the Hall effect may appear in the ab-plane10,31. Indeed, a recent calculation found that Mn3Sn may have a large anomalous Hall conductivity20, which can be estimated by integrating the Berry curvature of the occupied bands over the entire Brillouin zone9. More recently, the possibility of a Weyl metal has been proposed, where the bands crossing EF have several Weyl points, around which the Berry curvature diverges11.

Another intriguing quantity governed by the Berry curvature is the anomalous Nernst effect. While all of the occupied bands are relevant for the anomalous Hall conductivity, only the Berry curvature around the Fermi level determines the ANE or, more precisely, the transverse thermoelectric conductivity αji (refs 8,9). Therefore, αji is significantly enhanced when the Berry curvature takes a large value at EF. To see this in the case of Mn3Sn, we performed a first-principles calculation, confirming the Weyl points nearby EF (ref. 11). The calculated anomalous Hall conductivity −σzx is found as large as seen in experiment. Theoretically, the extra Mn in Sample 1(2) should dope the conduction electron (Supplementary Information) and thus increase EF by 0.04 (0.05)?eV. Since −σzx forms a peak at EEF 0.065?eV (Supplementary Fig. 4), a shift in EF from +0.04 to +0.05?eV leads to a slight increase in −σzx, but a substantial decrease in −αzx (Fig. 3e, f inset). This behaviour is consistent with the experimental observation, that is, Mn doping suppresses −αzx by 50% at low temperatures, while it enhances −σzx only by 10% (Fig. 3e, f). For a simple Weyl Hamiltonian, σji takes its maximum and αji becomes zero when EF is located exactly at the Weyl node and on the other hand, αji becomes strongly enhanced when EF moves slightly away32. Clearly, the real band structure of Mn3Sn should be more complicated. Interestingly, however, our theory also finds that −σzx forms a peak at EEF 0.065?eV, around which Weyl nodes exist11, and −αzx becomes large when EF is slightly away from the nodes (Supplementary Fig. 4). Our results thus indicate that the ANE in Mn3Sn is particularly enhanced because of the characteristic structure of the Berry curvature with several Weyl points nearby the Fermi level11,32. Our study further highlights the complementary roles of anomalous Hall conductivity σji and transverse thermoelectric conductivity αji in revealing the topological character of band structure.

Finally, from the viewpoint of application for thermoelectric power generation, ANE could be useful as it facilitates the fabrication of a module structurally much simpler than the conventional one using the Seebeck effect6. The orthogonal orientation of the voltage output to the thermal heat flow (Fig. 1a) enables a lateral series connection of a single kind of ferromagnet with alternating magnetization direction (Fig. 1a inset). This simplifies a thermopile structure to efficiently cover the surface of a heat source (Fig. 1a inset), in comparison with the conventional thermoelectric module using the Seebeck effect, which consists of a pillar structure of alternating p- and n-type semiconductors. To increase power density, a thermopile should ideally cover the entire surface of a heat source, and therefore, a micro-fabricated thermopile array has to be arranged as densely as possible. However, as long as a ferromagnet is used, their inherent stray fields may perturb magnetization direction of neighbouring modules, and limit the integration density.

Our discovery of a new class of material that produces almost no stray fields but exhibits a large ANE is highly important for the application toward thermoelectric power generation, and it should allow us to design a thermopile with a much denser integration of thermoelectric modules to efficiently cover a heat source than the ferromagnetic counterparts. While the observed values in this work would be still far from the size necessary for application, our study indicates that the magnetic Weyl metals such as Mn3Sn should be particularly useful to obtain a larger ANE by enhancing the Berry curvature at EF. Further studies to develop the technology for application such as thin-film growth and coercivity control of such magnets will be important to build a thermoelectric power generator.

Methods

Polycrystalline samples were made by melting the mixtures of manganese and tin in an alumina crucible sealed in an evacuated quartz ampoule in a box furnace at 1,050?°C for 6?h. In preparation for single-crystal growth, the obtained polycrystalline materials were crushed into powders, compacted into pellets, and inserted into an alumina crucible that was subsequently sealed in an evacuated silica ampoule. Single-crystal growth was performed using a single-zone Bridgman furnace with a maximum temperature of 1,080?°C and growth speed of 1.5?mm?h−1. Analysis using inductively coupled plasma spectroscopy showed that the composition of the single crystal is Mn3.06Sn0.94 for Sample 1 and Mn3.09Sn0.91 for Sample 2. Powder X-ray diffraction measurement was performed using a diffractometer (RAPID, Rigaku) with a rotating stage to remove preferred orientation in the data. The one-dimensional intensity pattern was extracted from two-dimensional Debye–Scherrer rings that were obtained using transmission mode. Rietveld analysis was performed using RIETAN-FP. All of the samples were shown to be single phase, with lattice parameters consistent with previous work (see Supplementary Information and Supplementary Tables 1 and 2). After the alignment made using Laue diffractometer, the as-grown single crystals were cut into bar-shaped samples by spark machining for transport and magnetization measurements.

The magnetization measurements were done using a commercial SQUID magnetometer (MPMS, Quantum Design). The associated measurement error is less than a few per cent. The specific heat measurement was performed using a commercial system (PPMS, Quantum Design) under zero magnetic field (Supplementary Information). Both longitudinal and Hall resistivities were measured by a standard four-probe method using a commercial system (PPMS, Quantum Design). Electrical contacts were made by spot-welding gold wires for the longitudinal and transverse voltage probes, both of which were placed 1.5?mm apart on the sample with a typical current cross-section area of 0.20?mm2. In addition, thermoelectric properties were jointly measured by the one-heater and two-thermometer configuration using a commercial system (PPMS, Quantum Design). For thermoelectric measurements, the samples have the typical dimension of 10 × 2 × 2?mm3 for Sample 1 (Mn3.06Sn0.94) and 5 × 1.5 × 1.5?mm3 for Sample 2 (Mn3.09Sn0.91). The thermal gradient −T was applied by a heater at one end of the bar-shaped sample toward a thermal bath at the other end, and was measured by monitoring two thermometers linked to the sample by strips of 0.5-mm-wide copper–gold plates along the longest direction of the sample. The distance between the two thermometers is approximately 5?mm for Sample 1 and 2.5?mm for Sample 2. The magnitude of the transverse voltage ΔV was found to be linearly increasing with the increase of applied temperature difference ΔT. Here, ΔT was typically set to be 1.5% 2.0% of the sample temperature for both Seebeck and Nernst measurements. By setting the temperature gradient −T along a bar-shaped single crystal (i axis), the thermoelectric longitudinal and transverse emf voltages Vi and Vj were measured in an open-circuit condition. The Seebeck coefficient Sii and Nernst signal Sji were then estimated as Sii = Ei/T and Sji = Ej/T, where Ei and Ej are the longitudinal and transverse electric field. The magnetic field dependence of the Hall resistivity and the Nernst signal was obtained after removing the longitudinal component of the respective transport properties, which is found to be approximately constant as a function of the magnetic field (Supplementary Information).

The measurement errors for the longitudinal resistivity ρii, Hall resistivity ρji, and Nernst signal Sji, are dominated by the uncertainties of their respective geometrical factors. The uncertainty of the longitudinal resistivity and Hall resistivity is 1–2% and smaller than the symbol sizes used in the figures in the main text and Supplementary Information. The uncertainty in the Nernst signal Sji is 10% for Sample 1 and 20% for Sample 2. The corresponding error bars for the Nernst signal and the transverse thermoelectric conductivity are given in Fig. 3 and Supplementary Fig. 2.

The transverse thermoelectric conductivity αzx was calculated using the Berry curvature formula8 with the first-principles electronic structure. The density functional theory calculation was performed within the generalized-gradient approximation37 as implemented in the quantum-ESPRESSO package38. A 7 × 7 × 7 k-point grid, ultrasoft pseudopotentials39 and plane wave basis sets with cutoff energies of 80 Ry for wavefunctions and 320 Ry for charge densities were used. For the Berry curvature calculation, a Wannier-interpolated band structure40 with a 40 × 40 × 40 k-point grid was employed.

The specimens for transmission electron microscopy were prepared by Ar ion milling using a JEOL Ion-Slicer operated at 5.5?kV and 150?μA under a low beam angle of 2.5°. Selected-area electron diffraction and high-resolution lattice images were obtained using a transmission electron microscope (JEOL JEM-2010F) operated at 200?kV. High-resolution lattice images were simulated by MacTempas software.

Figure 4 was made using the ANE results obtained for various ferromagnets well below their Curie temperatures, as reported in the literature including L10-FePt (300?K; ref. 7), L10-FePd (300?K; ref. 7), L10-MnGa (300?K; ref. 7), D022-Mn2Ga (300?K; ref. 7), Co/Ni films (300?K; ref. 7), Nd2Mo2O7 (T < Tc = 73 K, B = 1 T ǁ [111]; ref. 30), Fe (300?K; ref. 33), Co (300?K; ref. 33), Fe3O4 (300?K, B < 0.8 T; ref. 34), MnGe (140?K, B > 2 T; ref. 35), MnGe (100?K, B > 5 T; ref. 35), MnGe (20?K, B < 14 T; ref. 35), and Pt/Fe multilayer N = 1 9 (300?K, B < 5 T; ref. 36).

Data availability.

The data that support the plots within this paper and other findings of this study are available from the corresponding author on reasonable request.

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Acknowledgements

We thank A. Nugroho, A. Sakai, T. Higo and N. Kiyohara for useful discussions. This work is partially supported by CREST (Grant Number JPMJCR15Q5) and PRESTO, Japan Science and Technology Agency, and Grants-in-Aid for Scientific Research (16H02209), and Program for Advancing Strategic International Networks to Accelerate the Circulation of Talented Researchers (No. R2604) from the Japanese Society for the Promotion of Science, and by Grants-in-Aids for Scientific Research on Innovative Areas (15H05882, 15H05883, 26103002) of the Ministry of Education, Culture, Sports, Science, and Technology of Japan. The use of the facilities of the Materials Design and Characterization Laboratory at the Institute for Solid State Physics, The University of Tokyo, is gratefully acknowledged.

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Contributions

S.N. and Y.O. conceived the project. S.N. planned the experiments, and T.T., M.I. and S.N. performed experiments and analysed data. T.K., M.-T.S. and R.A. performed the first-principles calculations. D.N.-H. performed the TEM measurements and analyses. S.N. and R.A. wrote the main text. M.I., S.N., T.T., D.N.-H., T.K., M.-T.S. and R.A. prepared the Supplementary Information and figures; all authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Satoru Nakatsuji.

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Ikhlas, M., Tomita, T., Koretsune, T. et al. Large anomalous Nernst effect at room temperature in a chiral antiferromagnet. Nature Phys 13, 1085–1090 (2017). https://doi.org/10.1038/nphys4181

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