Abstract
Mn_{3}Sn has recently attracted considerable attention as a magnetic Weyl semimetal exhibiting concomitant transport anomalies at room temperature. The topology of the electronic bands, their relation to the magnetic ground state and their nonzero Berry curvature lie at the heart of the problem. The examination of the full magnetic Hamiltonian reveals otherwise hidden aspects of these unusual physical properties. Here, we report the full spin wave spectra of Mn_{3}Sn measured over a wide momentum—energy range by the inelastic neutron scattering technique. Using a linear spin wave theory, we determine a suitable magnetic Hamiltonian which not only explains the experimental results but also stabilizes the lowtemperature helical phase, consistent with our DFT calculations. The effect of this helical ordering on topological band structures is further examined using a tight binding method, which confirms the elimination of Weyl points in the helical phase. Our work provides a rare example of the intimate coupling between the electronic and spin degrees of freedom for a magnetic Weyl semimetal system.
Introduction
It is an intriguing question of how certain electronic band structures give rise to nontrivial topological properties. Initial work focused on how unique edge states form for particular symmetries, leading to the now broad and popular field of topological insulators.^{1} More recently, attention has concentrated on magnetic systems, where Dirac points at band crossings are broken into two sets of socalled Weyl points because of the broken timereversal symmetry. Systems with Weyl points exhibit a nontrivial Berry’s phase, which can be characterized by measuring transport anomalies such as the anomalous Hall effect. Mn_{3}Sn, a noncollinear metallic antiferromagnet whose magnetic ions sit on a kagome lattice, has recently drawn growing interest for its rather remarkable transport anomalies. Density functional calculations examining the Weyl points^{2,3} and transport measurements^{4,5,6,7,8} of Mn_{3}Sn show that this antiferromagnetic metal exhibits significant transport anomalies at room temperature. Subsequently, the presence of the Weyl points in the electronic band structure have been confirmed by angleresolved photoemission spectroscopy and magnetoresistance measurements.^{9} More recently, it was suggested that it may also have spin polarized current, which would make it an interesting candidate for spintronics applications.^{10}
According to studies done in the 1980s and 1990s, the magnetic structure of Mn_{3}Sn has a 120° structure with a negative vector chirality at room temperature. This ground state indicates the presence of significant Dzyaloshinskii–Moriya (DM) interactions mediated by a spin–orbit coupling^{11,12,13,14} which, together with an easyaxis anisotropy, stabilizes a weak ferromagnetic moment.^{13,15} On the other hand, the lowtemperature phase of Mn_{3}Sn is known to depend on the precise ratio between Mn and Sn contents (see ref. ^{16}). Some earlier results show that Mn_{3}Sn displays an additional incommensurate helical ordering along the caxis k_{z} ≃ 0.09 below 220 –270 K^{17,18} (we call it as the Atype Mn_{3}Sn), while others found a glass phase below 50 K, instead of the helical ordering^{19,20} (we call it as the Btype Mn_{3}Sn). While both types show the same transport anomalies at room temperature,^{4,7} the Atype Mn_{3}Sn does not show the transport anomaly at the lowtemperature phase with the helical ordering.^{21}
It is usually beneficial to carry out inelastic neutron scattering experiments at lower temperature because of thermal fluctuations. However, the complex glass phase of the Btype Mn_{3}Sn makes it difficult to use the inelastic neutron scattering technique at low temperature. Meanwhile, we note that both types are identical in terms of the crystal and magnetic structures at room temperature,^{9,18,20,21} and it is also known that the phase transition at 220–270 K in the Atype Mn_{3}Sn is not accompanied by any structural transition.^{22} Thus, we can gain insight into the magnetic Hamiltonian of Mn_{3}Sn at room temperature by measuring the Atype Mn_{3}Sn at low temperature, since there should be, a priori, no significant difference in the intrinsic magnetic Hamiltonian for both types of Mn_{3}Sn samples with the same crystal structure.
The magnetic excitation spectra of the Atype, helically ordered, Mn_{3}Sn were previously studied using inelastic neutron scattering (INS) in the early 1990s with limited success.^{23,24,25} Here, we report the full spin wave spectra of the Atype Mn_{3}Sn over a wide momentumenergy range measured by INS. The full magnetic Hamiltonian based on a local moment model is proposed in order to explain the data completely using a linear spin wave theory. Supported by density functional theory (DFT) calculations, we also present a lowtemperature magnetic structure which involves a helical ordering with some important details that have so far been neglected in previous studies, including its effect on the Weyl points of Mn_{3}Sn.
Results
Magnetic structure
A 120° magnetic structure with negative vector chirality (Γ_{5}) is used within each single a–b plane, which has been confirmed by both theoretical calculations and experiments^{11,12,13,14,15} (Fig. 1b). However, because our inelastic neutron scattering experiment was done at 5 K, extra effects from the lowtemperature phase must be considered rather than just using the room temperature phase. As we noted in the Introduction, there are two types of Mn_{3}Sn: A and B type. By measuring the magnetization of our sample, we observed a clear phase transition near 260 K but failed to observe any increasing magnetization below 50 K (see Supplementary Fig. 1). This result confirms that our sample has the helical order of the Atype at low temperature as shown in Fig. 1a.
There are a few other noteworthy features of this phase. First, the spin directions of two Mn atoms connected by inversion symmetry are no longer parallel in the helical phase. Instead, one of them is rotated by as much as half a turn (≃16.3°) in the a–b plane by the helical ordering (Fig. 1a). This helical ordering has significant effects on Weyl fermions and the corresponding Hall conductivity of Mn_{3}Sn, which will be dealt with in the Discussion section. The helical ordering of the Atype Mn_{3}Sn is rather complicated: a previous neutron diffraction result shows that there are multiple satellite peaks at (1, 0, τ_{1}), (1, 0, τ_{2}), and (1, 0, 3τ_{2}), with \(\tau _1 \simeq 0.07\) and \(\tau _2 \simeq 0.09\) (see ref. ^{17}). The first two peaks correspond to two propagating helices, and the third one is due to the anharmonicity of the second helix. We can safely ignore the contribution by τ_{1} for our spin waves calculations, since the diffraction peak of τ_{1} is much weaker than the other one.^{26} The existence of anharmonicity can also be well explained by our Hamiltonian, which will be shown later.
INS data and magnetic Hamiltonian
Our INS data show the spin wave spectra over the full Brillouin zone, which extends to almost 0.1 eV (Fig. 2a). These coherent spin wave spectra prove to have a welldefined magnetic structure at 5 K. To explain the data, we use the following Hamiltonian:
where Δk is the bond vector corresponding to the k^{th} nearest neighbor. The first two terms denote the usual isotropic exchange interaction and the DM interaction between Mn atom and its k^{th} nearest neighbor, while the last two terms denote a singleion anisotropy related to the local easy axes on the a–b plane and the sixthorder anisotropy term, respectively. The last term originates from crystal field effects, described in terms of Stevens operator equivalents.^{27} The necessity of this last term will be discussed in further detail later in this paper. With this Hamiltonian, we fitted the measured dispersion curves using a linear spin wave theory. Indicated by solid lines layered on Fig. 2a, the fitted result from the Hamiltonian with coupling constants stated in Table 1 is consistent with the data. For our fitting, we used seven isotropic exchange parameters:up to the second inplane nearest neighbor coupling and up to the third interplane nearest neighbor coupling.
We have also taken into account the effects of magnon damping and twins for our spin wave calculations. The calculated spin wave spectra from a linear spin theory yield very large spectral weight above 80 meV (See Supplementary Fig. 3), whereas our data show quite a uniform intensity throughout almost the whole energy range. To resolve this discrepancy, energydependent magnon damping is included to the calculation, which is often necessary to explain the spin waves for metallic magnets^{28,29} (see Supplementary Note 3 for theoretical background). The damping effect we considered is similar to that of refs ^{28,29}. By analyzing the elastic peak positions of the INS data, we found two kinds of twins rotated about ±3.5° from the original lattice orientation. The calculated spin wave spectra with both twins and damping effect included are quite consistent with the data, supporting our Hamiltonian (Figs. 2b and 3).
When fitting the magnetic Hamiltonian, we also performed Monte Carlo simulations before the spin wave calculations to check whether our Hamiltonian stabilizes the magnetic structure mentioned before. Indeed, our Hamiltonian successfully demonstrates a helical ordering as the ground state, with k_{z} of 0.0904. This stabilization is found in our model calculations to arise from exchange frustration, particularly due to the positive J_{6} and the negative J_{4} and J_{5}. Note that the optimized k_{z} value is found to be narrowly focused around 0.0904 ± 0.01, since the resulting k_{z} value varies sharply even if the J values are adjusted only slightly.
Spin model based on DFT calculations
In order to render further credence to the above study of the helical ordering in Mn_{3}Sn in terms of Eq. (1), we performed abinitio calculations of the exchange interactions. Indeed, the dominating interactions fall within a distance of 5–6 Å (see Fig. 4a) as predicted by our model Hamiltonian (Table 1). Though the corresponding values of the isotropic interactions from the fitting to the magnon spectrum and from abinitio calculations differs, there are crucial similarities between the two sets of parameters: (i) strong inplane antiferromagnetic interactions that stabilize the triangular inplane spinstructure and (ii) the abovementioned frustration of outofplane exchange interactions being the source of the helical modulation. This is evidenced in Fig. 4b showing the dispersion E(k_{z}) of the helically modulated Γ_{5} spin state of Mn_{3}Sn as calculated with the abinitio exchange interactions: a minimum of E(k_{z}) emerges at k_{z} = 0.086(2π/c) (blue curve in Fig. 4b), in very good agreement with the desired value of 0.0904.
DM interaction and singleion anisotropy
As shown in Table 1, we applied the DM interaction on the nearest inplane neighbor coupling (J_{2} and J_{3}) with its DM vector being parallel to the \(\hat z\) axis (with the direction from site i to site j defined counterclockwise in the triangle formed by J_{2}). The most important role of this term is to stabilize the magnetic structure with negative vector chirality (Γ_{5}) since this term lowers the energy of that structure.^{12,13,14} It also serves as an effective easyplane anisotropy, stabilizing the structure in which the spins lie in the a–b plane. For these reasons, we assumed that the easyplane anisotropy effect purely comes from our DM interaction term, rather than a singleion anisotropy term with its easy plane perpendicular to the caxis, similar to a recent theoretical study.^{30}
To explore the effect of DM interactions more quantitatively, we calculated the DM interactions in the system using relativistic DFT.^{31} The dominating zcomponents of the DM vectors are smaller at least by two orders of magnitude than the leading isotropic interactions (Fig. 4a), consistent with our fitting results (see Table 1). When including the DM interactions to the calculation of E(k_{z}), we observe that the curve is shifted downwards (at k_{z} = 0 by about 3 meV) stabilizing the Γ_{5} spinstate against the Γ_{3} state with different chirality (red line in Fig. 4b). As the DM energy E_{DM}(k_{z}) monotonously increases with k_{z} (inset of Fig. 4b), the outofplane DM interactions somewhat destabilize the helical ordering but this effect is marginal. Surprisingly, E_{DM}(k_{z}) shows a quadratic dependence for small k_{z}, in contrast to the usual linear dependence of spinspirals with a ferromagnetic order normal to the propagation. Analytic calculations with nearest neighbor outofplane DM vectors indeed show that \({E}_{{\mathrm{DM}}}\left( {{k}_{z}} \right) \simeq {\mathrm{cos}}\left( {{k}_{z}{c/2}} \right)\), which is fully confirmed numerically (inset of Fig. 4b).
When it comes to an analysis of the DM interaction in the data, the low energy dynamics of the spin wave is important since the DM interaction is relatively small and has little influence on the spectra in Fig. 2 (ref. ^{32}). Figure 5 shows the low energy INS data obtained from the incident neutron energies relatively lower than in Fig. 2, and the corresponding simulation results. The size of DM interaction is determined through implementing a gradually increasing mode at an energy value around 16 meV (see inset of Fig. 5d, indicated as a red arrow near 16 meV) which is also seen in the INS data of the previous study (ref. ^{25}). Meanwhile, the result from our relativistic DFT gives the magnitude of the inplane nearest neighbor DM interaction as 0.17 meV, the same order of magnitude as the fitted values.
We also found singleion anisotropy to be as crucial as the DM interaction in explaining several microscopic effects like an energy gap (~5 meV) observed at the magnetic zone center (Fig. 5a). It is already known that neither the ordinary easyaxis anisotropy term (the third term in Eq. (1)) nor the fourthorder anisotropy term can produce the energy gap since the effect of these terms coming from each Mn site cancel out.^{25,27} Considering symmetrically allowed twoion anisotropy is also unnecessary since this term would destroy the subtle spin canting observed experimentally^{13} (see Supplementary Note 4 for more details). Thus, we applied the sixthorder anisotropy term (the fourth term in Eq. (1)), and calculated spin waves near the magnetic zone center (Fig. 5c). For the details of this calculation, see Supplementary Note 4. The calculation result clearly reproduces the energy gap at the C point (Fig. 5c).
Moreover, the singleion anisotropy term related to the easy axes (the third term in Eq. (1)) can explain the anharmonicity of the helical order. As noted before, a magnetic Bragg peak also exists at (1, 0, 3τ_{2}) in addition to the primary satellite peak at (1, 0, τ_{2}).^{17} In terms of Fourier components, this can be interpreted as the thirdorder anharmonicity of the helical ordering; some part of which shows a tendency to rotate three times the normal angle per unit cell. However, finding a corresponding magnetic structure analytically is very difficult. Instead, we performed a Monte Carlo simulation with our Hamiltonian to obtain the energyminimized magnetic structure, and calculated the structure factor along the (1 0 L) line (See Methods). Interestingly enough, the simulation results show that the easyaxis anisotropy term clearly produces the thirdorder anharmonicity (Fig. 5e), which is also consistent with the data from the previous study (ref. ^{17}).
Discussion
It is worth pointing out the key difference between our data and the previous reported results, extending the comparison with our experimental results. Supported by our DFT calculations, taking exchange interactions up to the third nearest neighbor into account in our Hamiltonian is consistent with that in the previous INS study of Mn_{3}Sn (ref. ^{25}). However, the corresponding values do differ; especially for the bonds with the almost same bond length but different symmetry. For example, J_{4} should be almost half the value of J_{5} to fit the data, while ref. ^{25} assumed them as identical. We attribute the difference between J_{4} and J_{5} to a different symmetry due to the different Sn environments (see Supplementary Fig. 4) and the same feature holds also for J_{2} and J_{3}. Our DFT calculation indeed demonstrates the difference between J_{4} and J_{5} as well, giving multiple values for the bonds with the same bond length (see supplementary Fig. 5). Also, our fitting parameters show similar oscillatory behavior to that of the parameters obtained from DFT calculation of Mn_{3}Ir (refs ^{31,33}), which has a structure comparable to that of Mn_{3}Sn (See Supplementary Fig. 5). However, it is to be noted that both the parameters in Table 1 and those from the DFT calculations do not follow the Ruderman–Kittel– Kasuya–Yosida (RKKY) exchange curve. The discrepancy between those and the RKKY curve may come from the nonsphericity of Fermi surfaces and the multiband contributions, which are not negligible for Mn_{3}Sn (ref. ^{34}). Note also that in real metals, an RKKYtype distance dependence of the exchange interaction applies only in the asymptotic limit. Meanwhile, our Hamiltonian is also similar to that used in the recent theoretical paper about Mn_{3}Sn (ref. ^{30}) in terms of anisotropic contributions, except that we have additionally considered sixthorder anisotropy.
Given that the intrinsic magnetic Hamiltonian of the Atype and the Btype Mn_{3}Sn cannot differ greatly from each other at room temperature, and the intrinsic Hamiltonian of the Atype Mn_{3}Sn does not change significantly with temperature^{22} (see Supplementary Fig. 6), our work would be reasonably applicable to the intrinsic Hamiltonian of the Btype Mn_{3}Sn. Nevertheless, we would like to note that performing INS for the Btype Mn_{3}Sn will be definitely interesting to do. The distinct difference between the two types is the amount of excess Mn atoms occupying the 2c sites (Sn sites), which is indeed responsible for the emergence of the spin glass phase near 50 K rather than the helical ordering in the Btype Mn_{3}Sn.^{20} When the temperature is much higher than 50 K, the effect of these excess Mn atoms is marginal, and we expect the Btype Mn_{3}Sn to share much of what we found in the Atype Mn_{3}Sn since the helical ordering through the caxis has only a small effect on the inplane excitation spectra. When temperature is getting closer to 50 K, however, the whole excitation spectra will change due to the different spin configuration in the glass phase of the Btype Mn_{3}Sn. Also, spin wave stiffness and magnon lifetime would decrease, and some quasielastic peaks may appear, as observed in Fe_{x}Cr_{1−x} alloy system^{35} where the glass phase emerges under the situation similar to Mn_{3}Sn. Yet, this effect should be less dramatic in Mn_{3}Sn, since it partially retains the ferromagnetic longrange order in the glass phase.
The change of Weyl points and anomalous Hall conductivity in the lowtemperature phase with the helical ordering is quite an interesting point. Surprisingly, a recent study has shown that the anomalous Hall effect from the Berry curvature of Weyl points almost disappears in the lowT helical ordering phase of the Atype Mn_{3}Sn (ref. ^{21}). The simplest way to explain this change is to look at the symmetry of the magnetic structure. Due to the presence of nearly exact twofold screw symmetry with 5.5c fractional translation along the caxis, the anomalous Hall conductivity (σ_{xz} and σ_{yz}) should be almost zero, consistent with ref. ^{21} Of further interest, the helical ordering also breaks the inversion symmetry of the magnetic structure. If both inversion and timereversal symmetries are broken, Weyl points may disappear by pair annihilation or they might still exist with changed position.^{36,37} To examine the effect of the helical ordering on Weyl points, we performed a simple band calculation using the tight binding method (see Supplementary Fig. 7), following the theoretical model initially proposed for the stacked kagome lattice,^{38} which is equivalent to Mn_{3}Sn. The results clearly show that Weyl points disappear under the helical ordering, mainly due to both significant zone folding and spin–orbit coupling. Thus, although this calculation is too simple to be compared in full detail to the real band structure of Mn_{3}Sn, at least our result shows that the helical ordering is generally a sufficient perturbation to eliminate Weyl points in the electronic band structure and therefore cause the anomalous Hall effect to decrease significantly for the Atype Mn_{3}Sn.
To summarize, we report the spin waves measured over the full Brillouin zone for the intermetallic antiferromagnet Atype Mn_{3}Sn, and present the full spin Hamiltonian. Further supported by DFT calculations, our Hamiltonian produces the helical order with k_{z} = 0.0904 as a magnetic ground state, including subtle anharmonicity. Our subsequent analysis supported by theoretical calculations using a tight binding model reveals that this helical ordering of the Atype Mn_{3}Sn removes the Weyl point, and consequently reduces the anomalous Hall conductivity experimentally seen in the Btype Mn_{3}Sn. Our work offers a rare example of how a topological phenomenon, namely the Weyl point, is removed by introducing a subtle change to the magnetic ground state. Ultimately, it demonstrates an intricate coupling between the electronic and spin degrees of freedom in inducing the exotic topological phase in the Btype Mn_{3}Sn.
Methods
Sample preparation
The Mn_{3}Sn single crystal with mass ~6 g was prepared by the Bridgman method. Pure Mn (99.98%) and Sn (99.999%) pieces in a molar ratio 3:1.1 were placed into a point bottomshaped alumina crucible (99.8%) and sealed in quartz ampoules under 0.3 bar argon atmosphere (99.9999%). The material was first prereacted in a box furnace and then placed in a custommade Bridgeman furnace with vertical temperature gradient. Starting in the hot zone at 1000 C, the sample was slowly moved to the colder bottom zone of the furnace for 4 days. After this process, the ingot was characterized by Laue diffraction and scanning electron microscopy (MIRA, Tescan Cezch Republic) equipped with an energydispersive Xray spectroscopy (EDX) detector (XFlash, Bruker, Germany). A large single crystal of Mn_{3}Sn was obtained from the bottom part of the ingot, while intergrowths of Mn_{3}Sn_{2} were found in the top part; this part of the ingot was removed. Before further measurements, the high quality of the crystal was then confirmed again with an IPXRD Laue Camera (TRYIPYGR, TRY SE). The precise stoichiometric ratio between Mn and Sn was analyzed using an ICPAtomic Emission Spectrometer (OPTIMA 8300, PerkinElmer USA), giving a result of Mn_{2.983}Sn_{1.017}. Magnetization was also measured to check the bulk properties (MPMS3 EverCool, Quantum Design USA). The results were consistent with the previously reported results (see Supplementary Fig. 1).
Inelastic neutron scattering experiment
INS experiment was done on the single crystal using the MERLIN timeofflight spectrometer at ISIS, UK.^{39,40} We used highly pure Al sample holder with proper Cd shielding to minimize unnecessary background signals. The measurements were performed at 5 K. By using the repetitionratemultiplication (RRM) technique,^{41} we could collect data simultaneously with various incident neutron energies (23, 42, and 100 meV, with chopper frequency of 400 Hz) in order to obtain the wide energymomentum spectra with high quality. To cover the full spin wave spectra, the measurement with an incident neutron energy of 200 meV was done using a sloppy chopper (with chopper frequency of 500 Hz) in a single E_{i} mode. Following standard procedures, timeindependent background was removed from the raw data using the data collected between 15,000 μs and 18,000 μs, with the data corrections to compensate for the detector efficiency.^{42} All data were symmetrized while conserving the lattice symmetry to improve statistics. Then, we properly combined all the data from different neutron incident energies to obtain uniformly fine quality over a wide energy and momentum range. After the experiment, we analyzed the data using the Horace software.^{43}
Theoretical calculations
We used spinW to calculate theoretical spin wave spectra, which is based on a linear spin wave theory using the method of diagonalizing the magnetic Hamiltonian by applying the Holstein–Primakoff transformation after local coordinate rotation.^{44} For the simulation showing the emergence of anharmonicity in the helical ordering, we set a supercell of [1 1 6600] size and executed the energyminimizing algorithm built in spinW to obtain the corresponding magnetic structure. Omitting some constants, the following wellknown equation is used to calculate the structure factor.
where F(q) and \({p}_{{\lambda }_{k}}\) denote the magnetic form factor and polarization, respectively. The damping effect is also included by an additional convolution with a Lorentzian function in addition to the Gaussian convolution related to the limited instrumental resolution. The instrumental resolution was precisely determined by the MantidPlot program, with accurate consideration of the beamline’s specifications and chopper frequency.^{45} For the Lorentzian convolution, we assumed that the amount of damping depends only on magnon energy. Dependence of its half width at half maximum (Γ) on spin wave energy (E) was chosen as the following, where E is a variable with a step of 0.1 meV: Γ = 1 meV for energies less than 40 meV, Γ = E/64 for E = 40.1 to 85 meV, and Γ = E/26 for E = 85.1 to 110 meV. Those values are determined by fitting the calculated intensity with the data.
Scalarrelativistic abinitio calculations were carried out using Green’s function approach based on the linearized muffintin orbital (LMTO) method in the atomic sphere approximation,^{46} as implemented in the Questaal code (Questaal Electronic Structure Package, https://www.questaal.org). To compensate for the underestimation of the exchange splitting in itinerant Mn compounds typical for the local density approximation (LDA), we employed the LDA+U method with double counting correction in the fully local limit^{47} using the parameters U = 1.0 eV and J = 0.5 eV. The isotropic exchange interactions were then calculated in terms of the method of infinitesimal rotations^{48} from a ferromagnetic state of reference. The DM interactions were obtained by exploiting the spincluster expansion technique as implemented in combination with the relativistic scheme of Disordered Local Moments within the screened Korringa–Kohn–Rostoker Green’s function approach.^{30}
Data availability
All relevant data that support the findings of this study are available from the corresponding author on request.
References
Hasan, M. Z. & Kane, C. L. Colloquium: topological insulators. Rev. Mod. Phys. 82, 3045–3067 (2010).
Kübler, J. & Felser, C. Noncollinear antiferromagnets and the anomalous Hall effect. EPL (Europhys. Lett.) 108, 67001 (2014).
Chen, H., Niu, Q. & MacDonald, A. H. Anomalous Hall effect arising from noncollinear antiferromagnetism. Phys. Rev. Lett. 112, 017205 (2014).
Nakatsuji, S., Kiyohara, N. & Higo, T. Large anomalous Hall effect in a noncollinear antiferromagnet at room temperature. Nature 527, 212–215 (2015).
Zhang, Y. et al. Strong anisotropic anomalous Hall effect and spin Hall effect in the chiral antiferromagnetic compounds Mn_{3} X (X=Ge, Sn, Ga, Ir, Rh, and Pt). Phys. Rev. B 95, 075128 (2017).
Ikhlas, M. et al. Large anomalous Nernst effect at room temperature in a chiral antiferromagnet. Nat. Phys. 13, 1085–1090 (2017).
Li, X. et al. Anomalous Nernst and RighiLeduc effects in Mn_{3}Sn: Berry curvature and entropy flow. Phys. Rev. Lett. 119, 056601 (2017).
Guo, G.Y. & Wang, T.C. Large anomalous Nernst and spin Nernst effects in the noncollinear antiferromagnets Mn_{3} X (X=Sn; Ge; Ga). Phys. Rev. B 96, 224415 (2017).
Kuroda, K. et al. Evidence for magnetic Weyl fermions in a correlated metal. Nat. Mater. 16, 1090–1095 (2017).
Zelezny, J., Zhang, Y., Felser, C. & Yan, B. Spinpolarized current in noncollinear antiferromagnets. Phys. Rev. Lett. 119, 187204 (2017).
Tomiyoshi, S. & Yamaguchi, Y. Magnetic structure and weak ferromagnetism of Mn_{3}Sn studied by polarized neutron diffraction. J. Phys. Soc. Jpn. 51, 2478–2486 (1982).
Sticht, J., Höck, K. H. & Kübler, J. Noncollinear itinerant magnetism: the case of Mn_{3}Sn. J. Phys. Condens. Matter 1, 8155–8170 (1989).
Nagamiya, T., Tomiyoshi, S. & Yamaguchi, Y. Triangular spin configuration and weak ferromagnetism of Mn_{3}Sn and Mn_{3}Ge. Solid State Commun. 42, 385–388 (1982).
Brown, P. J., Nunez, V., Tasset, F., Forsyth, J. B. & Radhakrishna, P. Determination of the magnetic structure of Mn_{3}Sn using generalized neutron polarization analysis. J. Phys. Condens. Matter 2, 9409–9422 (1990).
Sandratskii, L. M. & Kübler, J. Role of orbital polarization in weak ferromagnetism. Phys. Rev. Lett. 76, 4963–4966 (1996).
Krén, E., Paitz, J., Zimmer, G. & Zsoldos, É. Study of the magnetic phase transformation in the Mn_{3}Sn phase. Phys. B + C 80, 226–230 (1975).
Cable, J. W., Wakabayashi, N. & Radhakrishna, P. A neutron study of the magnetic structure of Mn_{3}Sn. Solid State Commun. 88, 161–166 (1993).
Duan, T. F. et al. Magnetic anisotropy of singlecrystalline Mn_{3}Sn in triangular and helixphase states. Appl. Phys. Lett. 107, 082403 (2015).
Ohmori, H., Tomiyoshi, S., Yamauchi, H. & Yamamoto, H. Spin structure and weak ferromagnetism of Mn_{3}Sn. J. Magn. Magn. Mater. 70, 249–251 (1987).
Feng, W. J. et al. Glassy ferromagnetism in Ni_{3}Sntype Mn_{3}._{1}Sn_{0.9}. Phys. Rev. B 73, 205105 (2006).
Sung, N. H., Ronning, F., Thompson, J. D. & Bauer, E. D. Magnetic phase dependence of the anomalous Hall effect in Mn_{3}Sn single crystals. Appl. Phys. Lett. 112, 132406 (2018).
Zimmer, G. J. & Kren, E. Investigation of the magnetic phase transformation in Mn_{3}Sn. AIP Conf. Proc. 5, 513–516 (1972).
Radhakrishna, P. & Tomiyoshi, S. A neutron scattering study of the magnetic excitations in a triangular itinerant antiferromagnet, Mn_{3}Sn. J. Phys. Condens. Matter 3, 2523–2527 (1991).
Radhakrishna, P. & Cable, J. W. Magnetic excitations in the triangular antiferromagnet Mn_{3}Sn. J. Magn. Magn. Mater. 104–107, 1065–1066 (1992).
Cable, J. W., Wakabayashi, N. & Radhakrishna, P. Magnetic excitations in the triangular antiferromagnets Mn_{3}Sn and Mn_{3}Ge. Phys. Rev. B 48, 6159–6166 (1993).
Cable, J. W., Wakabayashi, N. & Radhakrishna, P. Structure of the Modulated Magnetic Phase of Mn _{3} Sn. United States https://www.osti.gov/servlets/purl/10104409 (1993).
J. Jensen & A. R. Mackintosh, RareEarth Magnetism, Ch. 5 (Clarendon, Oxford, 1991)
Diallo, S. O. et al. Itinerant magnetic excitations in antiferromagnetic CaFe_{2}As_{2}. Phys. Rev. Lett. 102, 187206 (2009).
Zhao, J. et al. Spin waves and magnetic exchange interactions in CaFe_{2}As_{2}. Nat. Phys. 5, 555–560 (2009).
Liu, J. & Balents, L. Anomalous Hall effect and topological defects in antiferromagnetic Weyl semimetals: Mn_{3}Sn/Ge. Phys. Rev. Lett. 119, 087202 (2017).
Szunyogh, L., Udvardi, L., Jackson, J., Nowak, U. & Chantrell, R. Atomistic spin model based on a spincluster expansion technique: application to the IrMn_{3} interface. Phys. Rev. B 83, 204401 (2011).
Jeong, J. et al. Temperaturedependent interplay of DzyaloshinskiiMoriya interaction and singleion anisotropy in multiferroic BiFeO_{3}. Phys. Rev. Lett. 113, 107202 (2014).
Szunyogh, L., Lazarovits, B., Udvardi, L., Jackson, J. & Nowak, U. Giant magnetic anisotropy of the bulk antiferromagnets IrMn and IrMn_{3} from first principles. Phys. Rev. B 79, 020403 (2009).
Alireza, A., Peter, T. & Ilya, E. Evolution of the multiband Ruderman–Kittel–Kasuya–Yosida interaction: application to iron pnictides and chalcogenides. New J. Phys. 15, 033034 (2013).
Fincher, C. R., Shapiro, S. M., Palumbo, A. H. & Parks, R. D. Spinwave evolution crossing from the ferromagnetic to spinglass regime of Fe_{x}Cr_{1−x}. Phys. Rev. Lett. 45, 474–477 (1980).
Zyuzin, A. A., Wu, S. & Burkov, A. A. Weyl semimetal with broken time reversal and inversion symmetries. Phys. Rev. B 85, 165110 (2012).
Chang, G. et al. Magnetic and noncentrosymmetric Weyl fermion semimetals in the RAlGe family of compounds (R=rare earth). Phys. Rev. B 97, 041104 (2018).
Ito, N. & Nomura, K. Anomalous Hall effect and spontaneous orbital magnetization in antiferromagnetic Weyl metal. J. Phys. Soc. Jpn 86, 063703 (2017).
Bewley, R., Guidi, T. & Bennington, S. MERLIN: a high count rate chopper spectrometer at ISIS. Not. Neutron. Luce Sincrotron. 14, 22–27 (2009).
Park, J.G. et al. 1600054, STFC ISIS Facility https://doi.org/10.5286/ISIS.E.79298815 (2016).
Russina, M. & Mezei, F. First implementation of repetition rate multiplication in neutron spectroscopy. Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrometers Detect. Assoc. Equip. 604, 624–631 (2009).
Windsor, C. G. Pulsed Neutron Scattering (Taylor & Francis Ltd, London, 1981).
Ewings, R. A. et al. Horace: Software for the analysis of data from single crystal spectroscopy experiments at timeofflight neutron instruments. Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrometers Detect. Assoc. Equip. 834, 132–142 (2016).
Toth, S. & Lake, B. Linear spin wave theory for singleQ incommensurate magnetic structures. J. Phys. Condens. Matter 27, 166002 (2015).
Arnold, O. et al. Mantid—Data analysis and visualization package for neutron scattering and μSR experiments. Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrometers Detect. Assoc. Equip. 764, 156–166 (2014).
Andersen, O. K. & Jepsen, O. Explicit, firstprinciples tightbinding theory. Phys. Rev. Lett. 53, 2571–2574 (1984).
Liechtenstein, A. I., Anisimov, V. I. & Zaanen, J. Densityfunctional theory and strong interactions: orbital ordering in MottHubbard insulators. Phys. Rev. B 52, R5467–R5470 (1995).
Liechtenstein, A. I., Katsnelson, M. I., Antropov, V. P. & Gubanov, V. A. Local spin density functional approach to the theory of exchange interactions in ferromagnetic metals and alloys. J. Magn. Magn. Mater. 67, 65–74 (1987).
Acknowledgements
We thank Kisoo Park, Jon Leiner, Taehun Kim, and L. Udvardi for helpful discussions, and M. Valiska and C. Draser for technical assistance. The work at the IBS CCES was supported by the research program of Institute for Basic Science (IBSR009G1). The work at the Materials Growth and Measurement Laboratory MGML was supported by the grant no. LM2011025 of the Ministry of Education, Youth and sports of Czech Republic. A.D. and L.S. acknowledge support provided by the BMENanotechnology FIKP grant of EMMI (BME FIKPNAT) and by the Hungarian National Scientific Research Fund (NKFIH) under project no. K115575 and PD124380. J.J. acknowledges support from the UK’s EPSRC under a service level agreement with STFC (core support for the CCP9 project).
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J.G.P. initiated and supervised the project. K.U. and V.S. grew the single crystal and J.O., H.L.K., H.C., and P.P. measured bulk properties. J.O., K.U., H.C.W., and D.A. conducted the inelastic neutron scattering experiments. P.P. and J.O. made the spin wave calculations, and P.P. and K.H.L. did tight binding calculations. DFT calculations were carried out by J.J., L.S., and A.D. All authors contributed to the discussion and writing the paper.
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Park, P., Oh, J., Uhlířová, K. et al. Magnetic excitations in noncollinear antiferromagnetic Weyl semimetal Mn_{3}Sn. npj Quant Mater 3, 63 (2018). https://doi.org/10.1038/s4153501801379
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DOI: https://doi.org/10.1038/s4153501801379
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