Session E05: Anomalous Transverse Transport in Mn3X Non-collinear Antiferromagnets

Large Transverse Responses at Room Temperature in the Weyl Antiferromagnets Mn3X

Recent observations of large anomalous Hall effects in the non-collinear antiferromagnets Mn₃X have opened new venues to study the effects of Berry curvature and their potential applications at room temperature. Our detailed experiments on the anomalous Hall and Nernst effects have clarified that large Berry curvature exists nearby the Fermi energy in the momentum space [1,2,3]. Besides, our measurements of ARPES, magnetoresistance, and their comparison with theory have provided strong evidence for the magnetic Weyl fermions in the ferro-ordered state of magnetic octupoles in Mn₃Sn [4]. In addition, we find that this Weyl antiferromagnet exhibits an anomalous type of spin Hall effect (magnetic spin Hall effect). If time permits, perspective for applications will be also discussed. This presentation is based on the collaboration with Takahiro Tomita, Tomoya Higo, Muhammad, Ikhlas, YoshiChika Otani, Motoi Kimata, Kouta Kondo, Kenta Kuroda, Takeshi Kondo, Shik Shin, Pallab Goswami, Hua Chen, Allan MacDonald, Ryotaro Arita, Michito Suzuki, Takashi Koretsune.

[1] S. Nakatsuji, N. Kiyohara and T. Higo, Nature 527, 212 (2015).
[2] N. Kiyohara, T. Tomita, S. Nakatsuji, Phys. Rev. Applied 5, 064009 (2016).
[3] M. Ikhlas, T. Tomita et al., Nature Physics 13, 1085 (2017).
[4] K. Kuroda, T. Tomita et al., Nature Materials 16, 1090 (2017).
  

Magnetic anti-skyrmions and triangular antiferromagnetism in Mn3X and Mn2XY compounds

It is well established that the anomalous Hall effect displayed by a ferromagnet scales with its magnetization. Therefore, an antiferromagnet that has no net magnetization should exhibit no anomalous Hall effect. We show that, rather, the non-collinear triangular antiferromagnet hexagonal Mn3Ge exhibits a large anomalous Hall effect comparable to that of ferromagnetic metals¹. Our theoretical calculations demonstrate that the Hall effect in Mn3Ge originates from a nonvanishing Berry curvature that arises from the chiral spin structure, and that also results in a large spin Hall effect comparable to that of platinum. In the chiral triangular antiferromagnet cubic Mn3Ir, we observe a large spin Hall effect but no evidence for any anomalous Hall effect². We speculate that this is because the sign of the spin Hall effect is independent of the chirality of the triangular antiferromagnet structure but that the sign of the anomalous Hall effect changes sign with the chirality of the antiferromagnetic structure. Finally, we discuss the discovery of anti-skyrmions in several tetragonal inverse Heusler compounds using Lorentz transmission electron microscopy³ and variable temperature magnetic force microscopy. We discuss the topological Hall effect that we observe in these systems.

1 Nayak, A. K. et al. Large anomalous Hall effect driven by non-vanishing Berry curvature in non-collinear antiferromagnet Mn₃Ge Sci. Adv. 2, e1501870, (2016).
2 Zhang, W. et al. Giant facet-dependent spin-orbit torque and spin Hall conductivity in the triangular antiferromagnet IrMn₃. Sci. Adv. 2, e1600759, (2016).
3 Nayak, A. K. et al. Magnetic antiskyrmions above room temperature in tetragonal Heusler materials. Nature 548, 561-566, (2017).   

Anomalous Nernst and Righi-Leduc Effects in Mn3Sn:Berry Curvature and Entropy Flow

In addition to the ordinary Hall effect, the transverse electric field generated by a longitudinal charge current in the presence of a magnetic field. In ferromagnetic solids, there is an additional component to the transverse current generated by an electric field in presence of magnetic field. This Anomalous Hall effect (AHE) is a result of the Berry curvature of the Bloch waves. Recently, following theoretical propositions [1], a large AHE has been observes in noncollinear antiferromagnets Mn₃Sn and Mn₃Ge [2-4].
We will present a study of thermoelectric and thermal response in Mn₃Sn beyond electric measurement. We measured Nernst and Righi-Leduc (or thermal hall) effects, and compared their anomalous components to their Hall counterpart. We found that the anomalous Nernst and Hall conductivities are strongly temperature-dependent and their ratio is 15 µV/K at room temperature and peaks to 50µV/K, close to k_B/e(= 86 µV/K) . We found that the thermal and electrical Hall conductivities respect the Wiedemann-Franz law, and in contrast to conventional ferromagnets, the anomalous Lorenz number remains close to the Sommerfeld number over the whole temperature range of study [5].
1.H. Chen, Q. Niu, and A. MacDonald, Phys. Rev. Lett. 112, 017205 (2014); J. Kübler and C. Felser, Europhys. Lett. 108, 67001 (2014).
2.S. Nakatsuji, N. Kiyohara, and T. Higo, Nature 527, 212 (2015).
3.A. K. Nayak et al., Sci. Adv. 2: e1501870 (2016).
4.N. Kiyohara, T. Tomita, and S. Nakatsuji, Phys. Rev. Appl. 5, 064009 (2016).
5.X. Li, L. Xu, L. Ding, J. Wang, M. Shen, X. Lu, Z. Zhu and K . Behnia, Phys. Rev. Lett. 119, 056601 (2017)   

Cluster multipole theory for anomalous Hall effect in antiferromagnets

Recently, it has been theoretically and experimentally shown that antiferromagnets such as Mn₃Ir, Mn₃Sn and Mn₃Ge exhibit extremely large anomalous Hall effect (AHE) [1,2,3,4,5], anomalous Nernst effect (ANE) [6,7], and magneto-optical Kerr effect (MOKE) [8]. While AHE/ANE/MOKE in ferromagnets usually scale with the magnetization of the system, it is a non-trivial problem which macroscopic order parameter determines these effects in antiferromagnets. In this talk, we introduce a new order parameter, which we call cluster multipole (CMP) moment, and show that CMP is useful to classify the magnetic structure and discuss the physical properties of antiferromagnets. In particular, we demonstrate that AHE in Mn₃Sn and that in elemental Fe can be described in the unified scheme using CMP [9]. We also discuss how CMP is useful for constructing database for magnets and designing functional antiferromagnets.

[1] H. Chen, Q. Niu, and A. MacDonald, Phys. Rev. Lett. 112, 017205 (2014).
[2] J. Kuebler and C. Felser, Europhys. Lett. 108, 67001 (2014).
[3] S. Nakatsuji, N. Kiyohara, and T. Higo, Nature 527, 212 (2015).
[4] N. Kiyohara et al., Phys. Rev. Applied 5, 064009 (2016).
[5] A. K. Nayak et al., Sci. Adv. 2, e1501870 (2016).
[6] M. Ikhlas, T. Tomita, T. Koretsune, M.-T. Suzuki, D. Nishio-Hamane, R. Arita, Y. Otani, S. Nakatsuji, Nature Physics (2017).
[7] X. Li et al., Phys. Rev. Lett. 119, 056601 (2017).
[8] W. Feng et al., Phys. Rev. B 92, 144426 (2015).
[9] M.-T. Suzuki, T. Koretsune, M. Ochi and R. Arita, Phys. Rev. B. 95, 094406 (2017).   

Interplay of transport and domain walls in nodal semimetals

Topological band structures are well-known to induce exotic bound states at their surfaces. With this in mind, it is natural to expect that magnetic domain walls can play a similar role *inside* a material. Here we report on theoretical studies of the transport in magnetic Weyl systems with domain walls, and how this leads to a number of physical effects. The applications to experiments on Mn3Sn and CeAlGe will be discussed.