Session A72: Magnetic Topological Materials I

Nonlinear optical domain imaging in magnetic Weyl semimetal CeAlSi

Topological magnets recently emerged as a new class of topological quantum materials. The interaction between topologically protected electronic states and long-range magnetic order not only gives rise to fundamentally new phenomena, but may also provide novel pathways for the electrical manipulation of magnetization with immediate implications for magnetoelectric devices. Exploring the intricate coupling between topology and magnetism, however, hinges critically on the ability to visualize and control the magnetic state. Here, we use optical second-harmonic spectroscopy and imaging to spatially resolve the complex multi-domain state in the magnetic Weyl semimetal CeAlSi. Second-harmonic generation (SHG) is a highly symmetry sensitive nonlinear optical process that couples linearly to the magnetic order parameter. Using SHG spectroscopy we identify components that are selectively sensitive either to the noncentrosymmetric crystal structure or to the magnetic order. Interference between the individual components allows us to uniquely distinguish all four magnetic domain states. Our results enable the further investigation of the intricate interplay between topology and magnetism.   

Observation of a phase transition within the domain walls of ferromagnetic Co3Sn2S2

The ferromagnetic phase of Co₃Sn₂S₂ is widely considered to be a topological Weyl semimetal, with evidence for momentum-space monopoles of Berry curvature from transportand spectroscopic probes. As the bandstructure is highly sensitive to the magnetic order, attention has focused on anomalies in magnetization, susceptibility and transport measurements that are seen well below the Curie temperature, leading to speculation that a “hidden” phase coexists with ferromagnetism. Here we report spatially-resolved measurements by Kerr effect microscopy that identify this phase. We find that the anomalies coincide with a deep minimum in domain wall (DW) mobility, indicating a crossover between two regimes of DW propagation. We demonstrate that this crossover is a manifestation of a 2D phase transition that occurs within the DW, in which the magnetization texture changes from continuous rotation to unidirectional variation. We propose that the existence of this 2D transition deep within the ferromagnetic state of the bulk is a consequence of a giant quality factor for magnetocrystalline anisotropy unique to this compound. This work broadens the horizon of the conventional binary classification of DWs into Bloch and Néel walls, and suggests new strategies for manipulation of domain walls and their role in electron and spin transport.   

Magnetic order, unusual exchange couplings and intermediate temperature electronic band structures in Co3Sn2S2

Co₃Sn₂S₂ is a newly discovered magnetic Weyl semimetal with a kagome lattice of cobalt ions and has triggered intense interest for rich fantastic phenomena. It was proposed that it may exhibit a coexistence of ferromagnetic order and antiferromagnetic order below T_C≈ 175 K, followed by a pure ferromagnetic order below T_A≈ 135 K. We employed half-polarized neutron technology and confirmed the ferromagnetic order along the c axis below T_C≈ 175 K. Inelastic neutron scattering revealed highly anisotropic magnon dispersions and linewidths below T_C, and paramagnetic excitations above T_C. The spin-wave spectra in the ferromagnetic order is dominated by ferromagnetic third-neighbor “across-hexagon” J_d coupling with a weak frustrated next-nearest-neighbor bond.  Our density functional theory calculations reveal that both the symmetry-allowed 120° antiferromagnetic orders support Weyl points in the intermediate temperature region T_AT_C, with distinct numbers and the locations of Weyl points. Our study unveils the unusual magnetic coupling and the interplay between various magnetic orders and electronic band topology in Co₃Sn₂S₂.   

Optical Characterization of Nonreciprocal Plasmons in a Magnetic Weyl Semimetal

The discovery of topological semimetals with nontrivial electronic surface states has caused an explosion of interest in the development of optical techniques to sample such states. In particular, the recent discovery of magnetic Weyl semimetals, which support non-closed topological Fermi arcs at their surfaces, has attracted attention to the time-reversal symmetry breaking properties of the resultant fermions, which are present even in the absence of an external magnetic field. Here we measure the full dielectric tensor of Co­₃Sn₂S₂, a Co-based half-metallic ferromagnet that supports Weyl points. Through identification of an epsilon-near-zero point at ~0.1 eV, we identify the MIR regime supporting nonreciprocal plasmons. We verify that the crystal is in a Voigt configuration with respect to the optical plane of incidence and its intrinsic magnetic moment through X-ray crystallographic studies of the exposed (110) facet. We also theoretically model the microscopic subsurface optical fields and compare them to the experimental characterization of the dielectric function and the angle-resolved reflection characteristic. Our results demonstrate the utility of magnetic Weyl semimetals in magneto-optical device configurations.   

Magnetic states in a triangular antiferromagnet – a model for CoNb3S6

CoNb₃S₆ (CNS) is an antiferromagnet (AFM) with triangular planes of Co intercalated with NbS₂ planes. The crystallographic unit cell breaks inversion symmetry, which allows for a Dzyaloshinskii-Moriya interaction. It was recently shown [1, 2] that CNS exhibits a large anomalous Hall effect (AHE) and a very small uniform magnetic out-of-plane moment below the Neel temperature; the magnetic susceptibility was also found to be very small, both in-plane and out-of-plane. The magnetic structure cannot be that of a collinear AFM because a collinear AFM with a small out-of-plane moment cannot yield a large AHE. We used finite-temperature atomistic modeling as well as a reduced 2D model to study the zero-temperature phase diagram. Increasing in-plane next-nearest neighbor interactions leads, together with a biquadratic interaction, to a transition from a single-wavevector spin spiral to a non-collinear AFM. While the former cannot yield a nonzero AHE, the latter does, and this is confirmed by first-principle calculations [3]. We will discuss our model and results in the context of experimental and first-principle electronic structure calculations.

[1] N. Ghimire et al, Nature Comm. 9, 1 (2018).

[2] G. Tenasini et al., Phys. Rev. Research 2, 023051 (2020)

[3] H.  Park, O. Heinonen, and I. Martin, arXiv:2110:03029   

Magnetic Domains in Rare-earth Element Weyl Semimetals RAlX (R = Rare-earth Element, X = Si, Ge)

Coupling between magnetic textures and carriers in Weyl semimetals is of a great interest as it can produce effective axial gauge fields and induce excess carriers near the domain walls. We use scanning SQUID microscopy to investigate magnetism in RAlX Weyl semimetals. [1–3]  We report on progress of observing the dynamics of the magnetic domain walls under external magnetic field, which provides a better understanding of the interplay between local magnetic and electronic properties in Weyl systems. Successful manipulation of magnetic domains with magnetic and electric fields in Weyl materials may lead to development of new magnetic quantum devices.

 

[1]     B. Xu, et al., Advanced Quantum Technologies 4, 2000101 (2021).

[2]     B. Xu, et al., ArXiv:2105.14384 [Cond-Mat] (2021).

[3]     H.-Y. Yang, et al., Phys. Rev. B 103, 115143 (2021).   

Topological magneto-elastic excitations in a two dimensional spin-nematic phase

Recent works [1,2] on spin-lattice coupling show that the hybridized magnon-phonon excitations can have non-zero Berry curvature and nontrivial band topology, despite the magnon and phonon systems individually being topologically trivial. In this work, we investigate the coupling between phonons and the nematic magnon degrees of freedom in spin-1 systems. We thoroughly explore the phase diagram of the square lattice bilinear-biquadratic spin-1 Heisenberg model coupled to phonons and external magnetic field, and discover a rich variety of topological phases with relatively high Chern numbers on the topological bands. In addition, we found that the magneto-elastic coupling leads to a robust power-law growth of thermal Hall conductivity (kxy/T) in the low temperature limit, which is a very distinct experimental signature of the phonon-magnon coupling despite kxy/T not being quantized.

Reference:

[1] Gyungchoon Go, et al. "Topological Magnon-Phonon Hybrid Excitations in Two-Dimensional Ferromagnets with Tunable Chern Numbers", Phys. Rev. Lett. 123, 237207 (2019).

[2] Shu Zhang, et al. "SU(3) Topology of Magnon-Phonon Hybridization in 2D Antiferromagnets", Phys. Rev. Lett. 124, 147204 (2020).   

Hard magnet topological semimetals in XPt3compounds with the harmony of Berry curvature

Topological magnetic semimetals, like Co₃Sn₂S₂ and Co₂MnGa, display exotic transport properties, such as large intrinsic anomalous (AHE) due to uncompensated Berry curvature. The highly symmetric XPt₃ compounds display anti-crossing gapped nodal lines, a driving mechanism in the intrinsic Berry curvature Hall effects. Uniquely, these compounds contain two sets of gapped nodal lines that harmoniously dominate the Berry curvature in this complex multi-band system. We calculate a maximum AHE of 1965 S/cm in the CrPt₃ by a state-of-the-art first principle electronic structure. We also find the topological structure and the magnetic interactions are significantly disturbed by strain. We have grown high-quality thin films by magnetron sputtering and measured a robust AHE of 1750 S/cm for different sputtering growth conditions. Additionally, the cubic films display a hard magnetic axis along [111] direction. The facile and scalable fabrication of these materials makes them ideal candidates for integration into topological devices   

Shubnikov-de Haas oscillations in the ferromagnetic Weyl semimetal Co3Sn2S2

We report the observation of Shubnikov-de Haas oscillations in high quality single crystals of the ferromagnetic Weyl semimetal Co₃Sn₂S₂ up to 45 T. The Fermi surfaces resolved in our experiments are three-dimensional and reflect an underlying trigonal crystallographic symmetry. Combined with density functional theoretical calculations, we identify that the majority of the Fermi surfaces in the system -- of both electron and hole nature -- arise from the strong energy dispersion of the mirror-protected nodal loops prior to the inclusion of spin-orbit coupling. We discuss the dependence of the Fermi surface on the ferromagnetic moment orientation. We anticipate our results to shed light on the interplay between the ferromagnetic order and topological electronic states in Co₃Sn₂S₂.   

Engineering Weyl Nodes in Ferrimagnetic Li2Fe3S4

In recent years, Weyl semimetals (WSM) have been intensely studied in view of their electronic transport properties that appear to be interesting for technological applications. These properties rest on low-energy quasi-particle excitations of electrons that mimic features of mass-less Weyl fermions. By means of density-functional calculations, we investigate Li₂Fe₃S₄ with monoclinic structure and nonsymmorphic symmetry. The magnetic ground state of this compound is found to be ferrimagnetic with a total magnetic moment of about 8 μB per unit cell. Our calculations suggest four pairs of Weyl nodes within an energy range of 40 meV around the Fermi level. Considering different magnetization directions, the positions of these nodes are significantly modified. Thus, Li₂Fe₃S₄ is a promising candidate for the tuning of WSM-related transport properties by rotating its direction of magnetization.   

Onset of multifractality in a ferromagnetic nodal line semimetal

The ferromagnet Fe3GeTe2 is predicted to have large Berry curvature due to its classification as a nodal line semi metal. Indeed, it shows an extremely large anomalous Hall effect [1] as well as a large anomalous Nernst angle [2]. We have investigated it using scanning tunneling microscopy (STM) and spectroscopic mapping. We find that it has a sharply suppressed density of states at the Fermi energy at low temperatures. We further find the signature of multifractality in the spatial fluctuations of the density of states that onset with decreasing temperatures and peaks at the Fermi energy. We discuss possible implications our observations have for the nodal line state.

[1] K. Kim et al. Large anomalous Hall current induced by topological nodal lines in a ferromagnetic van der Waals semimetal. Nature Material 17, 794 (2018).

[2] Jinsong Xu et al. Large Anomalous Nernst Effect in a van der Waals Ferromagnet Fe3GeTe2, Nano Letters 19, 8250 (2019).