Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session A72: Magnetic Topological Materials IFocus Recordings Available

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Sponsoring Units: DMP GMAG DCMP Chair: Anup Sakhya Pradhan, University of Central Florida Room: Hyatt Regency Hotel Jackson Park D 
Monday, March 14, 2022 8:00AM  8:36AM 
A72.00001: Signature of interfacial nontrivial spin texture in topological insulator/ferromagnetic insulator heterostructures Invited Speaker: Dhavala Suri

Monday, March 14, 2022 8:36AM  8:48AM 
A72.00002: Nonlinear optical domain imaging in magnetic Weyl semimetal CeAlSi Christian Tzschaschel, JianXiang Qiu, HungYu Yang, Anyuan Gao, YuFei Liu, Damien Bérubé, ShengChin Ho, Houchen Li, Qiong Ma, Fazel Tafti, Suyang Xu Topological magnets recently emerged as a new class of topological quantum materials. The interaction between topologically protected electronic states and longrange 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 secondharmonic spectroscopy and imaging to spatially resolve the complex multidomain state in the magnetic Weyl semimetal CeAlSi. Secondharmonic 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. 
Monday, March 14, 2022 8:48AM  9:00AM 
A72.00003: Observation of a phase transition within the domain walls of ferromagnetic Co_{3}Sn_{2}S_{2} Changmin Lee, Praveen Vir, Kaustuv Manna, Chandra Shekhar, Joel E Moore, Marc A Kastner, Claudia Felser, Joseph Orenstein The ferromagnetic phase of Co_{3}Sn_{2}S_{2} is widely considered to be a topological Weyl semimetal, with evidence for momentumspace 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 spatiallyresolved 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. 
Monday, March 14, 2022 9:00AM  9:12AM 
A72.00004: Magnetic order, unusual exchange couplings and intermediate temperature electronic band structures in Co_{3}Sn_{2}S_{2} Qiang Zhang, Satoshi Okamoto, German D Samolyuk, Matthew B Stone, Alexander I Kolesnikov, Masaaki Matsuda, Vasile O Garlea, Rui Xue, Jiaqiang Yan, Michael A McGuire, David G Mandrus, David A Tennant Co_{3}Sn_{2}S_{2} 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 halfpolarized 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 spinwave spectra in the ferromagnetic order is dominated by ferromagnetic thirdneighbor “acrosshexagon” J_{d }coupling with a weak frustrated nextnearestneighbor bond. Our density functional theory calculations reveal that both the symmetryallowed 120° antiferromagnetic orders support Weyl points in the intermediate temperature region T_{A}T_{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 Co_{3}Sn_{2}S_{2}. 
Monday, March 14, 2022 9:12AM  9:24AM 
A72.00005: Optical Characterization of Nonreciprocal Plasmons in a Magnetic Weyl Semimetal Arun Nagpal, Christopher J Ciccarino, Chandra Shekhar, Claudia Felser, Prineha Narang, Harry Atwater 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 nonclosed topological Fermi arcs at their surfaces, has attracted attention to the timereversal 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 Co_{3}Sn_{2}S_{2}, a Cobased halfmetallic ferromagnet that supports Weyl points. Through identification of an epsilonnearzero 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 Xray 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 angleresolved reflection characteristic. Our results demonstrate the utility of magnetic Weyl semimetals in magnetooptical device configurations. 
Monday, March 14, 2022 9:24AM  9:36AM 
A72.00006: Magnetic states in a triangular antiferromagnet – a model for CoNb_{3}S_{6} Olle Heinonen CoNb_{3}S_{6} (CNS) is an antiferromagnet (AFM) with triangular planes of Co intercalated with NbS_{2} planes. The crystallographic unit cell breaks inversion symmetry, which allows for a DzyaloshinskiiMoriya interaction. It was recently shown [1, 2] that CNS exhibits a large anomalous Hall effect (AHE) and a very small uniform magnetic outofplane moment below the Neel temperature; the magnetic susceptibility was also found to be very small, both inplane and outofplane. The magnetic structure cannot be that of a collinear AFM because a collinear AFM with a small outofplane moment cannot yield a large AHE. We used finitetemperature atomistic modeling as well as a reduced 2D model to study the zerotemperature phase diagram. Increasing inplane nextnearest neighbor interactions leads, together with a biquadratic interaction, to a transition from a singlewavevector spin spiral to a noncollinear AFM. While the former cannot yield a nonzero AHE, the latter does, and this is confirmed by firstprinciple calculations [3]. We will discuss our model and results in the context of experimental and firstprinciple electronic structure calculations. 
Monday, March 14, 2022 9:36AM  9:48AM 
A72.00007: Magnetic Domains in Rareearth Element Weyl Semimetals RAlX (R = Rareearth Element, X = Si, Ge) Bochao Xu, Jacob Franklin, HungYu Yang, Xiaohan Yao, Fazel Tafti, Ilya Sochnikov 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. 
Monday, March 14, 2022 9:48AM  10:00AM 
A72.00008: Topological magnetoelastic excitations in a two dimensional spinnematic phase Shuyi Li, Yan Han, Andriy H Nevidomskyy Recent works [1,2] on spinlattice coupling show that the hybridized magnonphonon excitations can have nonzero 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 spin1 systems. We thoroughly explore the phase diagram of the square lattice bilinearbiquadratic spin1 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 magnetoelastic coupling leads to a robust powerlaw growth of thermal Hall conductivity (kxy/T) in the low temperature limit, which is a very distinct experimental signature of the phononmagnon coupling despite kxy/T not being quantized. 
Monday, March 14, 2022 10:00AM  10:12AM 
A72.00009: Hard magnet topological semimetals in XPt_{3}compounds with the harmony of Berry curvature Jacob Gayles, Bushra Sabir, Anastasios Markou, Yan Sun, Claudia Felser Topological magnetic semimetals, like Co_{3}Sn_{2}S_{2} and Co_{2}MnGa, display exotic transport properties, such as large intrinsic anomalous (AHE) due to uncompensated Berry curvature. The highly symmetric XPt_{3} compounds display anticrossing 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 multiband system. We calculate a maximum AHE of 1965 S/cm in the CrPt_{3} by a stateoftheart first principle electronic structure. We also find the topological structure and the magnetic interactions are significantly disturbed by strain. We have grown highquality 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 
Monday, March 14, 2022 10:12AM  10:24AM 
A72.00010: Shubnikovde Haas oscillations in the ferromagnetic Weyl semimetal Co_{3}Sn_{2}S_{2} Jorge I Facio, Linda Ye, Madhav P Ghimire, Mun K Chan, JhihShih You, David C Bell, Manuel Richter, Jeroen van den Brink, Joseph G Checkelsky We report the observation of Shubnikovde Haas oscillations in high quality single crystals of the ferromagnetic Weyl semimetal Co_{3}Sn_{2}S_{2} up to 45 T. The Fermi surfaces resolved in our experiments are threedimensional 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 mirrorprotected nodal loops prior to the inclusion of spinorbit 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 Co_{3}Sn_{2}S_{2}. 
Monday, March 14, 2022 10:24AM  10:36AM 
A72.00011: Fermi arcs, drumhead states and nodal planes in CoS_{2} Iñigo Robredo Since the discovery of the first topological material 15 years ago, the search for material realizations of novel topological phases has become the driving force of the field. While oftentimes we search for new materials, we forget that well established materials can also display very interesting topological properties. In this work, we revisit CoS_{2}, a metallic ferromagnetic pyrite that has been extensively studied in the literature due to its magnetic properties. We study the topological features of its electronic band structure and identify protected crossings, a 4fold Fermion, Weyl nodes, Nodal lines and Nodal planes. Looking at different surface terminations, we find Fermi arcs stemming from Weyl node projections and drumhead states from Nodal line projections. 
Monday, March 14, 2022 10:36AM  10:48AM 
A72.00012: Engineering Weyl Nodes in Ferrimagnetic Li_{2}Fe_{3}S_{4} Gang B Acharya, Manuel Richter, Madhav P Ghimire 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 lowenergy quasiparticle excitations of electrons that mimic features of massless Weyl fermions. By means of densityfunctional calculations, we investigate Li_{2}Fe_{3}S_{4} 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, Li_{2}Fe_{3}S_{4} is a promising candidate for the tuning of WSMrelated transport properties by rotating its direction of magnetization. 
Monday, March 14, 2022 10:48AM  11:00AM 
A72.00013: Onset of multifractality in a ferromagnetic nodal line semimetal Mathimalar S, Ambikesh Gupta, Yotam Roet, Stanislaw Galeski, Nitesh Kumar, Walter Schnelle, Rafal Wawrzynczak, Claudia Felser, Johannes Gooth, Nurit Avraham, Haim Beidenkopf 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. 
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