Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session G54: Magnetic 2D van der Waals MaterialsFocus Recordings Available

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Sponsoring Units: GMAG Chair: Dustin Gilbert, University of Tennessee: Knoxville Room: McCormick Place W476 
Tuesday, March 15, 2022 11:30AM  11:42AM 
G54.00001: Moiré Skyrmions in Twisted CrX_{3} (X = I, Br, and Cl) Bilayers: Part (I) Ab Initio Calculations Jesse Kapeghian, Muhammad Akram, Harrison LaBollita, Dibyendu Dey, Onur Erten, Antia S Botana We present a comprehensive theory of the magnetic phases in twisted bilayer chromium trihalides through firstprinciples calculations and atomistic simulations [1]. In this talk, the firstprinciples simulations will be described. Our calculations permit to build an effective spin model for the bilayers with the isotropic and anisotropic CrCr exchange couplings, the single ion anisotropy, and the interlayer exchange coupling. The intralayer Heisenberg exchange coupling is found to be ferromagnetic and with a value that decreases from I to Cl, as expected. The correct (experimental) magnetic anisotropy (outofplane for CrI_{3} and CrBr_{3} and inplane for CrCl_{3}) is also reproduced. More importantly, we find that the stackingdependent interlayer exchange leads to an effective moiré field that is mostly ferromagnetic with antiferromagnetic patches in all three materials. As such, the competing stackingdependent interlayer interactions can give rise to novel magnetic states upon twisting. 
Tuesday, March 15, 2022 11:42AM  11:54AM 
G54.00002: Moiré Skyrmions in Twisted CrX3 (X = I, Br, and Cl) Bilayers: Part (II) Atomistic Simulations Muhammad Akram, Harrison LaBollita, Dibyendu Dey, Jesse Kapeghian, Onur Erten, Antia S Botana We present a comprehensive theory of the magnetic phases in twisted bilayer chromium trihalides through firstprinciples calculations and atomistic simulations [1]. In this talk, the atomistic simulations will be described. In order to find the ground state for the bilayer Hamiltonian, we introduce the Dzyaloshinskii−Moriya interaction (DMI) and solve the LandauLifshitzGilbert equations. As a result of the competing interlayer antiferromagnetic coupling and the energy cost for forming ferromagneticantiferromagnetic domain walls, a wide range of noncollinear magnetic phases can be stabilized as a function of the twist angle and DMI. In particular, for small twist angles, various skyrmion crystal phases can be stabilized in both CrI_{3} and CrBr_{3}. Our results provide an interpretation for the recent observation of noncollinear magnetic phases in twisted bilayer CrI_{3} and demonstrate the possibility of engineering further nontrivial magnetic ground states in twisted bilayer chromium trihalides. 
Tuesday, March 15, 2022 11:54AM  12:06PM 
G54.00003: Biskyrmion lattices in centrosymmetric magnetic films Daniel Capic, Dmitry A Garanin, Eugene M Chudnovsky We develop a theoretical framework to construct biskyrmion lattices which were observed in nonchiral magnetic films. Skyrmions and antiskyrmions in a pure twodimensional (2D) exchange model can be described in terms of a complex function that is analytic except for a simple pole. In the same 2D pure exchange model, biskyrmion lattices are naturally described by the Weierstrass ℘ and ζ elliptic functions. We study films of finite thickness containing up to 1000×1000×100 spins starting with the biskyrmion lattice as the initial state. We investigate how the biskyrmion lattice evolves toward a minimumenergy state in zero magnetic field in the presence of perpendicular magnetic anisotropy (PMA) and dipoledipole interaction (DDI). Metastable biskyrmion lattices exist at lower PMA strengths. At higher PMA strengths, we observe stable triangular lattices of biskyrmion bubbles containing Bloch lines, whose energies are lower than the energy of the uniformly magnetized state. 
Tuesday, March 15, 2022 12:06PM  12:18PM 
G54.00004: Revisiting static and dynamic magnetic correlations in the chiralhelimagnet Cr_{1/3}NbS_{2}. Lisa M DeBeerSchmitt, Lazar L Kish, Adam A Aczel, Travis J Williams, Huibo Cao, Timothy R Charlton, Nirmal Ghimire, Ling Li, Jacob Ruff, Michael A McGuire, Stephen J Kuhn, Morten R Eskildsen, David G Mandrus Transition metal dichalcogenides intercalated with magnetic ions can crystallize in a chiral, hexagonal space group. Competition between Heisenberg exchange, the DzyaloshinskiiMoriya interaction, magnetocrystalline anisotropy and an applied magnetic field in the monoaxial chiral magnet Cr_{1/3}NbS_{2} lead to interesting longperiod magnetic structures such as a helimagnetic and chiral soliton lattice states. We recently performed a smallangle neutron scattering experiment to characterize these states in detail. We observed higherorder peaks in our zero field data, which are unexpected for an ideal helical state with perfect sinusoidal modulation. These signatures of anharmonic distortion could be due to undiscovered magnetocrystalline anisotropy, or metastable behavior due to disorder effects. 
Tuesday, March 15, 2022 12:18PM  12:30PM 
G54.00005: Chiral Magnetism in Intercalated Transition Metal Dichalcogenides Studied by Nonlinear Optics Youngjun Ahn, Deepak Sapkota, Hasitha Suriya Arachchige, Rui Xue, Shirin Mozaffari, David G Mandrus, Liuyan Zhao Transition metal dichalcogenides (TMDCs) intercalated with magnetic atoms have been predicted to host exotic twodimensional spin textures owing to their strong spinorbit coupling. One of the anticipated spin textures is chiral helimagnetism in which a periodic rotation of magnetic moments emerges within the plane perpendicular to its propagation direction. While point groups lacking both mirror and inversion symmetries are prerequisites for realizing robust helical structure, systematic experimental studies of the full structural and magnetic symmetries remain missing. Optical second harmonic generation (SHG) probe, which is sensitive to crystallographic and magnetic point group symmetries and capable of coupling to complex spin textures, provides unique opportunities to explore the physics of twodimensional chiral magnets and serve as guidance to discover new magnetic materials with chirality. Here, we report our studies on helimagnet TMDC candidates, V and Crintercalated NbS_{2} and TaS_{2}, using static and dynamic optical SHG. We will show how we track the complete symmetry evolutions across their magnetic critical temperatures by performing temperaturedependent rotation anisotropy SHG spectroscopy measurements and image the structural and magnetic domains by conducting scanning SHG microscopy measurements. 
Tuesday, March 15, 2022 12:30PM  12:42PM 
G54.00006: Topologically nontrivial spintextures and multiferroicity in 2D nickel dihalides. Danila Amoroso, Paolo Barone, Silvia Picozzi There is currently an increasing enthusiasm towards longrange magnetic order and multiferroicity in twodimensional materials (2D) both from the fundamental and applicative point of view. In this respect, by means of firstprinciples calculations and Monte Carlo simulations, we investigate the nickel dihalides (NiX_{2}, with X= Cl, Br, I), which belong to the promising class of van der Waals materials, with a special focus on the exotic NiI_{2} case. NiBr_{2} and NiI_{2} monolayer exhibit, in fact, anisotropic exchange couplings related to the relevant spinorbit coupling of the heavy ligands, driving formation of spontaneous noncollinear magnetic configurations, supported by competing ferromagnetic and antiferromagnetic isotropic exchange. In particular, we predict intrinsic chiral topological spintextures in NiI_{2}, among other closely lying competing phases; the specific topology of the skyrmionic lattices is determined by the, so far overlooked, twoion anisotropy, which acts as an emerging chiral interaction. In addition, we also report interesting lowdimensional magnetoelectric and, eventual, multiferroic properties. 
Tuesday, March 15, 2022 12:42PM  12:54PM 
G54.00007: Temperature and FieldDependent Study of Magnetic Textures and Skyrmions in a Layered Van der Waals Magnet Reed Yalisove, Sandhya Susarla, Hongrui Zhang, Rui Chen, Xiang Chen, Robert J Birgeneau, Jie Yao, Mary Scott, Ramamoorthy Ramesh Twodimensional Van der Waals magnets offer a highly tunable platform for the study of novel topological textures such as skyrmions. Codoped Fe_{5}GeTe_{2} (FCGT) is such a system which exhibits zerofield, room temperature skyrmions when doped to precisely 50% Co concentration. The presence, structure, and organization of these skyrmions is highly dependent on sample geometry and environmental conditions and history. Previous study and simulation of this system have demonstrated stripe domains, isolated skyrmions, and skyrmion lattice at a variety of temperature and field conditions. Here, we use Lorentz transmission electron microscopy (LTEM) to map the behavior of magnetic domain structure in FCGT nanoflakes with temperature and applied field at a variety of nanoflake thicknesses. LTEM allows for domain imaging with insitu temperature control from 90K to room temperature and field control from fieldfree conditions to well above the saturation magnetization of the system. The resulting temperaturemagnetic field phase diagrams prescribe the necessary conditions to achieve magnetic skyrmions in FCGT. 
Tuesday, March 15, 2022 12:54PM  1:06PM 
G54.00008: Antiferromagnetic spin cycloids imaged with a Scanning NitrogenVacancy Magnetometer Peter Rickhaus, Hai Zhong, Johanna Fischer, Aurore Finco, Vincent Jacques, Vincent Garcia Multiferroics, such as BiFeO_{3}, in which antiferromagnetism and ferroelectricity coexist at room temperature, appear as a unique platform for spintronic and magnonic devices. The nanoscale structure of its ferroelectric domains has been widely investigated with piezoresponseforce microscopy (PFM) [1,2]. However, the BiFeO_{3} nanoscale magnetic textures and their potential for spinbased technology remain concealed. We present two different antiferromagnetic spin textures in BiFeO_{3} thin films with different epitaxial strains, measured with a commercial scanning NitrogenVacancy magnetometer (SNVM) [3]. Two BiFeO_{3} samples were grown on DyScO_{3}(110) andSmScO_{3}(110) substrates. The striped ferroelectric domains in both samples are first observed by the inplane PFM, and SNVM confirms the existence of the spin cycloid texture. At the local scale, the combination of PFM and SNVM allows to identify the relative orientation of the ferroelectric polarization and cycloid propagation directions on both sides of a domain wall. Our results show the potential for reconfigurable nanoscale spin textures on multiferroic systems by strain engineering. 
Tuesday, March 15, 2022 1:06PM  1:18PM 
G54.00009: First principles investigation of electronic and magnetic properties of Co and Ni doped Fe5GeTe2. Jonathan T Reichanadter, Jeffrey B Neaton The layered van der Waals itinerant roomtemperature ferromagnet Fe5GeTe2 is an attractive material for spintronic applications with tunable electronic and magnetic properties via transition metal and vacancy doping. Specific doping concentrations are reported to alter the structural and stacking behavior within the layered compound [1] which drives a transition to a polar structure with distinct electronic and magnetic properties in turn, including the stabilization of a zerofield neeltype skyrmion lattice state [2]. Here we perform abinitio density functional theory calculations across a myriad of structural phases with various Ni and Co doping ratios to elucidate the relationship between atomic geometry, electronic band structure, and magnetic order. We discuss our results in the context of an array of experimental measurements, including recent angleresolved photoemission data. 
Tuesday, March 15, 2022 1:18PM  1:30PM 
G54.00010: Topological protection and glassiness in a noncentroysmmetric magnet Sunil K Karna, John F DiTusa, David P Young, Frank Womack, G. Cao, William A Phelan, D. Tristant, Wei Tian, Adam A Aczel, Ilya Vekhter, William A Shelton, Philip W Adams Hexagonal and noncentrosymmetric ScFeGe was discovered to host an incommensurate helimagnetic state along the $c$axis below 36 K that matches well with a nesting condition found in its electronic structure. The result is a highly itinerant transition metal magnet whose physical properties that displays many similarities to the largely localmoment rare earth metals. The transition to a fully polarized magnetic state for fields perpendicular to the $c$axis is found to progress through a sharp metamagnetic transition to a fan state. The signature of this transition in the $dc$ magnetization at low temperatures is extraordinarily different from its low frequency $ac$ counterpart despite their quantitative agreement above 15 K. This change is accompanied by a distinct change in the specific heat from a steplike feature at low temperature to a secondorder phase transition above 15 K. These data are interpreted as revealing a local topological protection of the chiral helimagnetic state despite the likely existence of right and lefthanded helimagnetic domains that nucleate upon cooling in zero magnetic field. 
Tuesday, March 15, 2022 1:30PM  1:42PM 
G54.00011: Quantum skyrmions in twodimensional Heisenberg ferromagnets Thomas L Schmidt, Andreas Michels, Alireza Habibi, Andreas Haller, Solofo Groenendijk We study quantummechanical twodimensional Heisenberg Hamiltonians containing ferromagnetic exchange interactions, a magnetic field, DzyaloshinskiiMoriya interactions, uniaxial anisotropy and magnetic dipole interactions. Using a range of different numerical approaches, we are able to determine the phase diagram ranging from small to larger system sizes. This allows us to identify a phase hosting quantum skyrmions at intermediate magnetic fields, and we discuss its stability against perturbations like uniaxial anisotropy and dipolar interactions. Moreover, we calculate observable quantities, such as the static structure factor, as a possible experimental signature for the existence of quantum skyrmions. Finally, we predict the existence of quantum skyrmion lattices for large system sizes. 
Tuesday, March 15, 2022 1:42PM  2:18PM 
G54.00012: Noncollinear magnetic textures from quartic asymmetric exchange Invited Speaker: Ivan Ado Our work reveals a new potential source of noncollinear magnetic textures in certain classes of ferromagnets (FMs). Namely, in those that are described by the achiral tetrahedral symmetry (point group T_{d}) or by the trigonal prismatic symmetry (point group D_{3h}). The latter symmetry is realised in monolayer Fe_{3}GeTe_{2}, some transition metal dichalcogenides, and other twodimensional FMs. It is known that the DzyaloshinskiiMoriya interaction (DMI) does not contribute to the free energy density in such systems. We find all fourth order "chiral" contributions beyond DMI in T_{d} and D_{3h} classes. We study whether these "quartic asymmetric exchange" terms can explain spin spirals observed in a recent experiment on Fe_{3}GeTe_{2}. We also find that these asymmetric terms might stabilize bimerons — the inplane analog of skyrmions. Finally, we estimate the radius of such bimerons analytically. 
Tuesday, March 15, 2022 2:18PM  2:30PM 
G54.00013: Static and Dynamic Magnetic Properties of CuBased OrganicInorganic Hybrid Perovskite Single Crystals Andrew H Comstock, Haipeng Lu, Eric Vetter, RUYI SONG, Josh Chou, Joseph N Sklenar, Luqiao Liu, Wei Zhang, Volker Blum, Matthew C Beard, Dali Sun Low dimensional magnetism reported in Van der Waals (VdW) materials has sparked a renewed interest in spintronic applications in recent years, focusing on two dimensional spin waves and antiferromagnetic magnon generation. We investigate Cubased hybrid organicinorganic perovskites which represent an intriguing class of quasi twodimensional layered materials, with far greater chemical (and magnetic) variability than their VdW counterparts, making them potentially suited for spintronic applications. Here we examine the static and dynamic magnetic properties of (C_{6}H_{5}CH_{3}CH_{2}NH_{3})_{2}CuCl_{4}, (CH_{3}CH_{2}NH_{3})_{2}CuCl_{4}, and (C_{6}H_{5}CH_{3}CH_{2}NH_{3})_{2}CuBr_{4 }single crystals using SQUID magnetometry and ferromagnetic resonance (FMR) spectroscopy, respectively. Static magnetization and susceptibility measurements reveal the effects of organic cation and halide substitution on critical ordering behavior. Dynamic FMR measurements show spinwave propagation in the paramagnetic region above the critical temperature as a result of strong intralayer exchange coupling. At the critical temperature, asymmetric FMR absorption occurs as interlayer exchange becomes relevant and additional spinwave modes are manifested. 
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