Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session A55: Skyrmions and Chiral Spin Textures in Van der Waals and 2D MaterialsFocus
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Sponsoring Units: GMAG Chair: Nirmal Ghimire, George Mason University Room: Room 305 |
Monday, March 6, 2023 8:00AM - 8:36AM |
A55.00001: Quantum dynamics and response functions of skyrmions Invited Speaker: Sopheak Sorn Recent research activities have turned to studying quantum effects in skyrmion-hosting ordered phases within the context of numerical diagonalization of quantum spin Hamiltonians as well as semiclassical magnonic theories. One of the works on two-dimensional chiral magnets has unveiled a force due to magnons exerting on a skyrmion sitting in a field-polarized ferromagnetic background. It arises from the scattering between the magnons and the skyrmion. In this talk, it will be shown how magnons in a skyrmion gas scatter off multiple skyrmions, and how this effectively generates an inter-skyrmiom interaction. Various features of this novel interaction such as its temperature dependence and its dependence on skyrmion density will be discussed. Finally, results from the same semiclassical theory on various correlation functions, which lie at the heart of linear responses of skyrmions to external probes, will be reported. |
Monday, March 6, 2023 8:36AM - 8:48AM |
A55.00002: Designer Magnetic Textures by Strain Engineering Van der Waals Magnets Chong Wang, Jimin Qian, Ting Cao, Di Xiao Two-dimensional magnets provide a new playground for strain engineering of magnetism. In this work, we discover that novel magnetic textures emerge in two-dimensional magnets, induced by periodic strain patterning. By imposing a strain field in a square lattice geometry, meron-like and antimeron-like topological magnetic texture emerges due to periodic modulation of magnetic anisotropy locked onto the strain field. We show that the stability of topological magnetic textures relative to non-topological bubble-like magnetic texture can be easily controlled by the strain field profile. Furthermore, the topological magnetic texture and their fundamental excitations can be mapped to an effective spin Hamiltonian, where the strength of four-spin interaction is comparable to nearest and second-nearest two-spin exchange coupling. As non-uniform strain patterning has been achieved by controlled synthesis and substrate engineering of two-dimensional materials, our work demonstrates the possibility of creating topological magnetic textures with strain and designing higher-order spin Hamiltonians on demand. |
Monday, March 6, 2023 8:48AM - 9:00AM |
A55.00003: Magnetic domain phase in van der Waals ferromagnet Fe3GeTe2 coupled to Co magnetic vortex Tianye Wang, Andreas Scholl, Alpha T N'Diaye, Xiaoxi Huang, Hongrui Zhang, Xixiang Zhang, Chanyong Hwang, Ramamoorthy Ramesh, Michael F Crommie, Zi Q. Qiu Van der Waals ferromagnet Fe3GeTe2 (FGT) is known to possess spontaneous chiral stripe domains which can be transformed into topological skyrmions under certain conditions. Using photoemission electron microscopy (PEEM), we imaged FGT magnetic domain phase under the influence of magnetic vortices. FGT flakes in contact with micron-sized Co disks across a spacer layer were fabricated by dry transfer method. We found that the vortex state of the Co disk melts the FGT stripes into elongated bubble states which resemble the liquid crystal phase. Thickness dependence of the interlayer coupling effect was also investigated on various thicknesses of FGT. Micromagnetic simulation via MuMax3 reproduced experimental phenomena. Our findings provide another novel spin texture phase in van der Waals magnetic materials. |
Monday, March 6, 2023 9:00AM - 9:12AM |
A55.00004: First principles investigation of bulk skyrmion formation within the 50% Co-doped Fe5GeTe2 layered magnetic metal Jonathan T Reichanadter, Jeffrey B Neaton The van der Waals-layered itinerant magnetic metal Fe5GeTe2 (F5GT) has recently garnered significant interest for its high-temperature and readily tunable magnetic ordering behavior. Moreover, the structural phase and interlayer stacking are highly sensitive to specific doping concentrations of Cobalt, leading to the formation of a polar structure at 50% doping that admits a zero-field neel-type skyrmion lattice at room temperature [1,2]. Here we perform ab-initio density functional theory calculations across a variety of structural instances of Co-doped F5GT to elucidate the relationship between atomic geometry, and compositional and magnetic order. These calculations provide new insight into how Co-substitution results in uncompensated in-plane Dzyaloshinskii-Moriya interactions that lead to skyrmion formation. |
Monday, March 6, 2023 9:12AM - 9:24AM |
A55.00005: Imaging Skyrmions in a van der Waals Magnetic Heterostructure with Magnetic Force Microscopy Brian Francisco Magnetic Skyrmions are spatial spin textures with non-trivial topology. Its formation typically originates from the Dzyaloshinskii–Moriya interaction (DMI) which requires strong spin-orbit coupling and broken inversion symmetry. Recent developments of van der Waals (vdW) magnetic materials provide new opportunities to engineer DMI through interface effects. In this talk, we discuss our recent studies on vdW heterostructures of Fe3GeTe2 (FGT) and Cr2Ge2Te6 (CGT), both of which exhibit spin-orbit coupling and their heterostructure interface naturally breaks the inversion symmetry. We employ magnetic force microscopy (MFM) to image the magnetic domains and observe a periodic lattice of regularly spaced Skyrmion-like features that occur at both the CGT and FGT sides of the heterostructure below their respective magnetic transition temperatures. The MFM results are corroborated by the topological Hall effect observed in transport. Our results show promises for Skyrmionic devices based on vdW heterostructures hosting multiple Skyrmion phases. |
Monday, March 6, 2023 9:24AM - 9:36AM |
A55.00006: Magnetic topological quasiparticles in 2D van der Waals magnets engineered by layer stacking Kai Huang, Ding-Fu Shao, Evgeny Y Tsymbal Skyrmions, antiskyrmions, and bimerons are swirling magnetic textures with nontrivial topologies in magnetic materials. Unlike the widely investigated skyrmions, antiskyrmions and bimerons are rarely found due to the requirement of the anisotropic Dzyaloshinskii-Moriya interaction (DMI). Here we propose to exploit the recently demonstrated van der Waals (vdW) assembly of two-dimensional (2D) materials that are non-polar in the bulk form to break inversion symmetry and create conditions for the emergence of anisotropic DMI in 2D magnets.1 We demonstrate, based on symmetry analyses and first-principles calculations, that this strategy is a promising platform to realize antiskyrmions and bimerons. The polar layer stacking of two centrosymmetric magnetic monolayers, such as CrI3, can efficiently lower the symmetry, resulting in anisotropic DMI that supports antiskyrmions or bimerons depending on the magnetic anisotropy of the 2D magnet. The DMI is reversible by switching the ferroelectric polarization inherited from the polar layer stacking, offering the ability to control antiskyrmions or bimerons by an electric field. Moreover, we find that the magnetic anisotropy of CrI3 can be efficiently changed by Mn doping, creating a possibility to control the size of antiskyrmions and bimerons. Our work opens a new direction to generate and control magnetic quasiparticles other than skyrmions. |
Monday, March 6, 2023 9:36AM - 9:48AM |
A55.00007: Skyrmions in van der Waals centrosymmetric materials Hung B Tran, Yu-ichiro Matsushita Skyrmions can appear in non-centrosymmetric materials due to non-vanishing Dzyaloshinskii–Moriya interactions (DMIs). We investigate the magnetic properties of rhombohedral MX3 (M: V, Cr, Mn, Fe; X: Cl, Br, I) with van der Waals materials with centrosymmetric lattices. We found that the Dzyaloshinskii–Moriya vector acting between the second nearest neighbor sites of the intralayer is non-zero and large even in MX3, owing to the breaking of the local inversion symmetry. Large DMIs cause nanoscale magnetic vortices, the so-called skyrmions in MX3. We observe not only conventional skyrmions in CrCl3 and VCl3 but also antiferromagnetic skyrmions in FeCl3 and merons in MnCl3. Furthermore, the skyrmions in CrCl3 and VCl3 have different helicities, indicating the possibility of controlling the helicity by electron/hole doping in MX3 materials. Van der Waals materials have high degrees of freedom in heterostructures and twisted structures, demonstrating promising potential as skyrmion materials. |
Monday, March 6, 2023 9:48AM - 10:00AM |
A55.00008: Probing Spin-Excitations in an Ultra-Thin van der Waals Helimagnet Connor A Occhialini, Qian Song, Yi Tseng, Joshua J Sanchez, Luiz Gustavo Pimenta Martins, Valentina Bisogni, Jonathan Pelliciari, Riccardo Comin Materials with non-collinear magnetic structures host many unique phenomena, including multiferroic, magnetoelectric and topological phases. Among them, the discovery of non-collinear magnetism in the van der Waals (vdW) nickel dihalides (NiX2, X = Br, I) down to the few- and single-atomic-layer limit paves the way for the realization of these properties in 2D devices/heterostructures. In these materials, geometric frustration leads to a complex magnetic phase diagram where distinct magnetic states are determined by a delicate balance of intra-layer exchange interactions. However, accessing these interactions in the 2D limit is experimentally challenging due to the limited thickness. In this work, we address these challenges and present measurements of the dispersive magnetic excitations in NiI2 in the bulk and ultra-thin limit using resonant inelastic X-ray scattering (RIXS). We uncover the dispersion of a single-magnon branch in the bulk which shows a clear correspondence to the incommensurate helimagnetic order. Additional measurements performed on a two-layer sample reveal dispersive spin excitations with marked renormalization with respect to the bulk. Based on these results, we assess the evolution of the intra-layer exchange interactions and the helimagnetic order in the 2D limit of NiI2. More broadly, these measurements provide information regarding the nature of finite-momentum spin excitations in layered 2D magnets, which have thus far eluded direct observation, and establish RIXS as an ideal technique to investigate complex magnetic phases in vdW materials. |
Monday, March 6, 2023 10:00AM - 10:12AM |
A55.00009: First-principles calculation and theoretical study of 2D van der Waals magnets and their magneto-elastic coupling effect Jimin Qian, Chong Wang The discovery of magnetic van der Waals materials opens a new research area of realizing the novel quantum states and topological spin texture, such as skyrmion. As an important tuning knob of the 2D magnets, strain can play a critical role in controlling the magnetic anisotropy through magneto-elastic coupling and hence produces novel spin textures. In this study, we performed first-principles calculations to characterize the magneto-elastic effect and the magnetic anisotropy of 2D magnets. We implemented a continuum model to simulate the response of spin textures and magnon dynamics under various external strain field. This study reveals a new approach of controlling the 2D spin texture through the design of strain field. |
Monday, March 6, 2023 10:12AM - 10:24AM |
A55.00010: Spontaneous topological Hall effect induced by non-coplanar antiferromagnetic order in intercalated van der Waals materials CoTa3S6 and CoNb3S6 Shinichiro Seki, Hirotaka Takagi, Rina Takagi, Susumu Minami, Takuya Nomoto, Kazuki Ohishi, Michi-To Suzuki, Yuki Yanagi, Motoaki Hirayama, Nguyen D Khanh, Kosuke Karube, Hiraku Saito, Daisuke Hashizume, Ryoji Kiyanagi, Yoshinori Tokura, Ryotaro Arita, Taro Nakajima Recent theoretical studies predicted that non-coplanar antiferromagnetic (AFM) order with scalar spin chirality can often induce large spontaneous Hall effect even without net magnetization or external magnetic field. This phenomenon, i.e. spontaneous topological Hall effect, can potentially be used for the efficient electrical readout of the AFM states, but its experimental verification has long been elusive due to the lack of appropriate materials hosting such exotic magnetism. Here, we report the discovery of all-in-all-out type non-coplanar AFM order in triangular lattice compounds CoM3S6 (M = Nb, Ta), by performing the detailed magnetic structure analysis based on polarized neutron scattering experiments and DFT calculations. These compounds are reported to host unconventionally large spontaneous Hall effect despite their vanishingly small net magnetization, and our analysis revealed that it can be well explained in terms of topological Hall effect, which originates from the fictitious magnetic field associated with scalar spin chirality in non-coplanar AFM orders. The present results indicate that the scalar spin chirality mechanism can offer a promising route to realize giant spontaneous Hall response even in compensated antiferromagnets, and highlight intercalated van der Waals magnets as an unique quasi-two-dimensional material platform to enable various nontrivial manner of electrical reading/writing of non-coplanar AFM domains. |
Monday, March 6, 2023 10:24AM - 10:36AM |
A55.00011: Proliferation of topological magnetic defects under field Erxi Feng, Anjana Samarakoon, Xianghan Xu, Xiaojian Bai, Chaowei Hu, Lei Ding, Yaohua Liu, Ni Ni, Cristian Batista, David A Tennant, Sang-Wook Cheong, Huibo Cao Magnetic domain walls and their crossings as magnetic vortices are typical magnetic topological defects existing in many magnetic materials. While visualizing them and their evolution under field at the atomic level are rarely reported except for magnetic skyrmions. Here I will present the proliferation of topological magnetic defects under field, seen by neutrons. |
Monday, March 6, 2023 10:36AM - 10:48AM |
A55.00012: Observation of the Chiral Phonon Activated Spin Seebeck Effect Jun Liu, Dali Sun, Wei You, Lifa Zhang, Jun Zhou, Xiao Li, Yu Yang, Andrew H Comstock, Rui Sun, Cong Yang, Ziqi Wang, Liang Yan, Eric Vetter, Kyunghoon Kim Utilization of the interaction between spin and heat currents is the central focus of the field of spin caloritronics. The recent emergence of chiral phonons possessing angular momentum arising from the broken symmetry of the lattice creates the potential for generating spin currents at room temperature in a non-magnetic material in response to a thermal gradient, precluding the need for a ferromagnetic contact. In this talk, we show the observation of spin currents generated by chiral phonons in a two-dimensional layered hybrid organic-inorganic perovskite implanted with chiral cations when subjected to a thermal gradient. Identified by transient magneto-optical Kerr effect measurements, the generated spin current shows a strong dependence on the chirality of the film and external magnetic fields, of which the coefficient is orders of magnitude larger than that produced by the reported spin Seebeck effect. Our findings indicate the potential of chiral phonons for spin caloritronic applications and offer a new route toward spin generation in the absence of magnetic materials. |
Monday, March 6, 2023 10:48AM - 11:00AM |
A55.00013: Spectroscopic signature of 3D magnetism in the itinerant van der Waals ferromagnet Fe5GeTe2 Vivek Bhartiya, Jiemin Li, Yanhong Gu, Taehun Kim, Shiyu Fan, Xiaodong Xu, Dmitri N Basov, Jonathan Pelliciari, Andrew F May, Valentina Bisogni Recently discovered family of high-temperature itinerant van der Waals ferromagnets Fe5-xGeTe2. Moreover, Fe5GeTe2 displays the highest Tc ~ 315 K and has the potential for magnonic devices operating at room temperature [4,5]. However, it is not yet known whether magnons in Fe5GeTe2 are confined to 2D or propagate in 3D. Investigation of dispersing magnons via inelastic neutron scattering is not feasible due to the small size of available single crystals. In this study, we overcome this issue by employing a complimentary probe, resonant inelastic x-ray scattering (RIXS), on a high-quality mm-size single crystal of Fe5GeTe2 [5]. A broad magnetic continuum stretching up to 150 meV and displaying a strong intensity modulation along (00L) is observed [6]. We show that this intensity modulation is compatible with a dominant inter-slab magnetic interaction enabling 3D propagation of magnons in the room-temperature ferromagnet Fe5GeTe2 [6]. |
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