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 K54: Stabilization of Topological Spin TexturesFocus
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Sponsoring Units: GMAG Chair: Charles Reichhardt, Los Alamos National Laboratory Room: Room 306 |
Tuesday, March 7, 2023 3:00PM - 3:36PM |
K54.00001: Chiral magnetic textures in itinerant magnets Invited Speaker: Satoru Hayami A magnetic skyrmion, which is characterized by a nontrivial topological spin texture in magnets, have been attracting much interest in condensed matter physics. Since the discovery of a skyrmion crystal, which corresponds to a periodic array of the skyrmion spin textures, in 2009, noncentrosymmetric magnets with the spin-orbit coupling have been good platforms of hosting the skyrmion crystals. Meanwhile, recent theoretical and experimental studies have clarified several different origins of the skyrmion crystals in centrosymmetric magnets by introducing the frustrated exchange interaction in localized magnets and the multiple-spin interaction arising from the spin-charge coupling in itinerant magnets. In the presentation, we discuss the stabilization mechanisms of the skyrmion crystals on the basis of the microscopic spin model with the symmetric and antisymmetric anisotropic interactions [1,2]. We present important interactions and magnetic anisotropy to stabilize the square- and triangular-shaped skyrmion crystals from the ground state to the finite-temperature state in centrosymmetric magnets. |
Tuesday, March 7, 2023 3:36PM - 3:48PM |
K54.00002: Chiral domain coarsening in frustrated itinerant magnets Yunhao Fan, Sheng Zhang, Gia-Wei Chern Itinerant magnets are characterized by an electron-mediated spin-spin interaction that is long-ranged and dependent on the underlying electron Fermi surface. This effective interaction is also often frustrated, giving rise to complex spin textures at low temperatures. In particular, the kondo lattice model with classical Heisenberg spins on the triangular lattice exhibits an unusual non-coplanar magnetic structure, known as the all-out order, with a quadrupled unit cell at both 1/4 and 3/4 electron filling fractions. Moreover, the all-out order breaks the chiral symmetry, and the corresponding electronic state is an effective quantum Hall insulator as electrons acquire a nontrivial Berry phase from the non-coplanar spins. Importantly, while the long-range magnetic ordering is destroyed by thermal fluctuations, the chirality can persist up to a finite temperature. Here we combine Landau-Lifshitz dynamics with efficient electronic structure method to investigate the coarsening of the chiral domains after a thermal quench. As the chiral order is characterized by a Z2 symmetry-breaking, the growth of the chiral domains is expected to follow that of a non-conserved Ising-like order. We compare our numerical results against the expected scaling behaviors and discuss how the differences can be attributed to the unusual domain-wall structures and dynamics. |
Tuesday, March 7, 2023 3:48PM - 4:00PM |
K54.00003: Systematic parameter study of magnetic skyrmions and antiskyrmions stabilised by exchange interactions Stephan von Malottki, Geoffroy Hautier It is a major ongoing task to optimize the thermal stability of magnetic skyrmions (Sk) and antiskyrmions (ASk), which is in particular limited for sub-10 nm Sk and ASk in 2D magnetic materials. In contrast to previous studies [1], we do not focus on a stabilisation of Sk and ASk by Dzyaloshinskii-Moriya interaction (DMI) but by exchange frustration [2] and higher order exchange interactions (HOI) [3]. We explore the large interaction parameter space of the atomistic Heisenberg model, consisting of exchange interaction beyond nearest neighbours, DMI, magnetocrystalline anisotropy and HOI by means of highly automatised energetic optimisation and geodesic nudged elastic band method (GNEB) simulations. Here we present the resulting sizes and energy barriers for SK and ASK in ferromagnetic and antiferromagnetic lattices with hexagonal and square geometry. In contrast to past, more anecdotally studies of the effect of exchange frustration [2,4] and HOI [3], this enables us to identify systematically the areas of parameters space in which metastable Sk and ASk can exist and how much their energy barriers can be enhanced in the framework of the atomistic extended Heisenberg model beyond nearest neighbours. |
Tuesday, March 7, 2023 4:00PM - 4:12PM |
K54.00004: Neuroevolutionary optimization for magnetic ground state Qichen Xu, Olle Eriksson, Nezhat Pournaghavi, Manuel Pereiro, Pawel Herman, Anna Delin Stable magnetic states, including particle-like topologically protected magnetic textures like skyrmions and hopfions, have recently attracted a lot of attention due to their potential applications in spintronics. In order to identify such textures, we have developed a hybrid neuroevolutionary algorithm based on a shallow neural network to find stable magnetic states in the Heisenberg spin model. Our algorithm uses both meta-heuristic global search and local optimization by combining a genetic tunneling (GT) with the stochastic gradient descent (SGD) method. It has no need of prior knowledge (i.e., an initial guess). Its capability to escape from the local minimum of the potential energy surface (PES) relies on treating the topologically protected magnetic patterns as chromosomes and performing genetic tunneling. The algorithm then finds the optimal spin configuration of the ground state of the magnetic Hamiltonian. We show that our algorithm achieves substantial improvements in robustness and effectiveness compared to traditional approaches like heat-bath simulated annealing. These features make our algorithm a promising tool for solving global optimization problems in magnetic systems. |
Tuesday, March 7, 2023 4:12PM - 4:24PM |
K54.00005: Machine-Learning Recognition of Dzyaloshinskii-Moriya Interaction from Magnetometry Bradley J Fugetta, Zhijie Chen, Kun Yue, Kai Liu, Amy Y Liu, Gen Yin The Dzyaloshinskii-Moriya interaction (DMI), which is the antisymmetric part of the exchange interaction between neighboring local spins, winds the spin manifold and can stabilize non-trivial spin textures in topological magnets. Since topology is a robust information carrier, characterization techniques that can extract the DMI magnitude are important for the discovery and optimization of spintronic materials. Existing experimental techniques for determining the DMI, such as high-resolution imaging of spin textures and measurement of magnon or transport properties, are time consuming and require access to specialized instrumentation. Here we show that a convolutional neural network (CNN) can extract the DMI magnitude from the minor hysteresis loops, or magnetic "fingerprints," of a material. These hysteresis loops are typically obtained by magnetometry measurements, the most accessible characterization technique for magnets. Our customized CNN was able to accurately estimate the value of the DMI of samples with featureful magnetic fingerprints and provide a confidence level for its estimate. This provides a convenient tool to search for topological spin textures for next-generation information processing. |
Tuesday, March 7, 2023 4:24PM - 4:36PM |
K54.00006: Phase degree of freedom and topological properties of multiple-Q spin textures Kotaro Shimizu, Shun Okumura, Yasuyuki Kato, Yukitoshi Motome Topological spin textures, such as skyrmion lattices (SkLs) and hedgehog lattices (HLs) are approximately represented by superpositions of multiple spin density waves, and hence, called multiple-Q spin structures. In such structures, the phase degree of freedom of the superposed waves plays an important role in the topological properties as well as the symmetry of the magnetic textures [1], but the systematic investigation has not been performed thus far. In this study, we theoretically investigate the evolution of the two-dimensional SkL (3Q-SkL) and the three-dimensional HL (4Q-HL) while changing the phases as well as the magnetization. For the 3Q-SkL, we elucidate the topological phase diagram including the SkLs with the skyrmion number -2, -1, 1, and 2. In the case of the 4Q-HL, we find a variety of HLs with different numbers of topological objects called the magnetic hedgehogs and antihedgehogs connected by the Dirac strings. Analyzing the spin textures obtained for the specific Hamiltonians, we find that an external magnetic field can cause topological transitions with phase shifts. Our findings clarify that the phase is a key parameter to realize unprecedented magnetic and topological phases, and the emergent electromagnetic phenomena associated with the noncoplanar spin textures. |
Tuesday, March 7, 2023 4:36PM - 4:48PM |
K54.00007: Hopfions and Hopf Invariant: Twisted Geometry of Space Curves Avadh B Saxena, Radha Balakrishnan, Rossen Dandoloff Topological invariants such as winding numbers and linking numbers appear in diverse physical systems described by a three-component unit vector field on 1D, 2D and 3D manifolds. We map the vector field to tangents of appropriate space curves that depict the manifold concerned. We then invoke the concept of parallel transport of the Frenet-Serret frame of space curves and the associated anholonomy. Our analysis shows that various topological invariants emerge in a natural fashion, as integrals of distinct intrinsic geometric quantities described by torsions (signifying nonplanarity) of the above space curves, as well as their intrinsic twists. We find that the presence of intrinsic twists plays a crucial role in the nontrivial topological invariants such as Hopf invariants (H) that arise in 3D manifolds. We illustrate this by considering exact hopfion solutions of an inhomogeneous, anisotropic 3D Heisenberg ferromagnet and show that a certain intrinsic twist is indeed necessary to yield a nontrivial H. |
Tuesday, March 7, 2023 4:48PM - 5:24PM |
K54.00008: Role of the topological singularity in the dynamics of chiral spin textures Invited Speaker: Ki-Suk Lee Topology is one of the key concepts in modern condensed matter physics. Since the early discovery of the Kosterlitz-Thouless transition in vortices, the importance of topological protection in understanding novel physical phenomena has been increasingly widely recognized, as evidenced in the quantum Hall effect, topological insulators, and Weyl semimetals. In the case of chiral spin textures such as magnetic skyrmions bubbles, and magnetic vortices, the concept of topological protection is essential in not only understanding their fundamentals but also harnessing them to future spintronic devices. Recently, it has been known that topological protection in chiral spin textures can be broken and the topology can be switched through the atomical discontinuity of magnetic materials or confinement of magnetic medium [1]. In this mechanism, a unique spin texture, a Bloch point (BP) which is a topological singularity, is mainly related. Unlike other spin textures, a BP has a unique feature – the local magnetization at a BP completely vanishes [2]. While it has also been theoretically proposed to bear critical roles in dynamic phenomena of chiral spin textures such as a magnetic vortex and skyrmion switching dynamics, it has not yet been experimentally well explored. |
Tuesday, March 7, 2023 5:24PM - 5:36PM |
K54.00009: Skyrmion and vortex crystals in the Hubbard model Kaito Kobayashi, Satoru Hayami Mutual interaction between the charge and spin degrees of freedom in electrons leads to the instability towards various topological magnetic orderings such as magnetic skyrmion and vortex crystals [1], whose structures are characterized by the multiple-Q state. In centrosymmetric itinerant magnets, the stabilization mechanism has been extensively studied in the Kondo lattice model consisting of itinerant electrons and localized spins, where the contribution from the charge degrees of freedom of localized electrons is disregarded. In this study, we investigate a realization of the skyrmion crystal (SkX) with the Hubbard model on a centrosymmetric triangular lattice, in which electrons show the itinerant nature rather than the localized one. Based on the self-consistent Hartree-Fock mean-field calculations, we find two types of the SkX as the ground state even without the external magnetic field, one of which corresponds to the findings in the Kondo lattice model [2]. Furthermore, we obtain another noncoplanar vortex crystal phase, and observe the topological phase transition from the vortex crystal to a different SkX in a magnetic field. Our results provide a further possibility to search for exotic topological magnetism in itinerant magnets. |
Tuesday, March 7, 2023 5:36PM - 5:48PM |
K54.00010: Derivation of effective low energy models using machine learning Vikram Sharma, Zhentao Wang, Cristian Batista We introduce a machine learning protocol to extract an effective low-energy spin model from a Kondo Lattice Model (KLM) with classical localized moments. The resulting effective spin model reproduces the phase diagram obtained with the original KLM and uncovers the effective four-spin interactions that are responsible for the stability of the skyrmion crystal phase. It enables an efficient computation of static and dynamical properties that are numerically orders of magnitude faster than the original KLM. Even though information about the spin dynamics is not used as a part of the training dataset, comparison of dynamical spin structure factor in the fully polarized phase reveals a reasonable agreement for the magnon dispersion. |
Tuesday, March 7, 2023 5:48PM - 6:00PM |
K54.00011: Dzyaloshinskii transition in a classical chiral Heisenberg model and analogy to DQC Jun Takahashi, Yoshihiko Nishikawa Magnetic systems with broken chirality can display diverse spin textures with a rich phase diagram due to competing interactions and fields [1]. A similar phenomenon has been realized even in a nonmagnetic valence-bond solid (VBS) state, where the chiral term favors a winding of the VBS order parameter [2]. This leads to new perspectives on deconfined quantum criticality (DQC) [3], a quantum phase transition beyond the Ginzburg-Landau (GL) paradigm, as a multicritical point in the extended phase diagram. |
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