Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session J40: Magnetic Spin TexturesFocus Live
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Sponsoring Units: GMAG DMP Chair: Lazar Kish, University of Illinois at Urbana-Champaign |
Tuesday, March 16, 2021 3:00PM - 3:12PM Live |
J40.00001: Stability and formation of skyrmion in ferromagnetic nanodots ~20 nm lateral dimensions with high Ms and moderate DMI Md Mahadi Rajib, Walid Al Misba, Dhritiman Bhattacharya, Jayasimha Atulasimha Voltage Controlled Magnetic Anisotropy (VCMA) induced skyrmion mediated reversal of ferromagnet has been experimentally shown to be robust and energy efficient (~1 fJ/bit) while not requiring a bias magnetic field. While this scheme could lead to Magnetic Tunnel Junctions (MTJs) based devices [1,2], scalability could be an issue as ferromagnetic skyrmions ~20 nm cannot be formed with experimentally observed Dzyaloshinskii-Moriya interaction (DMI) [3]. Though small skyrmions ~10 nm can be formed in ferrimagnets due to their low saturation magnetization (Ms), it is only possible in infinite films and racetracks whose dimensions are much larger than those of the skyrmions [4]. In this study, we show with micromagnetic simulation that small ferromagnetic skyrmion ~20 nm size in patterned nanostructures can be formed by increasing Ms for experimentally available DMI, as opposed to following the trend of lowering Ms and forming small skyrmion in ferrimagnetic racetrack/infinite films. |
Tuesday, March 16, 2021 3:12PM - 3:24PM Live |
J40.00002: Extended Exchange Interactions in Cr1/3NbS2, MnBi, and VTe: What is Nature saying? David Parker Exchange interactions are well-known as fundamental to magnetism and magnetic phenomena, deriving ultimately from the interaction of the Pauli principle with electromagnetic and other atomic-level interactions. There is a community belief that such interactions generally fall off relatively quickly with interatomic distance (recognizing that at short distances, generally less than 3 Angstrom, an interaction sign change may occur). Here I present evidence from 3 recent theoretical studies that find evidence, generally supported by relevant experimental data, for substantial exchange interactions even at interatomic distances approaching 6 Angstroms, where one would ordinarily expect such exchange interactions to be minimal. What is Nature saying here? Is there operative a mechanism, or mechanisms, additional to the normal, well-known exchange, superexchange, and double exchange interactions? I will discuss these issues. |
Tuesday, March 16, 2021 3:24PM - 3:36PM Live |
J40.00003: Antiferromagnetism of Fe1/3NbS2 as Characterized by Resonant Ultrasound Spectroscopy Sylvia Lewin, Gabriel Perko-Engel, Shannon Haley, Eran Maniv, James Analytis Fe1/3NbS2 is an intercalated transition metal dichalcogenide with electrically switchable antiferromagnetic order below 42 K [1]. If demonstrated in a room-temperature antiferromagnet, this type of switching could be the foundation of faster and more compact memory storage devices. Neutron scattering and magnetic susceptibility measurements have revealed two distinct antiferromagnetic phases in this material, but have left some ambiguity about their nature and symmetry. We have used resonant ultrasound spectroscopy to further characterize the antiferromagnetism of Fe1/3NbS2. With RUS we can determine a material's elastic moduli from its mechanical resonances. As the material undergoes a phase transition, the response of the different elastic moduli reveals the symmetry of the order parameter. By precisely identifying these symmetries, we hope to shed new light on the mechanism of Fe1/3NbS2's electrical switching. |
Tuesday, March 16, 2021 3:36PM - 3:48PM Live |
J40.00004: Measurements of a novel magnetically induced structural phase in CoPS3 Thuc Mai, Kevin Garrity, Michael Susner, Benjamin S Conner, Amber McCreary, Michael A McGuire, Angela Hight Walker The MPX3 family (M - transition metal, P - phosphorus, X - sulfur or selenium) of magnetic van der Waals (vdW) materials has the potential to realize antiferromagnetism in the 2D limit. In CoPS3, a vdW material with antiferromagnetic ordering around 130 K, we perform magneto-Raman spectroscopy, magnetometry, calorimetric, and X-ray diffraction as a function temperature to reveal a new magnetically induced structural phase transition. This behavior is reminiscent to the case of FePS3, where subtle structural changes occurs below the Neel temperature. We explore the implications of these data on the symmetries of this yet to identified phase with the additional help of density functional theory. |
Tuesday, March 16, 2021 3:48PM - 4:00PM Not Participating |
J40.00005: A general, microscopic model of B20 magnets Kyle Hall, Stephanie Curnoe Materials belonging to the B20 family are prominent in the study of exotic magnetism, including notable examples of helical magnetism and skyrmion lattice phases. Contemporary research continues to reveal intricacies of the low-temperature magnetic ordering of these materials, some of which are observed in only a subset. To properly account for these complexities, one requires a more in-depth description of these materials than is typical. |
Tuesday, March 16, 2021 4:00PM - 4:12PM Live |
J40.00006: Rich Phase Behaviors in Kitaev Honeycomb Magnet Na2Co2TeO6 Wenjie Chen, Xintong Li, Zhenhai Hu, Li Yue, Ronny Sutarto, Feizhou He, Kazuki Iida, Kazuya Kamazawa, Xi Lin, Yuan Li Quantum spin liquids based on the exactly solvable Kitaev honeycomb model have attracted considerable interest. In real materials, the realization of Kitaev interactions requires particular combinations of crystal-field environment, spin-orbit coupling, and exchange interactions. A recently proposed honeycomb magnet, Na2Co2TeO6, provided a promising opportunity for realizing the Kitaev model. After performing a combination of spectroscopic and thermodynamic measurements on single crystals of this material, we found that Na2Co2TeO6, surprisingly, goes through several distinct phase transitions at low temperatures and in zero magnetic field. We attribute the complex phase behaviors to the complexity of the underlying interactions. Understanding these newly discovered magnetic phases is likely a prerequisite before the system can be tuned towards quantum spin liquids. |
Tuesday, March 16, 2021 4:12PM - 4:24PM Live |
J40.00007: Phonon-assisted Formation of Spin Density Wave Order in Cr Thin Film Jiaruo Li, Oleg Gorobtsov, Sheena Kusum Kishor Patel, Nelson Hua, Benjamin Gregory, Anatoly Shabalin, Rajasekhar Medapalli, Stjepan B Hrkac, Jim Wingert, Devin Cela, James M Glownia, Matthieu Chollet, Diling Zhu, Eric Fullerton, Oleg Shpyrko, Andrej Singer Electronic instabilities drive ordering transitions in condensed matter. Despite many advances in sophisticated microscopic models of the ordered state, a more nuanced, yet profound, question often remains unanswered: How do collective excitations influence the formation of electronic order? Using time-resolved free electron X-ray diffraction technique, we monitor the laser-induced quench and subsequent recovery of a spin density wave (SDW) in a 28 nm thick Cr film. In this talk, we will discuss how the formation of SDW in antiferromagnetic Cr thin film after its quench by a femtosecond (fs) laser is influenced by an acoustic phonon launched by the laser. Our results demonstrate how a coherent ionic lattice vibration markedly modifies the out-of-equilibrium pathway in energy diagram of an electronic phase transformation, highlighting the opportunity for using collective excitations for guiding electronic instabilities. |
Tuesday, March 16, 2021 4:24PM - 4:36PM Live |
J40.00008: Real-space Observation of Ferroelectrically Induced Magnetic Spin Crystal in SrRuO3 Samuel Seddon, Daniela Emilia Dogaru, Sam Holt, Dorin Rusu, Jon Peters, Ana Sanchez-Fuentes, Marin Alexe Unusual features in the Hall Resistivity of thin film systems are frequently associated with whirling spin textures such as Skyrmions. A host of recent investigations of Hall Hysteresis loops in SrRuO3 heterostructures have provided conflicting evidence for different causes for such features. We have constructed an SrRuO3-PbTiO3 (Ferromagnetic–Ferroelectric) bilayer that exhibits features in the Hall Hysteresis previously attributed to a Topological Hall Effect (THE), and Skyrmions. We have employed field dependent Magnetic Force Microscopy measurements throughout the key fields where the ‘THE’ presents, revealing not Skyrmions, but the emergence of two periodic chiral spin textures. First, a zero-field cycloidal phase, which then transforms into a ‘double-q’ incommensurate spin crystal over the ‘THE-like’ field region, finally developing into a ferromagnetic switching regime as the sample reaches saturation, and the ‘THE-like’ diminishes. Scanning Tunnelling Electron Microscopy and DFT was used to observe and analyse surface inversion symmetry breaking and confirm the role of an interfacial Dzyaloshinskii–Moriya interaction at the heart of the system. |
Tuesday, March 16, 2021 4:36PM - 4:48PM Live |
J40.00009: Signature of defect-induced symmetry breaking in magnetic neutron scattering Andreas Michels The antisymmetric Dzyaloshinskii-Moriya interaction (DMI) plays a decisive role for the stabilization and control of chirality of skyrmion textures in various magnetic systems exhibiting a noncentrosymmetric crystal structure. A less studied aspect of the DMI is that this interaction is believed to be operative in the vicinity of lattice imperfections in crystalline magnetic materials, due to the local structural inversion symmetry breaking. If this scenario leads to an effect of sizable magnitude, it implies that the DMI introduces chirality into a very large class of magnetic materials — defect-rich systems such as polycrystalline magnets. Here, we show experimentally that the microstructural-defect-induced DMI gives rise to a polarization-dependent asymmetric term in the small-angle neutron scattering (SANS) cross section of polycrystalline ferromagnets. The results are supported by theoretical predictions using the continuum theory of micromagnetics. Analysis of the scattering asymmetry allows one to determine the defect-induced DMI constant. Our study proves the generic relevance of the DMI for the magnetic microstructure of defect-rich ferromagnets. |
Tuesday, March 16, 2021 4:48PM - 5:00PM Live |
J40.00010: Microscopic magnetic Hamiltonian for exotic spin textures in metals Deepak Kathyat, Arnob Mukherjee, Sanjeev Kumar We derive and study a microscopic spin Hamiltonian on a lattice for Rashba-coupled double exchange metals. |
Tuesday, March 16, 2021 5:00PM - 5:12PM Live |
J40.00011: Estimation of magnetic parameters from domain images with convolutional neural networks Jian Feng Kong, Yuhua Ren, Xiaoye Chen, Nicholas Tey, Pin Ho, Constantin Ciprian Chirila, Nathaniel Ng, Khoong Hong Khoo, Anjan Soumyanarayanan Magnetic multilayer films are known to host a variety of novel magnetic configurations such as topological magnetic skyrmions with potential nano-electronic applications. However, characterizing these material systems can be time-consuming and expensive. Therefore, it is crucial to maximize the information extracted from results of experiments that are more accessible, such as domain images obtained from magnetic force microscopy. We show that deep convolutional neural networks are able to extract magnetic parameters such as the exchange interaction, Dzyaloshinskii-Moriya interaction, and uniaxial anisotropy from images of domain configurations. Experimentally realistic training and validation data were generated through micromagnetic simulations. The trained models were consistently able to reach R^2 values greater than 0.9 on validation data. By inspecting the intermediate feature maps of the neural network, we find that the network is able to learn features such as domain boundaries. Testing the models on actual experimental data yield values that were consistent with our knowledge of the material systems. Our work thus demonstrates the utility of developing machine models trained on simulation data as a means to accelerate the characterization of magnetic systems. |
Tuesday, March 16, 2021 5:12PM - 5:24PM Live |
J40.00012: Berry-Phase Curvature and Hall Effect in Co Nanoparticles Ahsan Ullah, Balamurugan Balasubramanian, Rabindra Pahari, Ralph Skomski, David J Sellmyer Berry-phase contributions to electron transport have attracted much attention in recent years. An intriguing example is the topological Hall effect (THE), but analogous phenomena also exist in magnetic nanostructures exhibiting noncoplanar noncollinear spin structures. We investigate the Berry-phase behavior of dense ensembles of Co nanoparticles using experiments and micromagnetic simulations. The nanoparticles, produced by a cluster-deposition method, have an average size of about 16 nm and exhibit magnetocrystalline anisotropy constants of about 3.5 Merg/cm3 at 5 K. Hall resistivity data show a significant THE, which decreases on decreasing temperature from 300 K to 5 K. Our micromagnetic simulations suggest that the competition between exchange and dipole interactions results in vortex-like spin textures, which increases the magnitude of THE at high temperatures. At low temperatures, the spin texture prefers parallel alignment in the nanoparticle core and possibly due to magnetic anisotropy, and a small THE contribution arises only from the surface of the nanoparticles due to inter-particle interactions. The effect of particle size on the topological Hall effect of Co nanoparticles will also be discussed. |
Tuesday, March 16, 2021 5:24PM - 5:36PM Live |
J40.00013: Sensing chiral spin orders with entangled neutrons Abu Ashik Md. Irfan, Gerardo Ortiz We present a quantum entangled-probe scattering framework that extends van Hove's standard scattering theory, and apply it to directly unveil chiral spin orders. Previously, it has been shown that a neutron beam that possesses entanglement in its different degrees of freedom [1], for instance, trajectory, spin, and energy, can be employed to successfully distinguish entangled from un-entangled states in the case of spin dimer orders [2]. Here, we consider a similar entangled neutron beam to study both short-and long-range chiral spin orders. This study has implications in the study of complex quantum materials such as antiferromagnetic Mott insulators. |
Tuesday, March 16, 2021 5:36PM - 5:48PM Live |
J40.00014: Monte Carlo studies of (T, H) phase diagram of quasi-2D skyrmion systems: Role of magnetic anisotropy Po-Kuan Wu, Mohit Randeria Recent topological Hall effect experiments [1] and MFM imaging in thin heavy metal/rare-earth iron garnet (Pt/TmIG) bilayers show that skyrmions are stable in a very specific temperature-field (T, H) regime, which is quite distinct from the much-studied skyrmion phase diagram in B20 thin plates and thin films. We show using Monte Carlo (MC) simulations that the main cause of this striking difference is the magnetic anisotropy: Pt/TmIG has easy-axis anisotropy while the B20 systems have a negligible uniaxial anisotropy. We present detailed MC results comparing these two cases, focusing on topological charge density, skyrmion size, |
Tuesday, March 16, 2021 5:48PM - 6:00PM Live |
J40.00015: Magnetic phase diagram of two-leg ladder iron-based systems: Exact numerical results Maksymilian Sroda, Elbio Dagotto, Jacek Herbrych Motivated by growing experimental evidence of exotic magnetism in low-dimensional iron-based systems, we study the magnetic phases of the multi-orbital Hubbard ladder in the orbital-selective Mott phase. To reduce computational effort, we use a low-energy description of the latter model: the generalized Kondo-Heisenberg Hamiltonian. Our main result is the doping- and interaction-dependent magnetic phase diagram. We reproduce the experimental results on the AFe2X3 family (A=Cs,Rb,Ba,K; X=S,Se), especially the block magnetism of BaFe2Se3 (AFM coupled 2x2 FM islands). In agreement with studies of the chain geometry, we also unveil block magnetism beyond the 2x2 pattern and an interaction-induced block-spiral magnetism (a spiral of rigidly rotating FM blocks). Moreover, we find phases not present on the chain: a robust regime of phase separation, incommensurate AFM, and a new regime of block-like magnetic patterns. Finally, we exploit the bonding/antibonding band occupations to give insight into the structure of the phase diagram. |
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