Showing 1–9 of 9 results for author: McCarter, M

X-ray Fractional Orbital Angular Momentum from Coherent Magnetic Scattering

Authors: P. D. Montgomery, J. S. Woods, M. R. McCarter, R. Divan, D. Czaplewski, W. -K. Kwok, U. Welp, R. V. Chopdekar, S. Roy, A. Barbour, C. Mazzoli, L. E. De Long, J. T. Hastings

Abstract: Artificial spin ice (ASI) based on a square lattice with a topological defect are known to generate orbital angular momentum (OAM) in diffracted X-ray beams. A previous investigation of ASI with even-charge topological defects showed both charge and magnetic X-ray scattering yield photon OAM, but these were confined to integer OAM values. However, the period of the square ASI's antiferromagnetic g… ▽ More Artificial spin ice (ASI) based on a square lattice with a topological defect are known to generate orbital angular momentum (OAM) in diffracted X-ray beams. A previous investigation of ASI with even-charge topological defects showed both charge and magnetic X-ray scattering yield photon OAM, but these were confined to integer OAM values. However, the period of the square ASI's antiferromagnetic ground state is twice the period of the structural ground state, which should lead to fractional OAM from magnetic scattering when the topological defect has odd-charge. We employed photoemission electron microscopy to confirm that these ASIs order into antiferromagnetic ground states with protected superdomain walls that provide the phase discontinuity required for fractional OAM. Resonant, coherent X-ray scattering from ASIs with topological defects of charge 1 yields integer-valued X-ray OAM at structural charge peaks and fractional X-ray OAM at magnetic peaks. For thermally active ASIs, the fractional OAM beam exhibits fluctuations in the position of the phase discontinuity and thus dynamic rotation of the beam. △ Less

Submitted 2 June, 2026; originally announced June 2026.

Comments: 24 pages, 6 figures

Terahertz-field activation of polar skyrons

Authors: Huaiyu Wang, Vladimir Stoica, Cheng Dai, Marek Paściak, Sujit Das, Tiannan Yang, Mauro A. P. Gonçalves, Jiri Kulda, Margaret R. McCarter, Anudeep Mangu, Yue Cao, Hari Padma, Utkarsh Saha, Diling Zhu, Takahiro Sato, Sanghoon Song, Mathias Hoffmann, Patrick Kramer, Silke Nelson, Yanwen Sun, Quynh Nguyen, Zhan Zhang, Ramamoorthy Ramesh, Lane Martin, Aaron M. Lindenberg , et al. (5 additional authors not shown)

Abstract: Unraveling collective modes arising from coupled degrees of freedom is crucial for understanding complex interactions in solids and developing new functionalities. Unique collective behaviors emerge when two degrees of freedom, ordered on distinct length scales, interact. Polar skyrmions, three-dimensional electric polarization textures in ferroelectric superlattices, disrupt the lattice continuit… ▽ More Unraveling collective modes arising from coupled degrees of freedom is crucial for understanding complex interactions in solids and developing new functionalities. Unique collective behaviors emerge when two degrees of freedom, ordered on distinct length scales, interact. Polar skyrmions, three-dimensional electric polarization textures in ferroelectric superlattices, disrupt the lattice continuity at the nanometer scale with nontrivial topology, leading to previously unexplored collective modes. Here, using terahertz-field excitation and femtosecond x-ray diffraction, we discovered subterahertz collective modes, dubbed 'skyrons', which appear as swirling patterns of atomic displacements functioning as atomic-scale gearsets. Momentum-resolved time-domain measurements of diffuse scattering revealed an avoided crossing in the dispersion relation of skyrons. We further demonstrated that the amplitude and dispersion of skyrons can be controlled by sample temperature and electric-field bias. Atomistic simulations and dynamical phase-field modeling provided microscopic insights into the three-dimensional crystallographic and polarization dynamics. The discovery of skyrons and their coupling with terahertz fields opens avenues for ultrafast control of topological polar structures. △ Less

Submitted 1 September, 2025; v1 submitted 15 May, 2025; originally announced May 2025.

Report number: ISSN 2041-1723

Journal ref: Nat Commun 16, 8994 (2025)

Artificial Spin Ice: A Tutorial on Design and Control of Geometry, Microstate, Magnon Dynamics & Neuromorphic Computing

Authors: Rawnak Sultana, Amrit Kumar Mondal, Vinayak Shantaram Bhat, Kilian Stenning, Yue Li, Daan M. Arroo, Aastha Vasdev, Margaret R. McCarter, Lance E. De Long, J. Todd Hastings, Jack C. Gartside, M. Benjamin Jungfleisch

Abstract: Artificial spin ice, arrays of strongly interacting nanomagnets, are complex magnetic systems with many emergent properties, rich microstate spaces, intrinsic physical memory, high-frequency dynamics in the GHz range and compatibility with a broad range of measurement approaches. This tutorial article aims to provide the foundational knowledge needed to understand, design, develop, and improve the… ▽ More Artificial spin ice, arrays of strongly interacting nanomagnets, are complex magnetic systems with many emergent properties, rich microstate spaces, intrinsic physical memory, high-frequency dynamics in the GHz range and compatibility with a broad range of measurement approaches. This tutorial article aims to provide the foundational knowledge needed to understand, design, develop, and improve the dynamic properties of artificial spin ice (ASI). Special emphasis is placed on introducing the theory of micromagnetics, which describes the complex dynamics within these systems, along with their design, fabrication methods, and standard measurement and control techniques. The article begins with a review of the historical background, introducing the underlying physical phenomena and interactions that govern artificial spin ice. We then explore standard experimental techniques used to prepare the microstate space of the nanomagnetic array and to characterize magnetization dynamics, both in artificial spin ice and more broadly in ferromagnetic materials. Finally, we introduce the basics of neuromorphic computing applied to the case of artificial spin ice systems with goal to help researchers new to the field grasp these exciting new developments. △ Less

Submitted 8 April, 2025; originally announced April 2025.

Journal ref: J. Appl. Phys. 138, 061101 (2025)

Mixed Delay/Nondelay Embeddings Based Neuromorphic Computing with Patterned Nanomagnet Arrays

Authors: Changpeng Ti, Usman Hassan, Sairam Sri Vatsavai, Margaret McCarter, Aastha Vasdev, Jincheng An, Barat Achinuq, Ulrich Welp, Sen-Ching Cheung, Ishan G Thakkar, J. Todd Hastings

Abstract: Patterned nanomagnet arrays (PNAs) have been shown to exhibit a strong geometrically frustrated dipole interaction. Some PNAs have also shown emergent domain wall dynamics. Previous works have demonstrated methods to physically probe these magnetization dynamics of PNAs to realize neuromorphic reservoir systems that exhibit chaotic dynamical behavior and high-dimensional nonlinearity. These PNA re… ▽ More Patterned nanomagnet arrays (PNAs) have been shown to exhibit a strong geometrically frustrated dipole interaction. Some PNAs have also shown emergent domain wall dynamics. Previous works have demonstrated methods to physically probe these magnetization dynamics of PNAs to realize neuromorphic reservoir systems that exhibit chaotic dynamical behavior and high-dimensional nonlinearity. These PNA reservoir systems from prior works leverage echo state properties and linear/nonlinear short-term memory of component reservoir nodes to map and preserve the dynamical information of the input time-series data into nondelay spatial embeddings. Such mappings enable these PNA reservoir systems to imitate and predict/forecast the input time series data. However, these prior PNA reservoir systems are based solely on the nondelay spatial embeddings obtained at component reservoir nodes. As a result, they require a massive number of component reservoir nodes, or a very large spatial embedding (i.e., high-dimensional spatial embedding) per reservoir node, or both, to achieve acceptable imitation and prediction accuracy. These requirements reduce the practical feasibility of such PNA reservoir systems. To address this shortcoming, we present a mixed delay/nondelay embeddings-based PNA reservoir system. Our system uses a single PNA reservoir node with the ability to obtain a mixture of delay/nondelay embeddings of the dynamical information of the time-series data applied at the input of a single PNA reservoir node. Our analysis shows that when these mixed delay/nondelay embeddings are used to train a perceptron at the output layer, our reservoir system outperforms existing PNA-based reservoir systems for the imitation of NARMA 2, NARMA 5, NARMA 7, and NARMA 10 time series data, and for the short-term and long-term prediction of the Mackey Glass time series data. △ Less

Submitted 5 December, 2024; originally announced December 2024.

Nematicity of a Magnetic Helix

Authors: Z. Tumbleson, S. A. Morley, E. Hollingworth, A. Singh, T. Bayaraa, N. G. Burdet, A. Us Saleheen, M. R. McCarter, D. Raftrey, R. J. Pandolfi, V. Esposito, G. L. Dakovski, F. -J. Decker, A. H. Reid, T. A. Assefa, P. Fischer, S. M. Griffin, S. D. Kevan, F. Hellman, J. J. Turner, S. Roy

Abstract: A system that possesses translational symmetry but breaks orientational symmetry is known as a nematic phase. While there are many examples of nematic phases in a wide range of contexts, such as in liquid crystals, complex oxides, and superconductors, of particular interest is the magnetic analogue, where the spin, charge, and orbital degrees of freedom of the electron are intertwined. The difficu… ▽ More A system that possesses translational symmetry but breaks orientational symmetry is known as a nematic phase. While there are many examples of nematic phases in a wide range of contexts, such as in liquid crystals, complex oxides, and superconductors, of particular interest is the magnetic analogue, where the spin, charge, and orbital degrees of freedom of the electron are intertwined. The difficulty of spin nematics is the unambiguous realization and characterization of the phase. Here we present an entirely new type of magnetic nematic phase, which replaces the basis of individual spins with magnetic helices. The helical basis allows for the direct measurement of the order parameters with soft X-ray scattering and a thorough characterization of the nematic phase and its thermodynamic transitions. We discover two distinct nematic phases with unique spatio-temporal correlation signatures. Using coherent X-ray methods, we find that near the phase boundary between the two nematic phases, fluctuations coexist on the timescale of both seconds and sub-nanoseconds. Additionally, we have determined that the fluctuations occur simultaneously with a reorientation of the magnetic helices, indicating that there is spontaneous symmetry breaking and new degrees of freedom become available. Our results provide a novel framework for characterizing exotic phases and the phenomena presented can be mapped onto a broad class of physical systems. △ Less

Submitted 30 April, 2025; v1 submitted 19 April, 2024; originally announced April 2024.

Antiferromagnetic real-space configuration probed by dichroism in scattered x-ray beams with orbital angular momentum

Authors: Margaret R. McCarter, Ahmad I. U. Saleheen, Arnab Singh, Ryan Tumbleson, Justin S. Woods, Anton S. Tremsin, Andreas Scholl, Lance E. De Long, J. Todd Hastings, Sophie A. Morley, Sujoy Roy

Abstract: X-ray beams with orbital angular momentum (OAM) are a promising tool for x-ray characterization techniques. Beams with OAM have a helicity--an azimuthally varying phase--which leads to a gradient of the light field. New material properties can be probed by utilizing the helicity of an OAM beam. Here, we demonstrate a novel dichroic effect in resonant diffraction from an artificial antiferromagnet… ▽ More X-ray beams with orbital angular momentum (OAM) are a promising tool for x-ray characterization techniques. Beams with OAM have a helicity--an azimuthally varying phase--which leads to a gradient of the light field. New material properties can be probed by utilizing the helicity of an OAM beam. Here, we demonstrate a novel dichroic effect in resonant diffraction from an artificial antiferromagnet with a topological defect. We found that the scattered OAM beam has circular dichroism at the antiferromagnetic Bragg peak whose sign is coupled to its helicity, which reveals the real-space configuration of the antiferromagnetic ground state. Thermal cycling of the artificial antiferromagnet can change the ground state, as indicated by reversal of the sign of circular dichroism. This result is one of the first demonstrations of a soft x-ray spectroscopy characterization technique utilizing the OAM of x-rays. This helicity-dependent circular dichroism exemplifies the potential to utilize OAM beams to probe matter in a way that is inaccessible using currently available x-ray techniques. △ Less

Submitted 12 June, 2023; v1 submitted 6 May, 2022; originally announced May 2022.

Journal ref: Phys. Rev. B 107, L060407 (2023)

Nonvolatile Electric-Field Control of Inversion Symmetry

Authors: Lucas Caretta, Yu-Tsun Shao, Jia Yu, Antonio B. Mei, Bastien F. Grosso, Cheng Dai, Piush Behera, Daehun Lee, Margaret McCarter, Eric Parsonnet, Harikrishnan K. P., Fei Xue, Ed Barnard, Steffen Ganschow, Archana Raja, Lane W. Martin, Long-Qing Chen, Manfred Fiebig, Keji Lai, Nicola A. Spaldin, David A. Muller, Darrell G. Schlom, Ramamoorthy Ramesh

Abstract: In condensed-matter systems, competition between ground states at phase boundaries can lead to significant changes in material properties under external stimuli, particularly when these ground states have different crystal symmetries. A key scientific and technological challenge is to stabilize and control coexistence of symmetry-distinct phases with external stimuli. Using BiFeO3 (BFO) layers con… ▽ More In condensed-matter systems, competition between ground states at phase boundaries can lead to significant changes in material properties under external stimuli, particularly when these ground states have different crystal symmetries. A key scientific and technological challenge is to stabilize and control coexistence of symmetry-distinct phases with external stimuli. Using BiFeO3 (BFO) layers confined between layers of the dielectric TbScO3 as a model system, we stabilize the mixed-phase coexistence of centrosymmetric and non-centrosymmetric BFO phases with antipolar, insulating and polar, semiconducting behavior, respectively at room temperature. Application of in-plane electric (polar) fields can both remove and introduce centrosymmetry from the system resulting in reversible, nonvolatile interconversion between the two phases. This interconversion between the centrosymmetric insulating and non-centrosymmetric semiconducting phases coincides with simultaneous changes in the non-linear optical response of over three orders of magnitude, a change in resistivity of over five orders of magnitude, and a change in the polar order. Our work establishes a materials platform allowing for novel cross-functional devices which take advantage of changes in optical, electrical, and ferroic responses. △ Less

Submitted 2 January, 2022; originally announced January 2022.

Comments: L Caretta and Y-T Shao contributed equally. 64 pages, 4 figures, 25 supplemental figures, 1 supplemental table

Electric Field Control of Chirality

Authors: Piush Behera, Molly A. May, Fernando Gómez-Ortiz, Sandhya Susarla, Sujit Das, Christopher T. Nelson, Lucas Caretta, Shang-Lin Hsu, Margaret R. McCarter, Benjamin H. Savitzky, Edward S. Barnard, Archana Raja, Zijian Hong, Pablo García-Fernandez, Stephen W. Lovesey, Gerrit van der Laan, Colin Ophus, Lane W. Martin, Javier Junquera, Markus B. Raschke, Ramamoorthy Ramesh

Abstract: Polar textures have attracted significant attention in recent years as a promising analog to spin-based textures in ferromagnets. Here, using optical second harmonic generation based circular dichroism, we demonstrate deterministic and reversible control of chirality over mesoscale regions in ferroelectric vortices using an applied electric field. The microscopic origins of the chirality, the path… ▽ More Polar textures have attracted significant attention in recent years as a promising analog to spin-based textures in ferromagnets. Here, using optical second harmonic generation based circular dichroism, we demonstrate deterministic and reversible control of chirality over mesoscale regions in ferroelectric vortices using an applied electric field. The microscopic origins of the chirality, the pathway during the switching, and the mechanism for electric-field control are described theoretically via phase-field modeling and second-principles simulations, and experimentally by examination of the microscopic response of the vortices under an applied field. The emergence of chirality from the combination of non-chiral materials and subsequent control of the handedness with an electric field has far-reaching implications for new electronics based on chirality as a field controllable order parameter. △ Less

Submitted 28 May, 2021; originally announced May 2021.

Comments: 19 pages, 4 figures Supplement: 27 pages, 13 figures

Subterahertz collective dynamics of polar vortices

Authors: Qian Li, Vladimir A. Stoica, Marek Paściak, Yi Zhu, Yakun Yuan, Tiannan Yang, Margaret R. McCarter, Sujit Das, Ajay K. Yadav, Suji Park, Cheng Dai, Hyeon Jun Lee, Youngjun Ahn, Samuel D. Marks, Shukai Yu, Christelle Kadlec, Takahiro Sato, Matthias C. Hoffmann, Matthieu Chollet, Michael E. Kozina, Silke Nelson, Diling Zhu, Donald A. Walko, Aaron M. Lindenberg, Paul G. Evans , et al. (7 additional authors not shown)

Abstract: The collective dynamics of topological structures have been of great interest from both fundamental and applied perspectives. For example, the studies of dynamical properties of magnetic vortices and skyrmions not only deepened the understanding of many-body physics but also led to potential applications in data processing and storage. Topological structures constructed from electrical polarizatio… ▽ More The collective dynamics of topological structures have been of great interest from both fundamental and applied perspectives. For example, the studies of dynamical properties of magnetic vortices and skyrmions not only deepened the understanding of many-body physics but also led to potential applications in data processing and storage. Topological structures constructed from electrical polarization rather than spin have recently been realized in ferroelectric superlattices, promising for ultrafast electric-field control of topological orders. However, little is known about the dynamics of such complex extended nanostructures which in turn underlies their functionalities. Using terahertz-field excitation and femtosecond x-ray diffraction measurements, we observe ultrafast collective polarization dynamics that are unique to polar vortices, with orders of magnitude higher frequencies and smaller lateral size than those of experimentally realized magnetic vortices. A previously unseen soft mode, hereafter referred to as a vortexon, emerges as transient arrays of nanoscale circular patterns of atomic displacements, which reverse their vorticity on picosecond time scales. Its frequency is significantly reduced at a critical strain, indicating a condensation of structural dynamics. First-principles-based atomistic calculations and phase-field modeling reveal the microscopic atomic arrangements and frequencies of the vortex modes. The discovery of subterahertz collective dynamics in polar vortices opens up opportunities for applications of electric-field driven data processing in topological structures with ultrahigh speed and density. △ Less

Submitted 10 February, 2021; originally announced February 2021.