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 C39: Spin Ice IFocus Live
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Sponsoring Units: GMAG DMP Chair: Chris Leighton, University of Minnesota |
Monday, March 15, 2021 3:00PM - 3:12PM Live |
C39.00001: Reservoir Computing with Frustrated Nanomagnet Arrays Alexander Edwards, Peng Zhou, Dhritiman Bhattacharya, Nathan R. McDonald, Felipe Garcia-Sanchez, Lisa Loomis, Clare D. Thiem, Jayasimha Atulasimha, Joseph S. Friedman Reservoir computing (RC) [1] is a subset of recurrent neural network where only the weights of the output layer are updated during training. This technique is therefore well suited for resource constrained hardware environments. We propose a novel reservoir comprising a planar arrangement of nanomagnets each having perpendicular magnetic anisotropy (PMA) [2]. The effect of nanomagnet magnetic fields upon adjacent nanomagnets exhibits two features: non-linear interaction and variable interaction strength, making the proposed implementation well suited for RC. Information is input by stimulating individual nanomagnets with spin-torque. The magnetizations of various nanomagnets are read electrically via magnetic tunnel junctions. A trained single layer circuit is used to perform vector-matrix multiplication on the magnetization values and the output weights to obtain the output vector. The nanomagnet reservoir was simulated in mumax3 with an input stream comprising triangle or square waves. The reservoir successfully identified the waveforms with 100% accuracy for both the training and testing data. |
Monday, March 15, 2021 3:12PM - 3:24PM Live |
C39.00002: Chiral switching and relaxation dynamics in artificial square ice Naëmi Leo, Sabri Koraltan, Matteo Pancaldi, Pedroáá Villalba González, Claas Abert, Christoph Vogler, Kevin Hofhuis, Florian Slanovc, Florian Bruckner, Paul Heistracher, Mattoe Menniti, Dieter Suess, Paolo Vavassori The relaxation kinetics and emerging correlations of arrays of interacting Ising-like nanomagnets are governed by the switching rates of the individual magnets. These rates depend, via the Arrhenius law, on the energy barrier for moment reversal in the nanomagnets. In this work, we consider how the switching behaviour of a nanomagnet in archetypical artificial square ice is modified by the interaction with its six nearest neighbours, and find that barrier energies for clockwise and counter-clockwise rotation can differ significantly. This effect is relevant for both exchange- as well as magnetostatically-dominated moment reversal. From string-method simulations we obtain further dynamical reductions of the switching barriers, as the micromagnetic structure evolves during reversal. The barrier splitting leads to an exponential enhancement of the transition rates compared to mean-field predictions. Finally, these modifications lead to faster relaxation dynamics and different spatial correlations in kinetic Monte Carlo simulations. |
Monday, March 15, 2021 3:24PM - 3:36PM Live |
C39.00003: Emergent Strings in Santa Fe Artificial Spin Ice Xiaoyu Zhang, Ayhan Duzgun, Yuyang Lao, Nicholas Bingham, Joseph N Sklenar, Hilal Saglam, Rajesh Chopdekar, Shayaan Subzwari, Joseph Batley, Justin Watts, Daniel Bromley, Chris Leighton, Liam O'Brien, Cristiano Nisoli, Peter Schiffer Artificial spin ice systems are two-dimensional arrays of interacting ferromagnetic nanoislands with uniaxial single-domain magnetization. By carefully designing the geometry of these arrays, some systems show collective phenomena associated with emergent higher-order frustration. Here we report a study on the Santa Fe Ice (SFI) system, which has an intrinsic population of excited vertices built into the geometry. Its magnetic configuration can be represented via emergent “strings” that connect all excited vertices, and thus moment fluctuations can be understood through the strings’ kinetics. SFI is predicted to have either a disordered ground state or a long-range ordered ground state associated with the distribution of excited vertices, depending on the local interactions. We have performed a magnetic force microscopy study on SFI after annealing near the Curie temperature and photoemission electron microscopy (PEEM) measurements on thermally active SFI. Our results indicate that SFI represents an unusual instance of emergent topological complexity. |
Monday, March 15, 2021 3:36PM - 3:48PM Live |
C39.00004: Field-Induced Magnetic Monopole Plasma in Artificial Spin Ice Mateusz Goryca, Xiaoyu Zhang, Jing Li, Andrew L Balk, Justin Watts, Chris Leighton, Cristiano Nisoli, Peter Schiffer, Scott Crooker Artificial spin ices (ASIs) are interacting arrays of lithographically-defined nanomagnets in which novel frustrated magnetic phases can be intentionally designed. A key emergent description of fundamental excitations in ASIs is that of magnetic monopoles - mobile quasiparticles that carry an effective magnetic charge. Here we demonstrate that the archetypal square ASI lattice can host, in specific regions of its magnetic phase diagram, plasma-like regimes containing a high density of mobile magnetic monopoles. These regimes result from the magnetic field-tunable tension on the Dirac strings connecting mobile monopoles. By passively "listening" to spontaneous monopole noise under conditions of strict thermal equilibrium, we reveal their intrinsic dynamics and show that monopole kinetics are most diffusive (that is, minimally correlated) in the plasma regime. These results open the door to on-demand monopole regimes having continuously field-tunable densities and dynamic properties, thereby providing a new paradigm for probing the physics of effective magnetic charges in synthetic matter. |
Monday, March 15, 2021 3:48PM - 4:24PM Live |
C39.00005: Complex Collective Effects in Vertex Frustrated Artificial Spin Ice Invited Speaker: Peter Schiffer Artificial spin ice systems are arrays of lithographically fabricated single-domain ferromagnetic elements with geometrically determined interactions that determine their collective properties. The ability to both design the lattice geometries and probe the individual moments in these systems allows the study of frustration in ways that are unavailable in natural materials. Our group has conducted a range of experimental studies of “vertex frustrated” lattices in which some fraction of the lattice vertices do not have moments arranged in their lowest energy configuration. This novel form of frustrated system results in unusual collective behavior, including topological charges (Shakti lattice), strings of excitations that are topologically protected (Santa Fe lattice), and reduced dimensionality associated with the placement of the excited vertices (Tetris lattice). We have probed these phenomena with magnetic force microscopy and photoemission electron microscopy, using thermalization to reach the low energy collective states of the systems. |
Monday, March 15, 2021 4:24PM - 4:36PM Live |
C39.00006: A Magnon Approach to Thermal Fluctuations in Mesospins Sam Sloetjes, Björgvin Hjörvarsson, Vassilios Kapaklis Systems consisting of single domain patterned magnetic elements, also mesospins, have proven to be a viable playground for studying ordering, phase transitions and dynamics at the mesoscale. One particularly interesting example of such a system is the 2D frustrated artificial spin ice (ASI). In analyzing and simulating the ASI, the mesospins are often approximated as point dipoles or ‘dumbbells’. When studying the effect of thermal fluctuations and thermally induced phase transitions however, the above description fails and leads to quantitative discrepancies. The reason behind this inaccuracy is rooted in the presence of an inner magnetic structure in the mesospins. Here, we address thermal fluctuations of mesospins by means of characterizing the spectrum of thermally excited magnons, and build a framework to study these excitations based on numerical simulations. We find that the spectrum of thermal magnons is discretized due to finite size effects, and that an important role is assigned to the fluctuations at the edges of the structure. The dynamics of these edge fluctuations is found to be governed by an energy barrier, which originates from the magnetic texture inside the mesospin. |
Monday, March 15, 2021 4:36PM - 4:48PM Live |
C39.00007: Avalanches from the ground state in artificial square ice Nicholas Bingham, Jungsik Park, Alejandro Simon, William Zhu, Justin Watts, Xiaoyu Zhang, Joseph Batley, Karin Andrea Dahmen, Chris Leighton, Peter Schiffer Avalanches are phenomena in which there is a cascade-like transition between two states, and are present in many material systems. Generally, avalanches are pinned at defects. These defects, however, are difficult to control, and thus the study of the avalanche process is rather difficult. In this work, we utilize the customizable nature of artificial spin ice to examine avalanche processes under well-controlled circumstances. Starting with the magnetic ground state of artificial square ice, magnetic force microscopy was performed at remanence after the islands were exposed to increasing magnetic fields. The results are used to study the avalanche processes, and effects of finite system size, as well as the relevant scaling of the avalanche process. |
Monday, March 15, 2021 4:48PM - 5:00PM Live |
C39.00008: Field-tunable correlations in perpendicular artificial spin ice arrays Susan Kempinger, Yu-Sheng Huang, Paul Lammert, Michael CS Vogel, John Pearson, Axel F Hoffmann, Vincent Henry Crespi, Peter Schiffer, Nitin Samarth Artificial spin ice (ASI) provides an effective platform for the study of custom designed frustration and its relationship with geometry, interaction, and stochasticity. Perpendicular ASI is particularly useful in this context, as the state of each element in a lattice is readily accessed using Kerr microscopy and the microstate of the entire lattice can be characterized through an applied field protocol. Unfortunately, studies of perpendicular ASI have been limited by weak interactions between elements. We have overcome this limitation by fabricating perpendicular ASI systems from Pt/Co islands on a soft-magnetic Ni80Fe20 (Py) underlayer to increase interactions. In the simplest case, the Py is saturated and serves to break the lateral symmetry in the arrays. We show that this configuration leads to a highly tunable system with unusual properties such as directionally-tunable interactions, preferred next-nearest neighbor coupling, and in situ adjustable coordination number. |
Monday, March 15, 2021 5:00PM - 5:12PM Live |
C39.00009: Magnetic Monopole Noise on Octahedral Spin Ice Jonathan Nilsson Hallén, Roderich Moessner, Claudio Castelnovo One of the most salient features of spin ice systems are emergent quasiparticle excitations that take the form of magnetic monopoles. The peculiar nature of these excitations reflects in many of their unusual properties, from thermodynamic quantities like the heat capacity to response and out of equilibrium properties like the susceptibility and magnetic avalanches. Recent SQUID measurements highlighted a further connection between the monopole excitations and the magnetic noise in these materials, exhibiting an anomalous power law behaviour that remains hitherto poorly understood. Here we consider a frustrated spin ice-like system , which we call octahedral spin ice (OSI), consisting of antiferromagnetically interacting Ising spins forming corner sharing octahedra. Using a combination of Monte Carlo simulations and effective modelling, we investigate how the creation, annihilation and propagation of the excitations in OSI contribute to the magnetic noise spectrum, shedding light on different factors that affect its anomalous behaviour in these materials. |
Monday, March 15, 2021 5:12PM - 5:24PM Live |
C39.00010: Proposal for Observing Magnetic Monopoles in Spin Ice via Electron Holography Ankur Dhar, Ludovic DC Jaubert, Tsumoru Shintake, Nicholas Shannon While the magnetic monopole proposed by Pierre Curie and Paul Dirac have remained elusive, emergent monopoles of the H field have been shown to exist in spin ice [1]. Signatures of these monopoles have been indirect so far, leaving their direct observation an open challenge since their discovery [2]. One such technique that could realize this direct observation is electron holography, due to the electron's sensitivity to magnetic fields via the Aharonov-Bohm effect [3]. We explore the possibility of imaging monopoles via electron holography through experimental measurements of monopole and spin ice analogs, and computational simulation of how monopoles would appear in a pyrochlore spin ice thin film [4]. These results suggest that imaging the dynamics of monopoles in thin films of spin ice through electron holography is a realistic possibility. |
Monday, March 15, 2021 5:24PM - 5:36PM Live |
C39.00011: Ground state transitions in Tetris artificial spin ice Hilal Saglam, Xiaoyu Zhang, Ayhan Duzgun, Nicholas Bingham, Yuyang Lao, Aikaterini Kargioti, Joseph N Sklenar, Ian J Gilbert, Cristiano Nisoli, Peter Schiffer Artificial spin ice systems comprised of interacting nanomagnets were introduced to mimic the frustration in naturally occurring spin ice materials [1]. Decimating the original square ice [1] has yielded a range of vertex-frustrated geometries, e.g., Tetris [2], Shatki [3]. We have studied the dynamics of Tetris ice, which can be decomposed into so-called staircase and backbone bands. We performed PEEM-XMCD experiments at various temperatures to understand the kinetics of Tetris. By analyzing the flipping rates, we previously showed that the backbones are stable against temperature, while the staircases are highly susceptible to thermal fluctuations. We have now studied the dynamics of Tetris by analyzing the transitions between different spin configurations and correlations both within and across staircases and backbones. We show that the low temperature dynamics of the system is associated with certain transitions within a low-energy manifold of collective states. |
Monday, March 15, 2021 5:36PM - 5:48PM Live |
C39.00012: Influence of dipolar interaction on the spin-wave mode propagation in artificial spin ice Nimisha Arora, Pintu Das Artificial Spin Ice1 systems are strongly dipolar coupled elongated magnetic nanoislands which are used as model systems to study the spin ice behavior observed in pyrochlore oxides as Dy2Ti2O7. Lately, there is an emphasis on the understanding of spin wave (sw) dynamics in such systems as they offer an intriguing possibility to tune sw behavior and manipulate sw band gaps. Hence, these systems are also considered as key candidates for reconfigurable magnonics2. We have thus performed a detailed analysis of the role of dipolar interaction on the sw dynamics and propagation in square artificial spin ice (S-ASI). We report that spin wave generation at particular frequencies can be realized even in the region of zero excitation due to strong dipolar coupling in such systems. This suggests the possibility of directional propagation of sw using reconfigurable ASI which can be employed in low power consumption based information processing devices. Additionally, our study suggests that S-ASI can be considered as a combination of four square rings of similar nanoislands of permalloy as a single square ring is adequate to understand sw dynamics in S-ASI. |
Monday, March 15, 2021 5:48PM - 6:00PM Live |
C39.00013: Monopole density and antiferromagnetic domains in spin-ice iridates Claudio Castelnovo, Attila Szabo, Matthew Pearce, Kathrin Goetze, Tycho Sikkenk, Martin Lees, Andrew Boothroyd, Prabhakaran Dharmalingam, Paul Goddard We investigate the behaviour of rare earth pyrochlore iridates Dy2Ir2O7 and Ho2Ir2O7 using dipolar Monte Carlo simulations for the rare earth moments and an effective representation of the Ir moments as local fields [Lefrancois et al, Nat. Comm. 2017]. We study in particular how the magnetisation and defect population respond to externally applied magnetic fields along different crystal directions. We uncover an intriguing interplay between the rare earth moments and the correlations in the underlying Ir moments, and we comment on their possible relevance to experimental results. |
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