arXiv subjects

Robert L. Stamps

Publications and source records attributed to Robert L. Stamps.

At least 19 recordsLinked to original sources

arXiv2026

Defect-mediated orientational ordering in stripe-forming magnetic systems

We investigate the nonequilibrium development of orientational order in a two-dimensional stripe-forming magnetic system with competing interactions. Following a quench into the stripe-ordered regime, horizontal and vertical stripes organize into orientational regions(super-domains) separated by characteristic defects. We show that the evolution towards a globally oriented stripe state is mediated by the motion and elimination of these defects, with T-junctions providing a direct microscopic measure of super-domain reorganization. The T-junction population reveals distinct dynamical regimes associated with super-domain formation, growth, and equilibration. Temperature, system size, and equilibrium stripe width modify the characteristic time scales of this evolution, while thermal fluctuations produce a finite defect population at higher temperatures. We further show that the defect dynamics observed in real space are reflected in the structure factor, providing a reciprocal-space signature of defect elimination. These results establish a microscopic defect-based description of orientational ordering in stripe-forming magnetic systems.

cond-mat.mes-hall↗

arXiv2025

Exceptional Antimodes in Multi-Drive Cavity Magnonics

Driven-dissipative systems provide a natural setting for the emergence of exceptional points -- i.e. non-Hermitian degeneracies where eigenmodes coalesce. These points are important for applications such as sensing, where enhanced sensitivity is required, and exhibit interesting and useful phenomena that can be controlled with experimentally accessible parameters. In this regard a four-port, three-mode, cavity-magnonics platform is demonstrated in which two microwave excitations can be precisely phase shifted and/or attenuated relative to one another. Destructive interference between the hybridised cavity-magnon modes is shown to give rise to antimodes (antiresonances) in the transmission spectrum, enabling coherent perfect extinction of the outgoing signals at selected ports. This interference can be used to actively tune the position and properties of exceptional points, without the fine tuning conventionally required to obtain exceptional points. Such controllable, interference-based engineering of exceptional points provides a practical and flexible pathway toward next-generation, high-sensitivity sensing devices operating at microwave frequencies.

cond-mat.mtrl-sci↗

arXiv2025

Monopole current control in artificial spin ice via localized fields

Artificial spin ice systems are metamaterials composed of interacting nanomagnets arranged on a lattice, exhibiting geometrical frustration and emergent phenomena such as monopole excitations. We explore magnetization dynamics and monopole current control in square artificial spin ice with added vertical control elements. Using Monte Carlo simulations, we examine how localized magnetic fields from these elements influence vertex configurations and domain propagation, enabling directional and polarity control of monopole currents. The control elements suppress monopole nucleation along one edge, steering monopole flow across the lattice, sometimes even against the applied field direction. These elements also reshape the system's energy landscape, producing tailored hysteresis and guided state transitions. Our results offer a strategy for manipulating collective behaviours in artificial spin ice using localized fields. This has implications for magnetic memory, physical reservoir computing, enabling reconfigurable magnetic logic and spin-based information processing, and device architectures requiring directional magnetic charge transport.

cond-mat.mes-hall↗

arXiv2024

Characterizing nanomagnetic arrays using restricted Boltzmann machines

Restricted Boltzmann machines are used for probabilistic learning and are capable of capturing complex dependencies in data. They are employed for diverse purposes such as dimensionality reduction, feature learning and can be used for representing and analyzing physical systems with minimal data. In this paper, we investigate a complex, strongly correlated magnetic spin system with multiple metastable states (magnetic artificial spin ice) using a restricted Boltzmann machine. Magnetic artificial spin ice is of interest because degeneracies can be specified leading to complex states that support unusual collective dynamics. We investigate two distinct geometries exhibiting different low-temperature orderings to evaluate the machine's performance and adaptability in capturing diverse magnetic behaviors. Data sets constructed with spin configurations importance-sampled from the partition function of square and pinwheel artificial spin ice Hamiltonians at different temperatures are used to extract features of distributions using a restricted Boltzmann machine. Results indicate that the restricted Boltzmann machine algorithm is sensitive to features that define the artificial spin ice configuration space and is able to reproduce the thermodynamic quantities of the system away from criticality - a feature useful for faster sample generation. Additionally, we demonstrate how the restricted Boltzmann machine can distinguish between different artificial spin ice geometries in data even when structural defects are present.

cond-mat.mes-hall↗

arXiv2024

Active Inference Demonstrated with Artificial Spin Ice

A numerical model of interacting nanomagnetic elements is used to demonstrate active inference with a three dimensional Artificial Spin Ice structure. It is shown that thermal fluctuations can drive this magnetic spin system to evolve under dynamic constraints imposed through interactions with an external environment as predicted by the neurological free energy principle and active inference. The structure is defined by two layers of magnetic nanoelements where one layer is a square Artificial Spin Ice geometry. The other magnetic layer functions as a sensory filter that mediates interaction between the external environment and the hidden Artificial Spin Ice layer. Spin dynamics displayed by the bilayer structure are shown to be well described using a continuous form of a neurological free energy principle that has been previously proposed as a high level description of certain biological neural processes. Numerical simulations demonstrate that this proposed bilayer geometry is able to reproduce theoretical results derived previously for examples of active inference in neurological contexts.

cond-mat.mes-hall↗

arXiv2022

Magnetic field driven dynamics in twisted bilayer artificial spin ice at superlattice angles

Geometrical designs of interacting nanomagnets have been studied extensively in the form of two dimensional arrays called artificial spin ice. These systems are usually designed to create geometrical frustration and are of interest for the unusual and often surprising phenomena that can emerge. Advanced lithographic and element growth techniques have enabled the realization of complex designs that can involve elements arranged in three dimensions. Using numerical simulations employing the dumbbell approximation, we examine possible magnetic behaviours for bilayer artificial spin ice (BASI) in which the individual layers are rotated with respect to one another. The goal is to understand how magnetization dynamics are affected by long-range dipolar coupling that can be modified by varying the layer separation and layer alignment through rotation. We consider bilayers where the layers are both either square or pinwheel arrangements of islands. Magnetic reversal processes are studied and discussed in terms of domain and domain wall configurations of the magnetic islands. Unusual magnetic ordering is predicted for special angles which define lateral spin superlattices for the bilayer systems.

cond-mat.mes-hall↗

arXiv2022

Mode attraction in Floquet systems with memory: application to magnonics

Level attraction is a type of mode hybridization in open systems where instead of forming a hybridization gap, the energy spectrum of two modes coalesce in a region bounded by exceptional points. We demonstrate that this phenomenon can be realized in a Floquet system with memory, which appears in describing linear excitations in a nonlinear driven system with a limit cycle. Linear response of the system in this state is different from its response near thermodynamic equilibrium. We develop a general formalism and provide an example in the context of cavity magnonics, where we show that magnetic excitations in systems driven far from the equilibrium may show level attraction with cavity photons. Our approach works equally well for quantum and semiclassical magnetic dynamics. The theory is formulated so that it can be used in combination with micromagnetic simulations to explore a wide range of experimentally interesting systems.

cond-mat.mes-hall↗

arXiv2021

Collective dynamics of domain walls: an antiferromagnetic spin texture in an optical cavity

Spin canting and complex spin textures in antiferromagnetic materials can often be described in terms of Dzyaloshinskii-Moriya interactions (DMI). Values for DMI parameters are not easily measurable directly, and often inferred from other quantities. In this work, we examine how domain wall dynamics in an antiferromagnetic optomagnonic system can display unique features directly related to the existence of DMI. Our results indicate that the presence of DMI enables spin interactions with cavity photons in a geometry which otherwise allows no magneto-optical coupling, and on the other hand modulates frequencies in a geometry where coupling is already realized in the absence of DMI. This result may be used to measure the DMI constant in optomagnonic experiments by comparing resonances obtained with different polarisations of the exciting field.

cond-mat.mtrl-sci↗

arXiv2020

Level attraction and exceptional points in a resonant spin-orbit torque system

Level attraction can appear in driven systems where instead of repulsion two modes coalesce in a region separated by two exceptional points. This behavior was proposed for optomechanical and optomagnonic systems, and recently observed for dissipative cavity magnon-polaritons. We demonstrate that such a regime exists in a spin-orbit torque system where a magnetic oscillator is resonantly coupled to an electron reservoir. An instability mechanism necessary for mode attraction can be provided by applying an electric field. The field excites interband transitions between spin-orbit split bands leading to an instability of the magnetic oscillator. Two exceptional points then appear in the oscillator energy spectrum and the region of instability. We discuss conditions under which this can occur and estimate the electric field strength necessary for reaching the attraction region for a spin-orbit torque oscillator with Rashba coupling. A proposal for experimental detection is made using magnetic susceptibility measurements.

cond-mat.mes-hall↗

arXiv2020

An Electromagnetic Approach to Cavity Spintronics

The fields of cavity quantum electrodynamics and magnetism have recently merged into \textit{`cavity spintronics'}, investigating a quasiparticle that emerges from the strong coupling between standing electromagnetic waves confined in a microwave cavity resonator and the quanta of spin waves, magnons. This phenomenon is now expected to be employed in a variety of devices for applications ranging from quantum communication to dark matter detection. To be successful, most of these applications require a vast control of the coupling strength, resulting in intensive efforts to understanding coupling by a variety of different approaches. Here, the electromagnetic properties of both resonator and magnetic samples are investigated to provide a comprehensive understanding of the coupling between these two systems. Because the coupling is a consequence of the excitation vector fields, which directly interact with magnetisation dynamics, a highly-accurate electromagnetic perturbation theory is employed which allows for predicting the resonant hybrid mode frequencies for any field configuration within the cavity resonator, without any fitting parameters. The coupling is shown to be strongly dependent not only on the excitation vector fields and sample's magnetic properties but also on the sample's shape. These findings are illustrated by applying the theoretical framework to two distinct experiments: a magnetic sphere placed in a three-dimensional resonator, and a rectangular, magnetic prism placed on a two-dimensional resonator. The theory provides comprehensive understanding of the overall behaviour of strongly coupled systems and it can be easily modified for a variety of other systems.

cond-mat.mes-hall↗

arXiv2020

Symmetry Approach to Chiral Optomagnonics in Antiferromagnetic Insulators

We discuss several aspects of chiral optomagnonics in antiferromagnetic insulators by considering common symmetries between the electromagnetic field and spin excitations. This approach allows us to look at optical and magnetic materials from similar perspectives, and discuss useful analogies between them. We show that spin waves in collinear antiferromagnets and the electromagnetic field in vacuum are both invariant under the same eight-dimensional algebra of symmetry transformations. By such analogy, we can extend the concept of optical chirality to antiferromagnetic insulators, and demonstrate that the spin-wave dynamics in these materials in the presence of a spin current is similar to that of the light inside chiral metamaterials. Photo-excitation of magnonic spin currents is also discussed from the symmetry point of view. It is demonstrated that a direct magnonic spin photocurrent can be exited by circularly polarized light, which can be considered as a magnonic analogue of the photogalvanic effect. We also note that the Zitterbewegung process should appear and may play a role in photo-excitation processes.

cond-mat.mes-hall↗

arXiv2020

Magnetization dynamics of weakly interacting sub-100 nm square artificial spin ices

Artificial Spin Ice (ASI), consisting of a two dimensional array of nanoscale magnetic elements, provides a fascinating opportunity to observe the physics of out of equilibrium systems. Initial studies concentrated on the static, frozen state, whilst more recent studies have accessed the out-of-equilibrium dynamic, fluctuating state. This opens up exciting possibilities such as the observation of systems exploring their energy landscape through monopole quasiparticle creation, potentially leading to ASI magnetricity, and to directly observe unconventional phase transitions. In this work we have measured and analysed the magnetic relaxation of thermally active ASI systems by means of SQUID magnetometry. We have investigated the effect of the interaction strength on the magnetization dynamics at different temperatures in the range where the nanomagnets are thermally active and have observed that they follow an Arrhenius-type Néel-Brown behaviour. An unexpected negative correlation of the average blocking temperature with the interaction strength is also observed, which is supported by Monte Carlo simulations. The magnetization relaxation measurements show faster relaxation for more strongly coupled nanoelements with similar dimensions. The analysis of the stretching exponents obtained from the measurements suggest 1-D chain-like magnetization dynamics. This indicates that the nature of the interactions between nanoelements lowers the dimensionality of the ASI from 2-D to 1-D. Finally, we present a way to quantify the effective interaction energy of a square ASI system, and compare it to the interaction energy calculated from a simple dipole model and also to the magnetostatic energy computed with micromagnetic simulations.

cond-mat.dis-nn↗

arXiv2019

Heisenberg pseudo-exchange and emergent anisotropies in field-driven pinwheel artificial spin ice

Rotating all islands in square artificial spin ice (ASI) uniformly about their centres gives rise to the recently reported pinwheel ASI. At angles around 45$^\mathrm{o}$, the antiferromagnetic ordering changes to ferromagnetic and the magnetic configurations of the system exhibit near-degeneracy, making it particularly sensitive to small perturbations. We investigate through micromagnetic modelling the influence of dipolar fields produced by physically extended islands in field-driven magnetisation processes in pinwheel arrays, and compare the results to hysteresis experiments performed in-situ using Lorentz transmission electron microscopy. We find that magnetisation end-states induce a Heisenberg pseudo-exchange interaction that governs both the inter-island coupling and the resultant array reversal process. Symmetry reduction gives rise to anisotropies and array-corner mediated avalanche reversals through a cascade of nearest-neighbour (NN) islands. The symmetries of the anisotropy axes are related to those of the geometrical array but are misaligned to the array axes as a result of the correlated interactions between neighbouring islands. The NN dipolar coupling is reduced by decreasing the island size and, using this property, we track the transition from the strongly coupled regime towards the pure point dipole one and observe modification of the ferromagnetic array reversal process. Our results shed light on important aspects of the interactions in pinwheel ASI, and demonstrate a mechanism by which their properties may be tuned for use in a range of fundamental research and spintronic applications.

cond-mat.mes-hall↗

arXiv2019

Microscopic origin of level attraction for a coupled magnon-photon system in a microwave cavity

We discuss various microscopic mechanisms for level attraction in a hybridized magnon-photon system of a ferromagnet in a microwave cavity. The discussion is based upon the electromagnetic theory of continuous media where the effects of the internal magnetization dynamics of the ferromagnet are described using dynamical response functions. This approach is in agreement with quantized multi-oscillator models of coupled photon-magnon dynamics. We demonstrate that to provide the attractive interaction between the modes, the effective response functions should be diamagnetic. Magneto-optical coupling is found to be one mechanism for the effective diamagnetic response, which is proportional to photon number. A dual mechanism based on the Aharonov-Casher effect is also highlighted, which is instead dependent on magnon number.

cond-mat.mes-hall↗

arXiv2019

Ice-rule made manifold: phase transitions, topological defects and manifold restoration in two-dimensional artificial spin systems

Artificial spin ices are arrays of correlated nano-scale magnetic islands that prove an excellent playground in which to study the role of topology in critical phenomena. Here, we investigate a continuum of spin ice geometries, parameterised by rotation of the islands. In doing so, we morph from the classic square ice to the recently studied pinwheel geometry, with the rotation angle acting as a proxy for controlling inter-island interactions. We experimentally observe a change in ground state magnetic order from antiferromagnetic to ferromagnetic across this class of geometries using Lorentz transmission electron microscopy on thermally annealed cobalt arrays. The change in ordering leads to an apparent change in the nature of the defects supported: from one-dimensional strings in the antiferromagnetic phase to two-dimensional vortex-like structures in the ferromagnetic one, consistent with the scaling predicted by the Kibble-Zurek mechanism. Our results show how magnetic order in artificial spin ices can be tuned by changes in geometry so that a truly frustrated ice-rule phase is possible in two-dimensional systems. Furthermore, we demonstrate this system as a testbed to investigate out-of-equilibrium dynamics across phases.

cond-mat.mes-hall↗

arXiv2019

Cavity Optomechanics of Topological Spin Textures in Magnetic Insulators

Collective dynamics of topological magnetic textures can be thought of as a massive particle moving in a magnetic pinning potential. We demonstrate that inside a cavity resonator this effective mechanical system can feel the electromagnetic radiation pressure from cavity photons through the magneto-optical inverse Faraday and Cotton-Mouton effects. We estimate values for the effective parameters of the optomechanical coupling for two spin textures -- a Bloch domain wall and a chiral magnetic soliton lattice. The soliton lattice has magnetic chirality, so that in circularly polarized light it behaves like a chiral particle with the sign of the optomechanical coupling determined by the helicity of the light and chirality of the lattice. Most interestingly, we find a level attraction regime for the soliton lattice, which is tunable through an applied magnetic field.

cond-mat.mes-hall↗

arXiv2018

Oriented Asymmetric Wave Propagation and Refraction Bending in Hyperbolic Media

Crystal quartz is a well-known anisotropic medium with optically active phonons in the THz region where hyperbolic phonon-polaritons can be excited. Here, we use this material to illustrate how the behavior of bulk and surface hyperbolic polaritons can be drastically modified by changing the orientation of the crystal's anisotropy axis with respect to its surface. We demonstrate, both theoretically and experimentally, phenomena associated with the orientation of hyperbolic media. We show the consequences of slight changes in the crystal's orientation in various ways, from the creation of hyperbolic surface phonon-polaritons to the demonstration of oriented asymmetric transmission of radiation passing through a hyperbolic medium.

cond-mat.mtrl-sci↗

arXiv2018

Excitation of magnon spin photocurrents in antiferromagnetic insulators

In the circular photogalvanic effect, circularly polarized light can produce a direct electron photocurrent in metals and the direction of the current depends on the polarization. We suggest that an analogous nonlinear effect exists for antiferromagnetic insulators wherein the total spin of light and spin waves is conserved. In consequence, a spin angular momentum is expected to be transfered from photons to magnons so that a circularly polarized electromagnetic field will generate a direct magnon spin current. The direction of the current is determined by the helicity of the light. We show that this resonant effect appears as a second order light-matter interaction. We find also a geometric contribution to the spin photocurrent, which appears for materials with complex lattice structures and Dzyaloshinskii-Moriya interactions.

cond-mat.mes-hall↗