arXiv subjects

Sarah Jenkins

Publications and source records attributed to Sarah Jenkins.

At least 19 recordsLinked to original sources

arXiv2026

Bridging the Semantic Chasm: Synergistic Conceptual Anchoring for Generalized Few-Shot and Zero-Shot OOD Perception

This manuscript presents a pioneering Synergistic Neural Agents Network (SynerNet) framework designed to mitigate the phenomenon of cross-modal alignment degeneration in Vision-Language Models (VLMs) when encountering Out-of-Distribution (OOD) concepts. Specifically, four specialized computational units - visual perception, linguistic context, nominal embedding, and global coordination - collaboratively rectify modality disparities via a structured message-propagation protocol. The principal contributions encompass a multi-agent latent space nomenclature acquisition framework, a semantic context-interchange algorithm for enhanced few-shot adaptation, and an adaptive dynamic equilibrium mechanism. Empirical evaluations conducted on the VISTA-Beyond benchmark demonstrate that SynerNet yields substantial performance augmentations in both few-shot and zero-shot scenarios, exhibiting precision improvements ranging from 1.2% to 5.4% across a diverse array of domains.

cs.CV↗

arXiv2024

Atomistic calculation of the f0 attempt frequency in Fe3O4 magnetite nanoparticles

The Arrhenius law predicts the transition time between equilibrium states in physical systems due to thermal activation, with broad applications in material science, magnetic hyperthermia and paleomagnetism where it is used to estimate the transition time and thermal stability of assemblies of magnetic nanoparticles. Magnetite is a material of great importance in paleomagnetic studies and magnetic hyperthermia but existing estimates of the attempt frequency $f_0$ vary by several orders of magnitude in the range $10^7-10^{13}$ Hz, leading to significant uncertainty in their relaxation rate. Here we present a dynamical method enabling full parameterization of the Arrhenius-N\'eel law using atomistic spin dynamics. We determine the temperature and volume dependence of the attempt frequency of magnetite nanoparticles with cubic anisotropy and find a value of $f_0 = 0.562 \pm 0.059$ GHz at room temperature. For particles with enhanced anisotropy we find a significant increase in the attempt frequency and a strong temperature dependence suggesting an important role of anisotropy. The method is applicable to a wide range of dynamical systems where different states can be clearly identified and enables robust estimates of domain state stabilities, with particular importance in the rapidly developing field of micromagnetic analysis of paleomagnetic recordings where samples can be numerically reconstructed to provide a better understanding of geomagnetic recording fidelity over geological time scales.

cond-mat.mtrl-sci↗

arXiv2023

Revealing the ultra-fast domain wall motion in Manganese Gold through permalloy capping

Antiferromagnets offer much faster dynamics compared to their ferromagnetic counterparts but their order parameter is extremely difficult to detect and control. So far, controlling the N\'eel order parameter electrically is limited to only very few materials where N\'eel spin-orbit torques are allowed by symmetry. In this work, we show that coupling a thin ferromagnet (permalloy) layer on top of an antiferromagnet (Mn$_2$Au) solves a major roadblock -- the controlled reading, writing, and manipulation of antiferromagnetic domains. We confirm by atomistic spin dynamics simulations that the domain wall patterns in the Mn$_2$Au are imprinted on the permalloy, therefore allowing for indirect imaging of the N\'eel order parameter. Our simulations show that the coupled domain wall structures in Mn$_2$Au-Py bilayers can be manipulated by either acting on the N\'eel order parameter via N\'eel spin-orbit torques or by acting on the magnetisation (the ferromagnetic order parameter) via magnetic fields. In both cases, we predict ultra-high domain wall speeds on the order of 8.5 km/s. Thus, employing a thin ferromagnetic layer has the potential to easily control the N\'eel order parameter in antiferromagnets even where N\'eel spin-orbit torques are forbidden by symmetry. The controlled manipulation of the antiferromagnetic order parameter provides a promising basis for the development of high-density storage and efficient computing technologies working in the THz regime.

cond-mat.mtrl-sci↗

arXiv2023

Local setting of spin textures in a granular antiferromagnet

Controlling the magnetic order of antiferromagnets is challenging due to their vanishing net magnetization. For this reason, the study of local spin textures in antiferromagnets is restricted by the difficulty in nucleating such states. Here, using atomistic simulations we demonstrate a method for nucleating localized spin textures in the grains of thin film antiferromagnet, $\gamma$-IrMn$_3$. Utilising the exchange bias coupling between a ferromagnet and an antiferromagnet, we set the spin texture in the latter from a predefined spin texture in the former by means of a thermal cycling procedure. The local textures set in the antiferromagnetic grains are shown to be stable against field perturbations. We also discuss how various material parameters affect the efficiency of the setting and the characteristics of these set textures. The setting of antiferromagnetic spin textures provides a potential route to antiferromagnetic spintronic devices with non-collinear spin states such as skyrmions, bubbles and domain walls.

cond-mat.mes-hall↗

arXiv2023

Single-shot laser-induced switching of an exchange biased antiferromagnet

Ultrafast manipulation of magnetic order has challenged our understanding the fundamental and dynamic properties of magnetic materials. So far single shot magnetic switching has been limited to ferrimagnetic alloys and multilayers. In ferromagnetic (FM)/antiferromagnetic (AFM) bilayers, exchange bias (He) arises from the interfacial exchange coupling between the two layers and reflects the microscopic orientation of the antiferromagnet. Here we demonstrate the possibility of single shot switching of the antiferromagnet (change of the sign and amplitude of He) with a single femtosecond laser pulse in IrMn/CoGd bilayers. We demonstrate the switching for a wide range of fluences for different layer thicknesses and compositions. Atomistic simulations predict ultrafast switching and recovery of the AFM magnetization on a timescale of 2 ps. These results provide the fastest and the most energy-efficient method to set the exchange bias and pave the way to potential applications for ultrafast spintronic devices.

cond-mat.mtrl-sci↗

arXiv2023

Multistep magnetization switching in orthogonally twisted ferromagnetic monolayers

The advent of twist-engineering in two-dimensional (2D) crystals enables the design of van der Waals (vdW) heterostructures exhibiting emergent properties. In the case of magnets, this approach can afford artificial antiferromagnets with tailored spin arrangements. Here, we fabricate an orthogonally-twisted bilayer by twisting 90 degrees two CrSBr ferromagnetic monolayers with an easy-axis in-plane anisotropy. The magneto-transport properties reveal multistep magnetization switching with a magnetic hysteresis opening, that is absent in the pristine case. By tuning the magnetic field, we modulate the remanent state and coercivity and select between hysteretic and non-hysteretic magneto-resistance scenarios. This complexity pinpoints spin anisotropy as a key aspect in twisted magnetic superlattices. Our results highlight the control over the magnetic properties in vdW heterostructures, leading to a variety of field-induced phenomena and opening a fruitful playground for creating desired magnetic symmetries and manipulating non-collinear magnetic configurations.

cond-mat.mes-hall↗

arXiv2022

Higher-order magnetic anisotropy in soft-hard magnetic materials

We have computationally studied the properties of higher-order magnetic anisotropy constants in an L10/A1-FePt coreshell system which is characterized by a strong second-order 2-ion Fe-Pt anisotropy component. We show that the coreshell structure induces an unexpected fourth-order anisotropy constant K2 the magnitude of which varies non-monotonically with the core-size ratio R reaching a peak at R = 0.50. Furthermore, we find that K2 scales with the normalized magnetization by (M/Ms)^2.2 at temperatures below the Curie temperature - a remarkable deviation from the established Callen-Callen theory which instead predicts a scaling exponent of 10. We construct an analytic model which demonstrates K2 arises from the canting of the core and shell magnetization, and successfully reproduces and justifies the scaling exponent obtained from numerical simulation.

cond-mat.mtrl-sci↗

arXiv2022

Breaking through the Mermin-Wagner limit in 2D van der Waals magnets

The Mermin-Wagner theorem states that long-range magnetic order does not exist in one- or two-dimensional (2D) isotropic magnets with short-ranged interactions. The theorem has been a milestone in magnetism and has been driving the research of recently discovered 2D van der Waals (vdW) magnetic materials from fundamentals up to potential applications. In such systems, the existence of magnetic ordering is typically attributed to the presence of a significant magnetic anisotropy, which is known to introduce a spin-wave gap and circumvent the core assumption of the theorem. Here we show that in finite-size 2D vdW magnets typically found in lab setups (e.g., within millimetres), short-range interactions can be large enough to allow the stabilisation of magnetic order at finite temperatures without any magnetic anisotropy for practical implementations. We demonstrate that magnetic ordering can be created in flakes of 2D materials independent of the lattice symmetry due to the intrinsic nature of the spin exchange interactions and finite-size effects in two-dimensions. Surprisingly we find that the crossover temperature, where the intrinsic magnetisation changes from superparamagnetic to a completely disordered paramagnetic regime, is weakly dependent on the system length, requiring giant sizes (e.g., of the order of the observable universe ~10$^{26}$ m) in order to observe the vanishing of the magnetic order at cryogenic temperatures as expected from the Mermin-Wagner theorem. Our findings indicate exchange interactions as the main driving force behind the stabilisation of short-range order in 2D magnetism and broaden the horizons of possibilities for exploration of compounds with low anisotropy at an atomically thin level.

cond-mat.mtrl-sci↗

arXiv2022

Spin-transfer and spin-orbit torques in the Landau-Lifshitz-Gilbert equation

Dynamic simulations of spin-transfer and spin-orbit torques are increasingly important for a wide range of spintronic devices including magnetic random access memory, spin-torque nano-oscillators and electrical switching of antiferromagnets. Here we present a computationally efficient method for the implementation of spin-transfer and spin-orbit torques within the Landau-Lifshitz-Gilbert equation used in micromagnetic and atomistic simulations. We consolidate and simplify the varying terminology of different kinds of torques into a physical action and physical origin that clearly shows the common action of spin torques while separating their different physical origins. Our formalism introduces the spin torque as an effective magnetic field, greatly simplifying the numerical implementation and aiding the interpretation of results. The strength of the effective spin torque field unifies the action of the spin torque and subsumes the details of experimental effects such as interface resistance and spin Hall angle into a simple transferable number between numerical simulations. We present a series of numerical tests demonstrating the mechanics of generalised spin torques in a range of spintronic devices. This revised approach to modelling spin-torque effects in numerical simulations enables faster simulations and a more direct way of interpreting the results, and thus it is also suitable to be used in direct comparisons with experimental measurements or in a modelling tool that takes experimental values as input.

physics.comp-ph↗

arXiv2021

Reservoir Computing with Magnetic Thin Films

Advances in artificial intelligence are driven by technologies inspired by the brain, but these technologies are orders of magnitude less powerful and energy efficient than biological systems. Inspired by the nonlinear dynamics of neural networks, new unconventional computing hardware has emerged with the potential to exploit natural phenomena and gain efficiency, in a similar manner to biological systems. Physical reservoir computing demonstrates this with a variety of unconventional systems, from optical-based to memristive systems. Reservoir computers provide a nonlinear projection of the task input into a high-dimensional feature space by exploiting the system's internal dynamics. A trained readout layer then combines features to perform tasks, such as pattern recognition and time-series analysis. Despite progress, achieving state-of-the-art performance without external signal processing to the reservoir remains challenging. Here we perform an initial exploration of three magnetic materials in thin-film geometries via microscale simulation. Our results reveal that basic spin properties of magnetic films generate the required nonlinear dynamics and memory to solve machine learning tasks (although there would be practical challenges in exploiting these particular materials in physical implementations). The method of exploration can be applied to other materials, so this work opens up the possibility of testing different materials, from relatively simple (alloys) to significantly complex (antiferromagnetic reservoirs).

cs.ET↗

arXiv2021

The atomistic origin of the athermal training effect in granular IrMn/CoFe bilayers

Anti-ferromagnetic materials have the possibility to offer ultra fast, high data density spintronic devices. A significant challenge is the reliable detection of the state of the antiferromagnet, which can be achieved using exchange bias. Here we develop an atomistic spin model of the athermal training effect, a well known phenomenon in exchange biased systems where the bias is significantly reduced after the first hysteresis cycle. We find that the setting process in granular thin films relies on the presence of interfacial mixing between the ferromagnetic and antiferromagnetic layers. We systematically investigate the effect of the intermixing and find that the exchange bias, switching field and coercivity all increase with increased intermixing. The interfacial spin state is highly frustrated leading to a systematic decrease in interfacial ordering of the ferromagnet. This metastable spin structure of initially irreversible spins leads to a large effective exchange coupling and thus large increase in the switching field. After the first hysteresis cycle these metastable spins drop into a reversible ground state that is repeatable for all subsequent hysteresis cycles, demonstrating that the effect is truly athermal. Our simulations provide new insights into the role of interface mixing and the importance of metastable spin structures in exchange biased systems which could help with the design an optimisation of antiferromagnetic spintronic devices.

cond-mat.mtrl-sci↗

arXiv2020

Properties and dynamics of meron topological spin textures in the two-dimensional magnet CrCl3

Merons are nontrivial topological spin textures highly relevant for many phenomena in solid state physics. Despite their importance, direct observation of such vortex quasiparticles is scarce and has been limited to a few complex materials. Here we show the emergence of merons and antimerons in recently discovered two-dimensional (2D) CrCl3 at zero magnetic field. We show their entire evolution from pair creation, their diffusion over metastable domain walls, and collision leading to large magnetic monodomains. Both quasiparticles are stabilized spontaneously during cooling at regions where in-plane magnetic frustration takes place. Their dynamics is determined by the interplay between the strong in-plane dipolar interactions and the weak out-of-plane magnetic anisotropy stabilising a vortex core within a radius of 8-10 nm. Our results push the boundary to what is currently known about non-trivial spin structures in 2D magnets and open exciting opportunities to control magnetic domains via topological quasiparticles.

cond-mat.mes-hall↗

arXiv2020

Quantum rescaling, domain metastability and hybrid domain-walls in two-dimensional CrI3 magnets

Higher-order exchange interactions and quantum effects are widely known to play an important role in describing the properties of low-dimensional magnetic compounds. Here we identify the recently discovered two-dimensional (2D) van der Waals (vdW) CrI3 as a quantum non-Heisenberg material with properties far beyond an Ising magnet as initially assumed. We find that biquadratic exchange interactions are essential to quantitatively describe the magnetism of CrI3 but requiring quantum rescaling corrections to reproduce its thermal properties. The quantum nature of the heat bath represented by discrete electron-spin and phonon-spin scattering processes induced the formation of spin fluctuations in the low temperature regime. These fluctuations induce the formation of metastable magnetic domains evolving into a single macroscopic magnetization or even a monodomain over surface areas of a few micrometers. Such domains display hybrid characteristics of Neel and Bloch types with a narrow domain wall width in the range of 3-5 nm. Similar behaviour is expected for the majority of 2D vdW magnets where higher-order exchange interactions are appreciable.

cond-mat.mes-hall↗

arXiv2020

The origin of exchange bias in multigranular non-collinear IrMn$_3$/CoFe thin films

Antiferromagnetic spintronic devices have the potential to outperform conventional ferromagnetic devices due to their ultrafast dynamics and high data density. A challenge in designing these devices is the control and detection of the orientation of the anti-ferromagnet. One of the most promising ways to achieve this is through the exchange bias effect. This is of particular importance in large scale multigranular devices. Due to the large system sizes, previously, only micromagnetic simulations have been possible, these have an assumed distribution of antiferromagnetic anisotropy directions. Here, we use an atomistic model where the distribution of antiferromagnetic anisotropy directions occurs naturally and the exchange bias occurs due to the intrinsic disorder in the antiferromagnet. We perform large scale simulations, generating realistic values of exchange bias. We find a strong temperature dependance of the exchange bias, which approaches zero at the blocking temperature while the coercivity has a peak at the blocking temeprature due to the superparamagnetic flipping of the antiferromagnet during the hysteresis loop. We find a large discrepancy between the exchange bias predicted from a geometric model of the antiferromagnetic interface indicating the importance of grain edge effects in multigranular exchange biased systems. The grain size dependence shows the expected peak due to a competition between the superparamagnetic nature of small grains and reduction in the statistical imbalance in the number of interfacial spins for larger grain sizes. Our simulations confirm the existence of single antiferromagnetic domains within each grain. The model gives insights into the physical origin of exchange bias and provides a route to developing optimised nanoscale antiferromagnetic spintronic devices.

cond-mat.mtrl-sci↗

arXiv2020

Spin wave excitations in exchange biased IrMn/CoFe bilayers

Using an atomistic spin model, we have simulated spin wave injection and propagation into antiferromagnetic IrMn from an exchange coupled CoFe layer. The spectral characteristics of the exited spin waves have a complex beating behavior arising from the non-collinear nature of the antiferromagnetic order. We find that the frequency response of the system depends strongly on the strength and frequency of oscillating field excitations. We also find that the strength of excited spin waves strongly decays away from the interfacial layer with a frequency dependent attenuation. Our findings suggest that spin waves generated by coupled ferromagnets are too weak to reverse IrMn in their entirety even with resonant excitation of a coupled ferromagnet. However, efficient spin wave injection into the antiferromagnet is possible due to the non-collinear nature of the IrMn spin ordering.

cond-mat.mtrl-sci↗

arXiv2020

Temperature scaling of two-ion anisotropy in pure and mixed anisotropy systems

Magnetic anisotropy plays an essential role in information technology applications of magnetic materials, providing a means to retain the long-term stability of a magnetic state in the presence of thermal fluctuations. Anisotropy consists of a single-ion contribution stemming from the crystal structure and two-ion terms attributed to the exchange interactions between magnetic atoms. A lack of robust theory crucially limits the understanding of the temperature dependence of the anisotropy in pure two-ion and mixed single-ion and two-ion systems. Here, we use Green's function theory and atomistic Monte Carlo simulations to determine the temperature scaling of the effective anisotropy in ferromagnets in these pure and mixed cases, from saturated to vanishing magnetization. At low temperature, we find that the pure two-ion anisotropy scales with the reduced magnetization as $k(m) \sim m^{2.28}$, while the mixed scenario describes the diversity of the temperature dependence of the anisotropy observed in real materials. The temperature dependence of the mixed anisotropy determined here is expected to considerably contribute to the understanding and the control of the thermal properties of magnetic materials.

cond-mat.mtrl-sci↗

arXiv2020

Role of anti-phase boundaries in the formation of magnetic domains in magnetite thin films

Anti-phase boundaries (APBs) are structural defects which have been shown to be responsible for the anomalous magnetic behaviour observed in different nanostructures. Understanding their properties is crucial in order to use them to tune the properties of magnetic materials by growing APBs in a controlled way since their density strongly depends on the synthesis method. With this aim, in this work we investigate their influence on magnetite (Fe$_3$O$_4$) thin films by considering an atomistic spin model, focusing our study on the role that the exchange interactions play across the APB interface. We conclude that the main atypical features reported experimentally in this material are well described by the model we propose here, confirming the new exchange interactions created in the APB as the responsible for this deviation from bulk properties.

cond-mat.mtrl-sci↗

arXiv2019

The role of faceting and elongation on the magnetic anisotropy of magnetite Fe3O4 nanocrystals

Fe3O4 nanoparticles are one of the most promising candidates for biomedical applications such as magnetic hyperthermia and theranostics due to their bio-compatibility, structural stability and good magnetic properties. However, much is unknown about the nanoscale origins of the observed magnetic properties of particles due to the dominance of surface and finite size effects. Here we have developed an atomistic spin model of elongated magnetite nanocrystals to specifically address the role of faceting and elongation on the magnetic shape anisotropy. We find that for faceted particles simple analytical formulae overestimate the magnetic shape anisotropy and that the underlying cubic anisotropy makes a significant contribution to the energy barrier for moderately elongated particles. Our results enable a better estimation of the effective magnetic anisotropy of highly crystalline magnetite nanoparticles and is a step towards quantitative prediction of the heating effects of magnetic nanoparticles.

cond-mat.mes-hall↗