arXiv subjects

Joseph Barker

Publications and source records attributed to Joseph Barker.

At least 19 recordsLinked to original sources

arXiv2026

Dynamical stability by spin transfer in nearly isotropic magnets

Spin transfer torques (STTs) control magnetisation by electric currents, enabling a range of nano-scale spintronic applications. They can destabilise the equilibrium magnetisation state by counteracting magnetic relaxation. Here, we maximise the STT effect through a dedicated growth-annealing protocol for CoFeB thin films, such that magnetic anisotropies originating from the interface and shape almost cancel each other. The nearly isotropic magnets enable low-current dynamical stabilisation of the magnetisation in the direction opposite to an applied magnetic field, thereby realising a spintronic analogue of the Kapitza pendulum. In an intermediate current regime, the STT drives large magnetisation vector fluctuations that cover the entire Bloch sphere. The continuous variable associated with the stochastic magnetisation direction may serve as a resource for probabilistic computing and neuromorphic hardware. Our results establish isotropic magnets as a platform to study as-yet-uncharted, far-from-equilibrium spin dynamics including anti-magnonics, with promising implications for unconventional computing paradigms.

cond-mat.mes-hall↗

arXiv2025

Probing frustrated metallo-molecular spin Kondo lattice interfaces through anomalous Nernst effect

Frustrated Kondo spin lattice (KSL) systems away from the antiferromagnetic (AFM) ground state have been found to display strange metal behaviour. A signature of strange metals in correlated systems is large Nernst response. Metallo-molecular interfaces of supramolecular lattices have been demonstrated as 2D KSL systems in STM studies. Here going beyond STM experiments we report a frustrated AFM state on molecular interfaces of Pt(111) and Pt(111)/Co films with around room temperature spin freezing transitions. Near these transitions we measure an anomalous Nernst coefficient of at least 3 $\mu$V/K.

cond-mat.str-el↗

arXiv2025

Explaining deep learning for ECG using time-localized clusters

Deep learning has significantly advanced electrocardiogram (ECG) analysis, enabling automatic annotation, disease screening, and prognosis beyond traditional clinical capabilities. However, understanding these models remains a challenge, limiting interpretation and gaining knowledge from these developments. In this work, we propose a novel interpretability method for convolutional neural networks applied to ECG analysis. Our approach extracts time-localized clusters from the model's internal representations, segmenting the ECG according to the learned characteristics while quantifying the uncertainty of these representations. This allows us to visualize how different waveform regions contribute to the model's predictions and assess the certainty of its decisions. By providing a structured and interpretable view of deep learning models for ECG, our method enhances trust in AI-driven diagnostics and facilitates the discovery of clinically relevant electrophysiological patterns.

cs.LG↗

arXiv2025

Data distribution impacts the performance and generalisability of contrastive learning-based foundation models of electrocardiograms

Contrastive learning is a widely adopted self-supervised pretraining strategy, yet its dependence on cohort composition remains underexplored. We present Contrasting by Patient Augmented Electrocardiograms (CAPE) foundation model and pretrain on four cohorts (n = 5,203,352), from diverse populations across three continents (North America, South America, Asia). We systematically assess how cohort demographics, health status, and population diversity influence the downstream performance for prediction tasks also including two additional cohorts from another continent (Europe). We find that downstream performance depends on the distributional properties of the pretraining cohort, including demographics and health status. Moreover, while pretraining with a multi-centre, demographically diverse cohort improves in-distribution accuracy, it reduces out-of-distribution (OOD) generalisation of our contrastive approach by encoding cohort-specific artifacts. To address this, we propose the In-Distribution Batch (IDB) strategy, which preserves intra-cohort consistency during pretraining and enhances OOD robustness. This work provides important insights for developing clinically fair and generalisable foundation models.

cs.LG↗

arXiv2025

Observing Differential Spin Currents by Resonant Inelastic X-ray Scattering

Controlling spin currents, i.e., the flow of spin angular momentum, in small magnetic devices is the principal objective of spin electronics, a main contender for future energy efficient information technologies. Surprisingly, a pure spin current has never been measured directly since the associated electric stray fields and/or shifts in the non-equilibrium spin-dependent distribution functions are too small for conventional experimental detection methods optimized for charge transport. Here we report that resonant inelastic x-ray scattering (RIXS) can bridge this gap by measuring the spin current carried by magnons -- the quanta of the spin wave excitations of the magnetic order -- in the presence of temperature gradients across a magnetic insulator. This is possible due to the sensitivity of the momentum- and energy-resolved RIXS intensity to minute changes in the magnon distribution under non-equilibrium conditions. We use the Boltzmann equation in the relaxation time approximation to extract transport parameters, such as the magnon lifetime at finite momentum, essential for the realization of magnon spintronics.

cond-mat.mes-hall↗

arXiv2025

Temperature gradient-driven motion of magnetic domains in a magnetic metal multilayer by entropic forces

We studied the displacement of magnetic domains under temperature gradients in perpendicularly magnetized Ta/[Pt/Co$_{68}$B$_{32}$/Ir]$_{\times 10}$/Pt multilayer tracks with microfabricated Pt heaters/thermometers by magnetic force microscopy (MFM). Subtracting out the effects of the Oersted field from the heating current reveals the pure temperature gradient-driven motion, which is always towards the heater. The higher the thermal gradient along the track (owing to proximity to the heater or larger heater currents), the greater the observed displacements of the domains, up to a velocity of around 1~nm/s in a temperature gradient of 20~K/$\mu$m. Quantitative estimates of the strength of different driving mechanisms show that entropic forces dominate over those arising from the spin Seebeck and spin-dependent Seebeck effects.

cond-mat.mes-hall↗

arXiv2025

Path integral spin dynamics with exchange and external field

In this work, we propose a path integral-inspired formalism for computing the quantum thermal expectation values of spin systems, when subject to magnetic fields that can be time-dependent and can accommodate the presence of Heisenberg exchange interactions between the spins. This is done by deriving an effective magnetic field from the quantum partition function of the system to use in classical atomistic spin dynamics simulations and generalises the formalism presented in our previous work [Phys. Rev. Research 5, 043075 (2023)]. In special cases where the effective field can be computed exactly we compare our results with exact/numerical diagonalisation methods for both ferromagnetic and antiferromagnetic coupling. We show that our method works well across a large temperature range and can reproduce quantum expectation values for antiferromagnetic coupling which is usually not possible with classical models.

quant-ph↗

arXiv2025

Learning to Predict Global Atrial Fibrillation Dynamics from Sparse Measurements

Catheter ablation of Atrial Fibrillation (AF) consists of a one-size-fits-all treatment with limited success in persistent AF. This may be due to our inability to map the dynamics of AF with the limited resolution and coverage provided by sequential contact mapping catheters, preventing effective patient phenotyping for personalised, targeted ablation. Here we introduce FibMap, a graph recurrent neural network model that reconstructs global AF dynamics from sparse measurements. Trained and validated on 51 non-contact whole atria recordings, FibMap reconstructs whole atria dynamics from 10% surface coverage, achieving a 210% lower mean absolute error and an order of magnitude higher performance in tracking phase singularities compared to baseline methods. Clinical utility of FibMap is demonstrated on real-world contact mapping recordings, achieving reconstruction fidelity comparable to non-contact mapping. FibMap's state-spaces and patient-specific parameters offer insights for electrophenotyping AF. Integrating FibMap into clinical practice could enable personalised AF care and improve outcomes.

cs.LG↗

arXiv2024

Origin of the laser-induced picosecond spin current across magnetization compensation in ferrimagnetic GdCo

The optical manipulation of magnetism enabled by rare earth-transition metal ferrimagnets holds the promise of ultrafast, energy efficient spintronic technologies. This work investigates laser-induced picosecond spin currents generated by ferrimagnetic GdCo via terahertz emission spectroscopy. A suppression of the THz emission and spin current is observed at magnetization compensation when varying the temperature or alloy composition in the presence of a magnetic field. It is demonstrated that this is due to the formation of domains in the GdCo equilibrium magnetic configuration. Without an applied magnetic field, the picosecond spin current persists at the compensation point. The experimental findings support the model for THz spin current generation based on transport of hot spin-polarized electrons, which is dominated by the Co sublattice at room temperature. Only at low temperature a comparable contribution from Gd is detected but with slower dynamics. Finally, spectral analysis reveals a blueshift of the THz emission related to the formation of magnetic domains close to magnetization compensation.

cond-mat.mes-hall↗

arXiv2024

Ultra-high spin emission from antiferromagnetic FeRh

An antiferromagnet emits spin currents when time-reversal symmetry is broken. This is typically achieved by applying an external magnetic field below and above the spin-flop transition or by optical pumping. In this work we apply optical pump-THz emission spectroscopy to study picosecond spin pumping from metallic FeRh as a function of temperature. Intriguingly we find that in the low-temperature antiferromagnetic phase the laser pulse induces a large and coherent spin pumping, while not crossing into the ferromagnetic phase. With temperature and magnetic field dependent measurements combined with atomistic spin dynamics simulations we show that the antiferromagnetic spin-lattice is destabilised by the combined action of optical pumping and picosecond spin-biasing by the conduction electron population, which results in spin accumulation. We propose that the amplitude of the effect is inherent to the nature of FeRh, particularly the Rh atoms and their high spin susceptibility. We believe that the principles shown here could be used to produce more effective spin current emitters. Our results also corroborate the work of others showing that the magnetic phase transition begins on a very fast picosecond timescale, but this timescale is often hidden by measurements which are confounded by the slower domain dynamics.

cond-mat.mtrl-sci↗

arXiv2023

Metadynamics calculations of the effect of thermal spin fluctuations on skyrmion stability

The stability of magnetic skyrmions has been investigated in the past, but mostly in the absence of thermal fluctuations. However, thermal spin fluctuations modify the magnetic properties (exchange stiffness, Dzyaloshinskii-Moriya interaction (DMI) and anisotropy) that define skyrmion stability. Thermal magnons also excite internal skrymion dynamics, deforming the skyrmion shape. Entropy has also been shown to modify skyrmion lifetimes in experiments, but is absent or approximated in previous studies. Here we use metadynamics to calculate the free energy surface of a magnetic thin film in terms of the topological charge and magnetization. We identify the free energy minima corresponding to different spin textures and the lowest energy paths between the ferromagnetic and single skyrmion states. We show that at low temperatures the lowest free energy barrier is a skyrmion collapse process. However, this energy barrier increases with temperature. An alternative path, where a singularity forms on the skrymion edge, has a larger free energy barrier at low temperatures but decreases with increasing temperature and eventually becomes the lowest energy barrier.

cond-mat.mtrl-sci↗

arXiv2023

Numerical Simulations of a Spin Dynamics Model Based on a Path Integral Approach

Inspired by path integral molecular dynamics, we build a spin model, in terms of spin coherent states, from which we can compute the quantum expectation values of a spin in a constant magnetic field, at finite temperature. This formulation facilitates the description of a discrete quantum spin system in terms of a continuous classical model and recasts the quantum spin effects within the framework of path integrals in a double $1/s$ and $\hbar s$ expansion, where $s$ is the magnitude of the spin. In particular, it allows for a much more direct path to the low- and high-temperature limits of the quantum system and to the definition of effective classical Hamiltonians that describe both thermal and quantum fluctuations. In this formalism, the quantum properties of the spins emerge as an effective anisotropy. We use atomistic spin dynamics to sample the path integral, calculate thermodynamic observables and show that our effective classical models can reproduce the thermal expectation values of the quantum system within temperature ranges relevant for studying magnetic ordering.

cond-mat.mtrl-sci↗

arXiv2023

Exploring interacting chiral spin chains in terms of black hole physics

In this paper we explore the properties of a 1-dimensional spin chain in the presence of chiral interactions, focusing on the system's transition to distinct chiral phases for various values of the chiral coupling. By employing the mean field theory approximation we establish a connection between this chiral system and a Dirac particle in the curved spacetime of a black hole. Surprisingly, the black hole horizon coincides with the interface between distinct chiral phases. We examine the chiral properties of the system for homogeneous couplings and in scenarios involving position dependent couplings that correspond to black hole geometries. To determine the significance of interactions in the chiral chain we employ bosonization techniques and derive the corresponding Luttinger liquid model. Furthermore, we investigate the classical version of the model to understand the impact of the chiral operator on the spins and gain insight into the observed chirality. Our findings shed light on the behavior of the spin chain under the influence of the chiral operator, elucidating the implications of chirality in various contexts, including black hole physics.

cond-mat.str-el↗

arXiv2023

Magnetic field-controlled lattice thermal conductivity in MnBi2Te4

Phonon properties and the lattice thermal conductivity are not generally understood to be sensitive to magnetic fields. Using an applied field to change MnBi2Te4 between antiferromagnetic (AFM), canted (CAFM) and ferromagnetic (FM) phases we discovered a new way to control the lattice thermal conductivity, generating both a positive and a negative magnetic field dependence. We report the field dependence of the thermal conductivity, k, in the in-plane direction under an applied magnetic field along the cross-plane direction in MnBi2Te4 from 2K to 30K. k decreases with field in the AFM phase, saturates in the CAFM phase, and increases with field in the FM phase. We explain this in terms of the field-induced changes of the magnon gap which modifies which magnon-phonon scattering processes are allowed by energy conservation. We also report magneto-Seebeck coefficient, Nernst coefficient and thermal Hall data measured in the same configuration. This finding may open a way to design magnetically controlled heat switches.

cond-mat.mtrl-sci↗

arXiv2022

The importance of the interface for picosecond spin pumping in antiferromagnet-heavy metal heterostructures

Interfaces between heavy metals (HMs) and antiferromagnetic insulators (AFIs) have recently become highly investigated and debated systems in the effort to create spintronic devices able to function at terahertz frequencies. Such heterostructures have great technological potential because AFIs can generate sub-picosecond spin currents which the HMs can convert into charge signals. In this work we demonstrate an optically induced picosecond spin transfer at the interface between AFIs and Pt using time-resolved THz emission spectroscopy. We select two antiferromagnets in the same family of fluoride cubic perovskites, KCoF3 and KNiF3, whose magnon frequencies at the centre of the Brillouin zone differ by an order of magnitude. By studying their behaviour with temperature we correlate changes in the spin transfer efficiency across the interface to the opening of a gap in the magnon density of states below the Néel temperature. Our observations are reproduced in a model based on the spin exchange between the localized electrons in the antiferromagnet and the free electrons in Pt. These results constitute an important step in the rigorous investigation and understanding of the physics of AFIs/HMs interfaces on the ultrafast timescale.

cond-mat.mes-hall↗

arXiv2022

Temperature gradient-driven magnetic skyrmion motion

The static and dynamic properties of skyrmions have recently received increased attention due to the potential application of skyrmions as information carriers and for unconventional computing. While the current-driven dynamics has been explored deeply, both theoretically and experimentally, the theory of temperature gradient-induced dynamics - Skyrmion-Caloritronics - is still at its early stages of development. Here, we move the topic forward by identifying the role of entropic torques due to the temperature dependence of magnetic parameters. Our results show that, skyrmions move towards higher temperatures in single-layer ferromagnets with interfacial Dzyaloshinski-Moriya interactions, whereas, in multilayers, they move to lower temperatures. We analytically and numerically demonstrate that the opposite behaviors are due to different scaling relations of the material parameters as well as a non-negligible magnetostatic field gradient in multilayers. We also find a spatially dependent skyrmion Hall angle in multilayers hosting hybrid skyrmions due to variations of the thickness dependent chirality as the skyrmion moves along the temperature gradient.

cond-mat.mes-hall↗

arXiv2022

Magnon Spectrum of the Amorphous Ferromagnet Co$_4$P from Atomistic Spin Dynamics

An anomaly in the magnon dispersion of the amorphous ferromagnet Co$_{4}$P, often referred to as a `roton-like' excitation, attracted much attention half a century ago. With the current interest in heat and spin currents in amorphous magnets, we apply modern simulation methods, combining reverse Monte Carlo to build the atomic structure and the stochastic Landau-Lifshitz equation for spin dynamics, to re-investigate the magnetic excitation spectrum. We find two magnon valleys, one at the origin and another at a finite wavenumber close to the observations, but without a magnon gap. We conclude that the second dip is due to Umklapp scattering caused by residual long-range order, which may be an alternative explanation of the putative roton excitation. Our study paves the way to study magnon transport in amorphous magnets and related spintronic applications.

cond-mat.mes-hall↗

arXiv2021

A review of modelling in ferrimagnetic spintronics

In this review we introduce computer modelling and simulation techniques which are used for ferrimagnetic materials. We focus on models where thermal effects are accounted for, atomistic spin dynamics and finite temperature macrospin approaches. We survey the literature of two of the most commonly modelled ferrimagnets in the field of spintronics--the amorphous alloy GdFeCo and the magnetic insulator yttrium iron garnet. We look at how generic models and material specific models have been applied to predict and understand spintronic experiments, focusing on the fields of ultrafast magnetisation dynamics, spincaloritronics and magnetic textures dynamics and give an outlook for modelling in ferrimagnetic spintronics.

cond-mat.mtrl-sci↗