arXiv subjects

Elizabeth M. Jefremovas

Publications and source records attributed to Elizabeth M. Jefremovas.

10 recordsLinked to original sources

arXiv2026

Polarized neutron scattering as a probe for vortex-type spin correlations in iron oxide multicore assemblies

We report an experimental investigation of the magnetic microstructure of iron oxide multicore assemblies by means of polarized small-angle neutron scattering (SANS). Guided by a recently developed analytical theory for vortex-state magnetic nanoparticles, we provide a quantitative comparison between the measured and calculated cross sections, revealing signatures that are consistent with vortex-type magnetization configurations at low applied magnetic fields. In particular, the field evolution and the characteristic isotropic ring-type feature of the spin-flip scattering intensity at intermediate momentum transfers are in line with the formation of flux-closure states. The latter are stabilized by the interplay of exchange, Zeeman, and magnetostatic energies. The methodology allows for a statistically significant characterization of vortex states in densely packed nanoparticle systems, thereby complementing surface-sensitive techniques that are commonly limited to the observation of spin structures in individual particles.

cond-mat.mes-hall↗

arXiv2026

Skyrmion Lattice Domain Formation in a Non-Flat Energy Landscape

Magnetic skyrmions are chiral spin structures with non-trivial topology that comprise two-dimensional quasi-particles and are promising information carriers for data storage and processing devices. Skyrmion lattices in magnetic thin films exhibit Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) phase transitions and have garnered significant interest for studying emergent 2D phase behavior. In experimental skyrmion lattices, the main factor limiting the quasi-long-range order in thin films has been the non-flat energy landscape - often referred to as pinning effects. We demonstrate direct control of the skyrmion lattice order by effectively tuning the energy landscape employing magnetic field oscillations. By quantifying lattice order and dynamics, we explore how domain boundaries form and evolve due to pinning effects in Kerr microscopy experiments and in Brownian dynamics simulations, offering a pathway to control and study emergent skyrmion lattice properties and 2D phase behavior.

cond-mat.mes-hall↗

arXiv2026

Angular anisotropy landscape of vortex ensembles in polarized small-angle neutron scattering

We present a symmetry-resolved classification of two-dimensional spin-flip small-angle neutron scattering (SANS) patterns arising from dilute ensembles of spherical nanoparticles hosting magnetic vortex states. Based on a linear vortex ansatz with an axially symmetric distribution of vortex axes and the corresponding analytical expression for the orientationally averaged spin-flip SANS cross section, we show that the angular scattering patterns organize into four distinct symmetry regimes: a four-fold anisotropy corresponding to coherent field-aligned magnetization, vertical and horizontal two-fold anisotropies associated with aligned and isotropically distributed vortex ensembles, and an isotropic ring-like condition separating the two two-fold regimes. The corresponding symmetry boundaries are obtained analytically and define a compact symmetry landscape in the parameter space of vortex amplitude and vortex-axis distribution width. Comparison with a nonlinear vortex profile shows that these symmetry regions are robust with respect to the detailed radial structure of the vortex core. The angular anisotropies are therefore governed primarily by rotational symmetry and by the statistical distribution of vortex axes, providing a compact and model-transparent classification framework of experimental polarized SANS data.

cond-mat.mes-hall↗

arXiv2026

Nanoscale mapping of internal magnetization dynamics reveals how disorder shapes heat generation in magnetic particle hyperthermia

Magnetic particle hyperthermia relies on the efficient conversion of magnetic field energy into heat in biomedical applications, yet the microscopic mechanisms governing heat generation within individual particles remain poorly understood. In this study, AC magnetometry experiments are combined with dynamic micromagnetic simulations to connect microstructural features, magnetization dynamics, and macroscopic heat dissipation. Beyond macroscopic heating metrics, the heat generation is resolved at the intra-particle level, uncovering a heterogeneous landscape of localized ''hot spots'' with nanometer spatial and nanosecond temporal resolution. The results demonstrate that grain size acts as a key experimentally tunable parameter, balancing anisotropy disorder and pinning strength, thereby controlling both the magnitude and spatio-temporal distribution of heat release within the particle. In particular, nanoflower architectures composed by larger grains deliver larger heat generation, while the smaller grains offer a deeper intra-particle pinning landscape, which effectively redistributes the heat generation over extended time windows. Together, our results provide a mechanistic framework linking nanoparticle microstructure to magnetic heating and establish design principles for optimizing nanoflowers as magnetic hyperthermia transducers.

cond-mat.mes-hall↗

arXiv2025

Controlling Skyrmion Lattice Orientation with Local Magnetic Field Gradients

Precise control over the formation and arrangement of magnetic skyrmion lattices is essential for understanding their emergent behavior and advancing their integration into spintronic and magnonic devices. We report on a simple and minimally invasive technique to nucleate and manipulate skyrmion lattices in soft magnetic CoFeB using single-pass magnetic force microscopy (MFM). By tuning the scan-line spacing to match the intrinsic stripe domain periodicity, the stray field gradient from the MFM tip induces reversible transitions from stripe domains to isolated skyrmions and locally ordered lattices. The resulting skyrmion positions are extracted to compute the local orientational order parameter $ψ_6$, enabling quantitative evaluation of lattice ordering. A systematic improvement in $\langle |ψ_6| \rangle$ is observed with repeated scanning, indicating a transition from a disordered state to ordered hexagonal lattices. Furthermore, we demonstrate that the lattice orientation can be deterministically rotated by changing the scanning direction, as confirmed by both real-space analysis and fast Fourier transformations. This method enables the controlled creation, reordering, and deletion of metastable skyrmion textures on demand. Our approach establishes a practical and accessible platform for studying two-dimensional phase behavior in topological spin systems, offering direct and reconfigurable control over lattice symmetry, order, and orientation.

cond-mat.mes-hall↗

arXiv2025

Skyrmion Lattice Order Controlled by Confinement Geometry

Magnetic skyrmions forming two-dimensional (2D) lattices provide a versatile platform for investigating phase transitions predicted by Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory. While 2D melting in skyrmion systems has been demonstrated, achieving controlled ordering in skyrmion lattices remains challenging due to pinning effects from a non-uniform energy landscape, which often results in polycrystalline structures. Skyrmions in thin films, however, offer thermal diffusion with high tunability and can be directly imaged via Kerr microscopy, enabling real-time observation of their dynamics. To regulate lattice order in such flexible systems, we introduce geometric confinements of varying shapes. Combining Kerr microscopy experiments with Thiele model simulations, we demonstrate that confinement geometry critically influences lattice order. Specifically, hexagonal confinements commensurate with the skyrmion lattice stabilize monodomain hexagonal ordering, while incommensurate geometries induce domain formation and reduce overall order. Understanding these boundary-driven effects is essential for advancing the study of 2D phase behavior and for the design of skyrmion-based spintronic applications, ranging from memory devices to unconventional computing architectures.

cond-mat.mes-hall↗

arXiv2025

Micromagnetic structure of oxidized magnetite nanoparticles: sharp structural versus diffuse magnetic interface

The oxidation of magnetite to maghemite is a naturally occurring process that leads to the degradation of the magnetic properties of magnetite nanoparticles. Despite being systematically observed with traditional macroscopic magnetization measurement techniques, a detailed understanding of this process at the microscale is still missing. In this study, we track the evolution of the magnetic structure of magnetite nanoparticles during their oxidation to maghemite through numerical micromagnetic simulations. To capture realistic interparticle effects, we incorporate dipolar interactions by modeling the nanoparticles arranged in chains. Our computational results are benchmarked against experimental data from magnetotactic bacteria, studied over a time scale of years. To resolve the magnetization at the interface between both oxide phases, we propose spin-polarized small-angle neutron scattering (SANS), an experimental technique capable of probing magnetization textures at nanometer length scales. By analyzing the pair-distance distribution function extracted from SANS, we identify distinct signatures of magnetic disorder. Specifically, our findings suggest that the magnetization from the non-oxidized core region varies smoothly across the (structurally sharp) interface into the oxidized shell. The existence of such a diffuse magnetic interface may account for the superior magnetic properties of partially oxidized magnetite nanoparticles compared to fully converted maghemite samples.

cond-mat.mes-hall↗

arXiv2025

Realizing Quantitative Quasiparticle Modeling of Skyrmion Dynamics in Arbitrary Potentials

We demonstrate fully quantitative Thiele model simulations of magnetic skyrmion dynamics on previously unattainable experimentally relevant large length and time scales by ascertaining the key missing parameters needed to calibrate the experimental and simulation time scales and current-induced forces. Our work allows us to determine complete spatial pinning energy landscapes that enable quantification of experimental studies of diffusion in arbitrary potentials within the Lifson-Jackson framework. Our method enables us to ascertain the time scales, and by isolating the effect of ultra-low current density (order $10^6 A/m^2$) generated torques we directly infer the total force acting on the skyrmion for a quantitative modelling.

cond-mat.stat-mech↗

arXiv2025

Imaging Topological Defect Dynamics Mediating 2D Skyrmion Lattice Melting

Topological defects are the key feature mediating 2D phase transitions. However, both resolution and tunability have been lacking to access the dynamics of the transitions. With dynamic Kerr microscopy, we directly capture the melting of a confined 2D magnetic skyrmion lattice with high resolution in real-time and -space. Skyrmions in magnetic thin films are two-dimensional, topologically non-trivial quasi-particles that provide rich dynamics as well as unique tunability as an essential ingredient for controlling phase behavior: We tune the skyrmion size and effective temperature on the fly to drive the two-step melting through an intermediate hexatic regime between the solid lattice and the isotropic liquid. We quantify the characteristic occurrence of topological defects mediating the transitions and reveal the so-far inaccessible dynamics of the lattice dislocations. The full real-time and -space imaging reveals the diffusion coefficient of the dislocations, which we find to be orders of magnitudes higher than that of the skyrmions.

cond-mat.stat-mech↗

arXiv2024

Experimental realization of metastable target skyrmion states in continuous films

Target skyrmions (TSks) are topological spin textures where the out-of-plane component of the magnetization twists an integer number of $m$-$π$ rotations. Based on a magnetic multilayer stack in the form of $n\times$[CoFeB/MgO/Ta], engineered to host topological spin textures via dipole and DMI energies, we have successfully stabilized 1$π$, 2$π$ and 3$π$ target skyrmions by tuning material properties and thermal excitations close to room temperature. The nucleated textures, imaged via Kerr and Magnetic Force Microscopies, are stable at zero magnetic field and robust within a range of temperatures (tens of Kelvin) close to room temperature (RT = 292 K) and over long time scales (months). Under applied field (mT), the TSks collapse into the central skyrmion core, which resists against higher magnetic fields ($\approx$ 2 $\times$ TSk annihilation field), as the core is topologically protected. Micromagnetic simulations support our experimental findings, showing no TSk nucleation at 0 K, but a $\approx$ 30 $\%$ probability at 300 K for the experimental sample parameters. Our work provides a simple method to tailor spin textures in continuous films, enabling free movement in 2D space, creating a platform transferable to technological applications where the dynamics of the topological textures can be exploited beyond geometrical confinements.

cond-mat.mtrl-sci↗