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Vassilios Kapaklis

Publications and source records attributed to Vassilios Kapaklis.

At least 19 recordsLinked to original sources
arXiv2026

Mapping Reversal Pathways and Interaction Fields in Artificial Spin Ice

In artificial spin ice (ASI), magnetic interactions between nanomagnets determine both the stable states and the switching pathways under an applied field. Here, first-order reversal curve (FORC) measurements are used to map how these interactions govern magnetization reversal in square arrays as the element shape and spacing are varied. The FORC diagrams show that some geometries reverse more uniformly, whereas others exhibit broader, more asymmetric responses, indicating stronger interaction effects and more complex reversal pathways. Combined FORC analysis and micromagnetic simulations also capture subtle changes in internal magnetization textures during switching, linking local behavior within individual elements to collective behavior across the array. These results establish FORC as a practical tool for mapping and engineering interaction landscapes, with direct relevance to reconfigurable magnetic reservoirs and neuromorphic functionality.

cond-mat.mtrl-sci↗
arXiv2026

Long-Range Magnetic Order in Structurally Embedded Mesospin Metamaterials

Engineered assemblies of interacting magnetic elements-magnetic metamaterials-provide a powerful route to tailor collective magnetic order and dynamics. By structuring matter at the mesoscale, they bridge atomic magnetism and macroscopic functionality, enabling emergent behaviour inaccessible in conventional materials. However, realizing large-area metamaterials that combine high morphological uniformity with intrinsic long-range order has remained challenging, largely due to the structural disorder inherent to lithographic fabrication. Here we demonstrate a scalable route to structurally and magnetically coherent metamaterials by embedding iron-ions to form mesospins within a non-magnetic thin film palladium host matrix. Using controlled implantation, we realize morphologically uniform arrays that spontaneously develop extended antiferromagnetic order in the as-fabricated state - without the need of external annealing or field cycling. Resonant X-ray scattering and microscopy reveal sharp magnetic Bragg peaks modulated by the mesospin form factor, evidencing long-range antiferromagnetic order coupled to structural coherence. This embedded architecture establishes a platform for exploring coherent spin-photon interactions and functional X-ray scattering in magnetic metamaterials free from lithographic topography and disorder.

cond-mat.mes-hall↗
arXiv2026

GenL: An extensible fitting program for Laue oscillations and whole pattern fitting

GenL is a flexible program that can be used to simulate and/or fit X-ray reflectivity and X-ray diffraction data from epitaxial thin films exhibiting, for example, Laue oscillations. It utilizes a differential evolution within a genetic algorithm for fitting data and uses a modular approach based on either the kinematic theory of diffraction or the dynamic theory. Effects of polarization, absorption, the Lorentz factor, as well as instrumental resolution and lattice vibrations are taken into account. Useful parameters that can be extracted after fitting include atomic interplanar spacings, number of coherently scattering atomic planes, strain profiles along the film thickness, and crystal roughness. The program has been developed in MATLAB and employs a graphical user interface. The deployment strategy is twofold, whereby the software can either be obtained in source code form and executed within the MATLAB environment, or as a pre-compiled binary for those who prefer not to run it within MATLAB. Finally, GenL can be easily extended to simulate multilayered film systems, superlattices, and films with atomic steps. The program is released under the GNU General Public License.

cond-mat.mtrl-sci↗
arXiv2025

Quantitative measurement of magnetic dichroic signals at sub-nanometer to atomic scale resolution conditions

When aiming for atomic resolution electron magnetic circular dichroism (EMCD) in STEM mode, the high convergence angle of the electron probe can lead to unforeseen artefacts and strong reductions of the signal to noise ratio (SNR) in the measurement of magnetic magnitudes. In this work, the EMCD signal is measured in STEM mode at semi-convergence angles ranging from α=\ 2-10\ mrad\ using iron samples with high structural perfection. We observe that the relative EMCD signal remains very similar for all convergence angles, which is in good agreement with simulations. The improvement in the signal-to-noise ratio as compared to earlier works is due to better crystalline quality of the Fe sample, high control of the sample orientation, better control over sample thickness, and factors such as higher beam current and the use of a fast and sensitive hybrid-pixel electron detector. One of the key factors to remove experimental ambivalences is the use of an acquisition geometry where the two conjugate EMCD spectra are acquired simultaneously. Furthermore the high crystalline quality Fe sample enables the acquisition of statistical entities of spectra at the same sample orientation, which in turn results in high SNR EMCD signals. We obtain high SNR EMCD signals for all semi convergence angles and could measure the orbital to spin magnetic moment ratio ml/ms with high precision. We observed that at high convergence angles, the ml/ms ratio gets sensitive to minute changes in sample orientation.

physics.ins-det↗
arXiv2025

Magnetic texture control in ion-implanted metamaterials

We study experimentally the impact of the additive fabrication method on the magnetic properties of Fe$^+$-implanted Pd square artificial spin ice lattices. Our findings show that the lattices exhibit a higher ordering temperature than their continuous film counterparts. This behavior is attributed to the additive fabrication process, which induces an inhomogeneous Fe concentration within the lattice building blocks. Moreover, the implantation process creates a magnetic depth profile, enabling temperature-dependent tunability of the magnetic thickness. These additional internal degrees of freedom broaden the design possibilities for magnetic metamaterials, allowing precise fine-tuning of their static and dynamic properties to achieve complex and customizable behaviors.

cond-mat.mes-hall↗
arXiv2025

Exponential Dependence of Interlayer Exchange Coupling in Fe/MgO(001) Superlattices on Temperature

The interlayer exchange coupling in Fe/MgO(001) superlattices is found to increase exponentially with decreasing temperature. Around 150~K, the field induced response changes from discrete switching, governed by field-driven domain propagation, to a collective rotation of the magnetic layers. This transition is accompanied by a change in the magnetic ground state from 180$^{\circ}$ (antiferromagnetic) to 90$^{\circ}$ alignment between adjacent Fe layers. These effects are argued to arise from quantum well states, defined by the total thickness of the samples.

cond-mat.mes-hall↗
arXiv2024

Magnetic Order and Long-Range Interactions in Mesoscopic Ising Chains

We investigate the design of magnetic ordering in one-dimensional mesoscopic magnetic Ising chains by modulating long-range interactions. These interactions are affected by geometrical modifications to the chain, which adjust the energy hierarchy and the resulting magnetic ground states. Consequently, the magnetic ordering can be tuned between antiferromagnetic and dimer antiferromagnetic phases. These phases are experimentally observed in chains fabricated using both conventional electron-beam lithography and ion implantation techniques, demonstrating the feasibility of controlling magnetic properties at the mesoscale. The ability of attaining these magnetic structures by thermal annealing, underlines the potential of using such systems instead of simulated annealers in tackling combinatorial optimization tasks.

cond-mat.mes-hall↗
arXiv2024

Magnetic metamaterials by ion-implantation

We present a method for the additive fabrication of planar magnetic nanoarrays with minimal surface roughness. Synthesis is accomplished by combining electron-beam lithography, used to generate nanometric patterned masks, with ion implantation in thin films. By implanting $^{56}$Fe$^{+}$ ions, we are able to introduce magnetic functionality in a controlled manner into continuous Pd thin films, achieving 3D spatial resolution down to a few tens of nanometers. Our results demonstrate the application of this technique in fabricating square artificial spin ice lattices, which exhibit well-defined magnetization textures and interactions among the patterned magnetic elements.

cond-mat.mtrl-sci↗
arXiv2024

Terahertz emission from $α$-W/CoFe epitaxial spintronic emitters

We report efficient terahertz (THz) generation in epitaxial $α$-W/Co$_{60}$Fe$_{40}$ spintronic emitters. Two types of emitters have been investigated; epitaxial $α$-W$(110)$/Co$_{60}$Fe$_{40}(110)$ and $α$-W$(001)$/Co$_{60}$Fe$_{40}(001)$ deposited on single crystalline Al$_{2}$O$_{3}$($11\bar{2}0$) and MgO($001$) substrates, respectively. First principle calculations of the electronic band structure at the W$(001)$ surface reveal Dirac-type surface states, similar to that reported previously for the W$(110)$ surface. The generated THz radiation is about $10\%$ larger for $α$-W$(110)$/Co$_{60}$Fe$_{40}(110)$ grown on single crystalline Al$_{2}$O$_{3}$($11\bar{2}0$), which is explained by the fact that the $α$-W$(110)$/Co$_{60}$Fe$_{40}(110)$ interface for this emitter is more transparent to the spin current due to the presence of Ångstr\" om-scale interface intermixing at the W/CoFe interface. Our results also reveal that the generation of THz radiation is larger when pumping with the laser light from the substrate side, which is explained by a larger part of the laser light due to interference effects in the film stack being absorbed in the ferromagnetic Co$_{60}$Fe$_{40}$ layer in this measurement configuration.

physics.app-ph↗
arXiv2024

Demagnetization dynamics after noncollinear dual optical excitation

We explore the impact of optical excitation using two interfering ultrashort optical pulses on ultrafast magnetization dynamics. Our investigation focuses on Pt/Co/Pt multilayers and TbCo alloy samples, employing a dual pump approach. We observe significant variations in the dynamics of magnetization suppression and subsequent recovery when triggered with two optical pulses of the same polarization-essentially meeting conditions for interference. Conversely, dynamics triggered with cross-polarized pump beams exhibit expected similarity to that triggered with a single pulse. Delving into the underlying physical processes contributing to laser-induced demagnetization and recovery dynamics, we find that our current understanding cannot elucidate the observed trends. Consequently, we propose that optical excitation with interfering light possesses not previously acknowledged capacity to induce long-lasting alterations in the dynamics of angular momentum.

physics.optics↗
arXiv2024

Modelling nanomagnet vertex dynamics through Coulomb charges

We investigate the magnetization dynamics in nanomagnet vertices often found in artificial spin ices. Our analysis involves creating a simplified model that depicts edge magnetization using magnetic charges. We utilize the model to explore the energy landscape, its associated curvatures, and the fundamental modes. Our study uncovers specific magnonic regimes and transitions between magnetization states, marked by zero-modes, which can be understood within the framework of Landau theory. To verify our model, we compare it with micromagnetic simulations, demonstrating a noteworthy agreement.

cond-mat.mes-hall↗
arXiv2024

One-shot pair distribution functions of thin films using lab-based x-ray sources

We demonstrate the feasibility of obtaining accurate pair distribution functions of thin amorphous films down to 80 nm, using modern laboratory-based x-ray sources. The pair distribution functions are obtained using a single diffraction scan (one-shot) without the requirement of additional scans of the substrate or of the air. By using a crystalline substrate combined with an oblique scattering geometry, most of the Bragg scattering of the substrate is avoided, rendering the substrate Compton scattering the primary contribution. By utilizing a discriminating energy filter, available in the latest generation of modern detectors, we demonstrate that the Compton intensity can further be reduced to negligible levels at higher wavevector values. We minimize scattering from the sample holder and the air by the systematic selection of pixels in the detector image based on the projected detection footprint of the sample and the use of a 3D printed sample holder. Finally, x-ray optical effects in the absorption factors and the ratios between the Compton intensity of the substrate and film are taken into account by using a theoretical tool that simulates the electric field inside the film and the substrate, which aids in planning both the sample design and measurement protocol.

cond-mat.mtrl-sci↗
arXiv2024

Boundary-induced phase in epitaxial iron layers

We report the discovery of a boundary-induced body-centered tetragonal (bct) iron phase in thin films deposited on MgAl$_{2}$O$_{4}$ ($001$) substrates. We present evidence for this phase using detailed x-ray analysis and ab-initio density functional theory calculations. A lower magnetic moment and a rotation of the easy magnetisation direction are observed, as compared to body-centered cubic (bcc) iron. Our findings expand the range of known crystal and magnetic phases of iron, providing valuable insights for the development of heterostructure devices using ultra-thin iron layers.

cond-mat.mtrl-sci↗
arXiv2023

Epitaxy enhancement in oxide/tungsten heterostructures by harnessing the interface adhesion

The conditions whereby epitaxy is achieved are commonly believed to be mostly governed by misfit strain. We report on a systematic investigation of growth and interface structure of single crystalline tungsten thin films on two different metal oxide substrates, Al$_{2}$O$_{3}$ ($11\bar{2}0$) and MgO ($001$). We demonstrate that despite a significant mismatch, enhanced crystal quality is observed for tungsten grown on the sapphire substrates. This is promoted by stronger adhesion and chemical bonding with sapphire compared to magnesium oxide, along with the restructuring of the tungsten layers close to the interface. The latter is supported by ab initio calculations using density functional theory. Finally, we demonstrate the growth of magnetic heterostructures consisting of high-quality tungsten layers in combination with ferromagnetic CoFe layers, which are relevant for spintronic applications.

cond-mat.mtrl-sci↗
arXiv2023

Optically Induced Ferromagnetic Order in a Ferrimagnet

The parallel or antiparallel arrangement of electron spins plays a pivotal role in determining the properties of a physical system. To meet the demands for innovative technological solutions, extensive efforts have been dedicated to exploring effective methods for controlling and manipulating this arrangement [1]. Among various techniques, ultrashort laser pulses have emerged as an exceptionally efficient tool to influence magnetic order. Ultrafast suppression of the magnetic order [2,3], all-optical magnetization switching [4, 5, 6, 7], and light-induced magnetic phase transitions [8] are just a few notable examples. However, the transient nature of light-induced changes in the magnetic state has been a significant limitation, hindering their practical implementation. In this study, we demonstrate that infrared ultrashort laser pulses can induce a ferromagnetic arrangement of magnetic moments in an amorphous TbCo alloy, a material that exhibits ferrimagnetism under equilibrium conditions. Strikingly, the observed changes in the magnetic properties persist for significantly longer durations than any previously reported findings. Our results reveal that ultrashort optical pulses can generate materials with identical chemical composition and structural state but entirely distinct magnetic arrangements, leading to unique magnetic properties. This breakthrough discovery marks a new era in light-driven control of matter, offering the exciting potential to create materials with properties that were once considered unattainable.

cond-mat.mtrl-sci↗
arXiv2023

Polymerization in magnetic metamaterials

We numerically study a mesoscopic system consisting of magnetic nanorings in the presence of thermal magnetization fluctuations. We find the formation of dipolar-field-mediated ``bonds" promoting the formation of annuli clusters, where the amount of bonds between two rings varies between zero and two. This system resembles the formation of polymers from artificial atoms, which in our case are the annuli and where the valency of the atom is set by the ring multipolarity. We investigate the thermodynamic properties of the resulting structures, and find a transition associated with the formation of the bonds. In addition, we find that the system has a tendency to form topological structures, with a distinct critical temperature in relation to the one for bond formation.

cond-mat.mes-hall↗
arXiv2023

Plasmon-enhanced optical control of magnetism at the nanoscale via the inverse Faraday effect

The relationship between magnetization and light has been the subject of intensive research for the past century, focusing on the impact of magnetic moments on light polarization. Conversely, the manipulation of magnetism through polarized light is being investigated to achieve all-optical control of magnetism in spintronics. While remarkable discoveries such as single pulse all-optical switching of the magnetization in thin films and sub-micrometer structures have been reported, the demonstration of local optical control of magnetism at the nanoscale has remained elusive. Here, we show that exciting gold nanodiscs with circularly polarized femtosecond laser pulses leads to the generation of sizeable local magnetic fields that enable ultrafast local control of the magnetization of an adjacent magnetic film. In addition, we find that the highest magnetic fields are generated when exciting the sample at a wavelength larger than that of the actual plasmonic resonance of the gold nanodiscs, so avoiding undesired heating effects due to absorption. Our study paves the way for light-driven control in nanoscale spintronic devices and provides important insights into the generation of magnetic fields in plasmonic nanostructures.

physics.optics↗