arXiv subjects

Arjen van den Berg

Publications and source records attributed to Arjen van den Berg.

7 recordsLinked to original sources

arXiv2025

Chirality Imprinting and Spin-texture Tunability in Conformally Coated 3D Magnetic Nanostructured Metamaterials

Three-dimensional (3D) magnetic nanostructures offer unprecedented opportunities for engineering emergent spin textures, but controlling their configuration remains a central challenge. Here we show that conformally coated Ni Nanotubes arranged in a woodpile geometry with lattice spacings ranging from 800 to 1200 nm, realised by two-photon lithography and atomic layer deposition, exhibit a geometry-tuneable balance between chiral and axial states. Magnetic force microscopy on the top layer of the 3D woodpile reveals that few-layer systems exhibit a chiral contrast whilst increasing the number of stacked layers drives a transition to an axial configuration with the change in state populations depending strongly on lattice spacing. Micromagnetic simulations demonstrate that chirality is not intrinsic to isolated tubes but is imprinted by spin textures formed in the substrate sheet film, which couple into the 3D network. As the sheet-film influence diminishes with increasing layer number, dipolar interactions dominate and stabilise the axial state. This two-stage mechanism of chirality imprinting followed by increasingly dominant dipolar interactions, provides clear control parameters for tailoring spin-texture populations. Our results establish conformally coated woodpiles as a reconfigurable 3D ferromagnetic metamaterial platform that can be exploited for data storage, magnonics, and neuromorphic computing.

cond-mat.mes-hall↗

arXiv2025

Direct Visualization of the Magnetic Monopole Field in a 3D Artificial Spin Ice

Magnetic monopoles, long hypothesised as fundamental particles carrying isolated magnetic charge, emerge in spin-ice systems as fractionalised excitations governed by the ice rule. Yet their three-dimensional field structure has never been directly visualised. Here, we use two-photon lithography and processing to fabricate a fully three-dimensional artificial spin-ice lattice with diamond-bond geometry. We then use scanning nitrogen-vacancy magnetometry to directly measure the stray magnetic fields of both charge-neutral and monopole vertices. We find that ice-rule vertices produce antivortex textures directly above their vertices, stabilised by the local frustrated two-in/two out ordering principle. Direct imaging of the monopole stray field shows a highly divergent profile. By correlating experiment with micromagnetic simulations and performing a multipole expansion of the reconstructed magnetisation, we reveal that monopoles in 3DASI are non-trivial micromagnetic entities, carrying both magnetic charge and an intrinsic moment, giving rise to anisotropic interactions that are dependent upon the quasiparticles position on the lattice. Results suggest that as monopoles separate under an applied field, the dipolar contribution to their interaction reorients relative to the underlying Coulombic field, revealing that monopole coupling is tunable through geometry, being set by the local vertex topology. These findings establish 3DASI as a programmable magnetic metamaterial in which nanoscale geometry governs the energetics and dynamics of emergent magnetic charges.

cond-mat.mes-hall↗

arXiv2025

Structural and Magnetic Properties of Barium Hexaferrite Nanoplatelets

Combining unique geometric and magnetic anisotropy in barium hexaferrites has the potential to enhance the performance of advanced technological applications, such as data storage, spintronics, and medicine. Here, we report the synthesis and deposition of barium hexaferrite nanoplatelets, followed by comprehensive structural and magnetic microscopies. The topographic, physical, and morphological properties of individual nanoplatelets, clusters, and aggregates are analyzed using atomic force microscopy (AFM) and electron microscopy, while magnetic properties are analysed using magnetic force microscopy (MFM). Relevant size distributions and nanoparticle configurations for magnetic interactions have been experimentally identified and numerically analyzed. For dense nanoparticle arrangements, direct exchange coupling dominates with parallel magnetisation configuration in overlapping particles. In contrast, separated particles exhibit anti-parallel coupling. Detailed micro-magnetic modelling elucidates the length scales over which these two interaction regimes are dominant, paving the way for macroscopic two-dimensional (2D) and three-dimensional (3D) structures with tuned magnetic ordering.

cond-mat.mtrl-sci↗

arXiv2024

Direct visualization of domain wall pinning in sub-100nm 3D magnetic nanowires with cross-sectional curvature

The study of 3D magnetic nanostructures has uncovered a range of rich phenomena including the stabilization and control of topological spin textures using nanoscale curvature, dynamic effects allowing controlled spin-wave emission, and novel ground states enabled by collective 3D frustrated interactions. From a technological perspective, 3D nanostructures offer routes to ultrahigh density data storage, massive interconnectivity within neuromorphic devices, as well as applications within health technologies, such as mechanical induction of stem cell differentiation. However, the fabrication of 3D nanomagnetic systems with feature sizes down to 10 nm poses a significant challenge. In this work we present a means of fabricating sub-100 nm 3D ferromagnetic nanowires, with both cross-sectional and longitudinal curvature, using two-photon lithography at a wavelength of 405 nm, combined with conventional deposition. Physical characterization illustrates that nanostructures with lateral features as low as 70 nm can be rapidly and reproducibly fabricated. A range of novel domain walls, with anti-vortex textures, coupled transverse textures, and hybrid vortex/anti-vortex textures are found to be enabled by the cross-sectional curvature of the system, as demonstrated by finite-element micromagnetic simulations. Magnetic force microscopy experiments in an externally applied magnetic field are used to image the injection and pinning of domain walls in the 3D magnetic nanowire. At specific field values, domain walls are observed to hop from trap to trap, providing a direct means to probe the local energy landscape. A simple model is presented demonstrating that thickness gradients and local roughness dictate the variation of pinning probability across the wire.

cond-mat.mes-hall↗

arXiv2023

Spin-wave spectral analysis in crescent-shaped ferromagnetic nanorods

The research on the properties of spin waves (SWs) in three-dimensional nanosystems is an innovative idea in the field of magnonics. Mastering and understanding the nature of magnetization dynamics and binding of SWs at surfaces, edges, and in-volume parts of three-dimensional magnetic systems enables the discovery of new phenomena and suggests new possibilities for their use in magnonic and spintronic devices. In this work, we use numerical methods to study the effect of geometry and external magnetic field manipulations on the localization and dynamics of SWs in crescent-shaped (CS) waveguides. It is shown that changing the magnetic field direction in these waveguides breaks the symmetry and affects the localization of eigenmodes with respect to the static demagnetizing field. This in turn has a direct effect on their frequency. Furthermore, CS structures were found to be characterized by significant saturation at certain field orientations, resulting in a cylindrical magnetization distribution. Thus, we present chirality-based nonreciprocal dispersion relations for high-frequency SWs, which can be controlled by the field direction (shape symmetry) and its amplitude (saturation).

cond-mat.mes-hall↗

arXiv2021

Exploiting femtosecond laser exposure for additive and subtractive fabrication of functional materials: A Route to designer 3D Magnetic Nanostructures

Three-dimensional nanostructured functional materials are important systems, allowing new means to intricately control electromagnetic properties. A key problem is realising a 3D printing methodology upon the nanoscale that can yield a range of functional materials. In this letter, it is shown that two-photon lithography when combined with femtosecond machining of sacrificial layers, can be used to realise such a vision and produce 3D functional nanomaterials of complex geometry. This is demonstrated by fabricating 3D magnetic nanowires that exhibit controlled domain wall injection and propagation. Secondly, we fabricate large scale 3D artificial spin-ice structures whose complex switching can be probed using optical magnetometry. We show that by careful analysis of the magneto-optical Kerr effect signal and by comparison with micro-magnetic simulations, depth dependent switching information can be obtained from the 3DASI lattice. The work paves the way to new materials, which exploit additional physics provided by non-trivial 3D geometries.

cond-mat.mes-hall↗

arXiv2021

Magnetic Charge Propagation upon a 3D Artificial Spin-ice

Magnetic charge propagation in bulk frustrated materials has yielded a paradigm-shift in science, allowing the symmetry between electricity and magnetism to be studied. Recent work is now suggesting magnetic charge dynamics upon the spin-ice surface may have important implications in determining the ordering and associated phase space. Here we detail a 3D artificial spin-ice, a 3D nanostructured array of magnetic islands which captures the exact geometry of bulk systems, allowing field-driven dynamics of magnetic charge to be directly visualized upon the surface. Using magnetic microscopy, we observe vastly different magnetic charge dynamics along two principle directions. These striking differences are found to be due to the surface-termination and associated coordination which yields different energetics and interaction strengths for magnetic charges upon the surface.

cond-mat.mes-hall↗