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Exploring the physics of 3D magnetic nanowires and 3D artificial spin-ice lattices

Van Den Berg, Arjen ORCID: 2023. Exploring the physics of 3D magnetic nanowires and 3D artificial spin-ice lattices. PhD Thesis, Cardiff University.
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Three-dimensional magnetic nanostructures have become subject to recent intense interest due to the availability of new fabrication techniques. 3D nanostructured materials provide access to a host of new phenomena such as novel spin textures established by exotic 3D geometries and curvature, ultrafast domain walls beating the Walker limit, controlled spin-wave emission, and a plethora of technological applications. Two-photon lithography (TPL) is a powerful tool facilitating the fabrication of 3D magnetic nanostructures, as has been demonstrated in recent work realizing a 3D lattice of nanowires arranged in a diamond bond structure. Initial experimental work has shown that this technique, when combined with thermal evaporation, can be used to produce 3D artificial spin-ice (3DASI) systems. After providing a background to the relevant physics and experimental techniques, chapter 4 outlines a detailed micromagnetic study of key geometries that make up the experimental 3DASI lattice. This study provides a detailed understanding of switching in individual wires, coordination-two bipod structures present on the surface and coordination-four tetrapod structures present within the bulk. These studies provide a deeper understanding of measurements performed upon the system. TPL with line-of-sight (LOS) deposition results in magnetic nanowires with a crescent-shaped cross-section where non-uniform thickness and curvature leads to novel switching mechanisms and perturbs domain wall structure. Simulations show that the individual wires in the lattice are Ising-like, single domain with sharp reversals between two well-defined states. The crescent-shaped cross-section perturbs domain wall structure and introduces novel edge states impacting switching. The Ising-like condition continues to hold in more complex geometries comprising these wires. Simulations exploring the switching in single wires, bipod systems, and tetrapod systems are explored and compared to experimental optical magnetometry. Every permutation of magnetization within coordination-two and coordination-four vertices are simulated to obtain spin textures, energies, and magnetic surface charge density of conventional artificial spin-ice vertex types. The energies of ice-rule states are found to be almost degenerate, and high energy singly charged monopole states are shown to be stable. Doubly charged monopole states are not stable within the simulation geometries. Computed magnetic surface charge density aids the identification of vertex types measured using magnetic force microscopy, enabling the identification of magnetic charges propagating through the lattice. The energy associated with a monopole excitation upon the surface coordination-two vertices of the 3DASI is shown to be a factor of ~3 higher than an excitation in the coordination-four vertices of the bulk. The utilisation of the calculated energies within Monte Carlo simulations performed by collaborators allowed a reasonable agreement to be obtained with experimental results. Despite the success of TPL and LOS deposition as a tool for magnetic nanostructure fabrication, a limitation in the methodology comes from a thin film of the functional material being deposited on the substrate. In the case of magnetic materials, the substrate film may interact with the functional components and unwanted signals in measurements using MOKE or other techniques where relatively large spot sizes capture background film. The presence of substrate film is then a limiting factor in the types of structures that may be fabricated using TPL and studied. Chapter 5 explores a modification to the TPL fabrication procedure to include a poly(acrylic acid) sacrificial layer compatible with TPL and laser ablation to create a process that removes the substrate film. The novel sacrificial layer process is used to produce isolated magnetic nanowires with no detectable material upon the substrate. MOKE measurements upon a simple nanowire show hysteresis loops with a sharp transition at 9.9 mT, the introduction of a large nucleation pad reduces the wire switching field to 1.63 mT, demonstrating controlled domain wall injection into the nanowire. We present a proof-of-principle of using 3D nanostructuring to introduce out-of-plane perturbations to control domain wall motion in the wires. Finite difference simulations elucidate the pinning mechanism at the proposed perturbation, and MOKE magnetometry suggests a 3mT pinning field. The validity of the pinning measurements is discussed. MOKE measurements performed on 3DASI lattices fabricated using the sacrificial layer approach show that previously detected low-field features due to background film were eliminated; this shows potential for MOKE as a technique to obtain depth-dependent switching information from our lattice. In particular, it is shown that the experimental parameters associated with MOKE, such as polarization and analyzer angle, can be used to help elucidate switching taking place upon the different sublattices. The successful implementation of a sacrificial layer enables the use of TPL with line-of-sight deposition to produce a wide variety of interesting 3D geometries that have been explored within the literature. Examples include gaussian surfaces, which stabilize skyrmions and other topological spin textures. Experimental feasibility of domain wall injection into a 3D nanowire system opens the possibility of realizing more complex domain wall circuits, approaching racetrack like devices. Finally, the work upon sacrificial layers and depth-dependent switching also has important implications for the study of 3DASI systems. Soon the group intends to study thermal systems. Here the removal of the sheet film will eliminate possible spurious signals from the substrate, whilst depth-dependent switching will allow the dynamic route to ground state to be studied.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Physics and Astronomy
Subjects: Q Science > QC Physics
Funders: EPSRC
Date of First Compliant Deposit: 22 June 2023
Last Modified: 22 Jun 2023 09:58

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