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Realising 3D artificial spin-ice systems using two-photon lithography and line-of-sight deposition

May, Andrew 2020. Realising 3D artificial spin-ice systems using two-photon lithography and line-of-sight deposition. PhD Thesis, Cardiff University.
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The core purpose of this thesis is to examine if two-photon lithography (TPL), coupled with a line-of-sight deposition, can facilitate the production of ferromagnetic nanowire arrays in three dimensional (3D) frustrated geometries, and to characterise the fabricated structures. Specifically, nanowire arrays possessing a diamond-bond lattice geometry are of interest herein, as this emulates the arrangement of rare-earth magnetic moments in bulk spin-ice. Simple, planar structures are first studied to form an understanding of the fabrication and characterisation processes, before progressing to investigate frustrated 3D nanowire lattices (3DNLs), fabricated upon polymer scaffolds. These complex structures extend 50 x 50 x 10 microns cubed, whilst individual nanowires are 1000 nm in length, 200 nm in lateral width, and have a peak thickness of 50 nm. Here, permalloy wires exhibit a crescent shaped cross-section, due to the ellipsoidal geometry of the voxel during TPL. Shadowing effects during the line-of-sight permalloy deposition limit the 3DNL to one unit cell in height, although scaffolds are defined as 5 unit cells high to isolate the 3DNL from the surrounding sheet film. Atomic force microscopy (AFM) measurements indicate that the upper two sub-lattices (L1 and L2) exhibit RMS surface roughness of (10.8 +/- 4.3)nm and (16.1 +/- 3.2)nm respectively. Micromagnetic simulations of a single wire, bipod, and tetrapod, representing a system of 1, 2, and 4 spins respectively, indicated the wires to be single domain at remanence and reverse via the propagation of vortex domain walls (DWs). A near-degenerate ice rule manifold is seen by considering the energetics of the possible tetrapod configurations, where the energy minimum is type 2. MOKE magnetometry, in a longitudinal configuration, is sensitive to the upper two sub-lattices of the 3DNL, these measurements indicate a coercive field of 8.0 mT. However, the intense laser beam used in this technique can cause significant deformation due to the poor conductivity of the polymer scaffold. Coating the polymer sidewalls with gold prior to the permalloy deposition is found to be one solution to this concern. Wires in the 3DNL are experimentally confirmed to be Ising-like via magnetic force microscopy (MFM) images captured after applying an external field parallel and perpendicular to the uppermost sub-lattice. MFM signal associated with the 3DNL topography is then proven to be magnetic in origin by inverting the tip magnetisation, as this process yields an inversion in the observed MFM contrast. Field-driven MFM experiments facilitate the identification of various vertex states. Furthermore, monopole excitations on the upper-most sub-lattice are seen to propagate via long cascades of reversing wires and are only seen as isolated charges, which are never observed upon two-wire surface terminations (bipods). In contrast, sub-surface monopole excitations propagate via short chains of wire reversals and frequently appear in closely correlated, charge neutral pairs. Field-driven micromagnetic simulations demonstrate the energy difference between the low energy and excited states to be a factor of 3.23 greater for a bipod, compared with a tetrapod. This indicates that enhanced surface energetics are present, due to the broken lattice symmetry at the upper-boundary. In addition, the vertex spin texture significantly impacts the pinning of monopole excitations, which is studied as a function of applied field, where the precise domain wall structure plays a key role. Monte-Carlo simulations are used to recreate the field-driven experiments, both with and without enhanced surface energetics. Simulations bear a far closer resemblance to experimental observations when surface energetics are considered. These results can be understood in the context of an effective chemical potential. Sub-surface charge pairs nucleate across one, sub-surface wire, leading to an effective chemical potential of 1.18. Whereas the enhanced energy barrier for the reversal of a surface wire, originating from the broken lattice symmetry at the upper boundary, yields an enhanced effective chemical potential upon the surface. This is calculated to be within the range of 2.50 - 4.39, where the exact value depends upon the method that is used to define the surface energetics factor. It is therefore determined that the broken lattice symmetry induces an enhanced effective chemical potential at the surface which is responsible for the striking differences in monopole dynamics observed on different sub-lattices.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Physics and Astronomy
Subjects: Q Science > QC Physics
Uncontrolled Keywords: Two-photon lithography, TPL, 3D, Nanomagnetism, Spin-ice, Artificial spin-ice,Nanotechnology, Magnetic materials, Diamond lattice, Permalloy, Magnetic force microscopy, MFM, Magnetic monopole
Funders: EPSRC, Cardiff University, School of Physics and Astronomy
Date of First Compliant Deposit: 16 June 2021
Last Modified: 07 Jan 2022 02:11

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