Thickness-Driven Dimensional Crossover in the Static and Dynamic Properties of Permalloy Nanomagnet Arrays

This thesis presents a systematic experimental and micromagnetic simulation study of the static and dynamic magnetic properties of large-area Permalloy Ni81Fe19 nanomagnet arrays as the film thickness is increased from the quasi-two-dimensional thin-film limit into a low-aspect-ratio three-dimensional regime. Elliptical nanomagnets with fixed lateral dimensions of 480 nm x 250 nm were fabricated using deep ultraviolet lithography at four thicknesses (20, 50, 100, and 250 nm), extending well beyond the thickness range commonly reported in the literature, in three array configurations: Isolated elements and dipolar-coupled linear chains along the nanomagnet short and long axes.

Vibrating sample magnetometry and micromagnetic simulations reveal that the remanent spin textures evolve from quasi-uniform single-domain states at low thicknesses to fully three-dimensional vortex textures with significant out-of-plane magnetisation components at 100 nm and 250 nm, establishing the static dimensional crossover between 50 nm and 100 nm. A growth-induced out-of-plane anisotropy, linked to a dominant (111) crystallographic texture, is identified as a source of systematic discrepancy between experiment and simulation.

The dynamic response, characterised by broadband ferromagnetic resonance spectroscopy, micro-focused Brillouin light scattering, and simulated spatial mode profiles, reveals that the onset of three-dimensional dynamics occurs at a lower thickness threshold than the static crossover. At 50 nm with the bias field along the easy axis, mode profiles already exhibit hybridisation between the Kittel-like uniform mode and the first perpendicular standing spin-wave mode. This occurs despite the equilibrium texture remaining quasi-two-dimensional at this thickness. The threshold for three-dimensional dynamics is shown to depend on the precession-plane geometry, with an approximately 2:1 lateral aspect ratio translating directly into an approximately 2:1 difference in the critical thickness between the two field orientations. Inter-element dipolar coupling modifies the resonance conditions, most notably rotating the effective easy axis in short-axis-coupled arrays, but does not fundamentally alter the nature of the thickness-driven dimensional crossover.