Justin Woods

Ferromagnetic resonance study of eightfold artificial ferromagnetic quasicrystals

We have performed broadband (10 MHz–18 GHz) and narrowband (9.7 GHz) ferromagnetic resonance (FMR) measurements on permalloy thin films patterned with quasiperiodic Ammann tilings having eightfold rotational symmetry. We observed highly reproducible mode structures in the low-frequency, hysteretic regime in which domain walls and unsaturated magnetization textures exist. A minimum of 10 robust modes were observed in patterned samples, compared to the single uniform mode observed in unpatterned permalloy films. The field dependence and approximate eightfold rotational symmetry of the FMR spectra are in good agreement with micromagnetic simulations that confirm the importance of patterning for controlling static and dynamic magnetic response.

Magnetic response of aperiodic wire networks based on Fibonacci distortions of square antidot lattices

The static and dynamic magnetic responses of patterned ferromagnetic thin films are uniquely altered in the case of aperiodic patterns that retain long-range order (e.g., quasicrystals). We have fabricated permalloy wire networks based on periodic square antidot lattices (ADLs) distorted according to an aperiodic Fibonacci sequence applied to two lattice translations, d1 = 1618 nm and d2 = 1000 nm. The wire segment thickness is fixed at t = 25 nm, and the width W varies from 80 to 510 nm. We measured the DC magnetization between room temperature and 5 K. Room-temperature, narrow-band (9.7 GHz) ferromagnetic resonance (FMR) spectra were acquired for various directions of applied magnetic field. The DC magnetization curves exhibited pronounced step anomalies and plateaus that signal flux closure states. Although the Fibonacci distortion breaks the fourfold symmetry of a finite periodic square ADL, the FMR data exhibit fourfold rotational symmetry with respect to the applied DC magnetic field direction.

Nonstochastic magnetic reversal in artificial quasicrystalline spin ice

We have measured the isothermal DC magnetization of Penrose P2 tilings (P2T) composed of wire segments of permalloy thin film. Micromagnetic simulations reproduce the coercive fields and “knee anomalies” observed in experimental data and show magnetic shape anisotropy constrains segments to be single-domain (Ising spins) at low fields, similar to artificial spin ice (ASI). Mirror symmetry controls the initial reversal of individual segments oriented parallel to the applied field, followed by complex switching of multiple adjacent segments (“avalanches”) of various orientations such that closed magnetization loops (“vortices”) are favored. Ferromagnetic P2T differ from previously studied ASI systems due to their aperiodic translational symmetry and numerous inequivalent pattern vertices, which drive nonstochastic switching of segment polarizations.

Observation of Robust FMR in Permalloy Quasiperiodic Arrays

We have used ferromagnetic resonance (FMR) at fixed frequency 9.7 GHz to investigate permalloy thin films patterned with antidots to form a five-fold quasicrystal pattern. For DC applied fields H below 3.5 kOe, we observed robust FMR spectra exhibiting two-fold rotational symmetry instead of the five-fold symmetry expected for quasicrystals. Dynamic micromagnetic simulations of the FMR spectra performed at H = 1 kOe and 12 kOe exhibit two-fold and ten-fold rotational symmetry, respectively. The ten-fold symmetry observed in simulations is consistent with a saturated state of isolated write fields. We infer the two-fold rotational symmetry observed for H = 1 kOe is due to an unsaturated state of five-fold write fields closely spaced on a square lattice.

FMR Study of Permalloy Films Patterned Into Square Lattices of Diamond Antidots

We have performed broadband (10 MHz-14 GHz) ferromagnetic resonance measurements on permalloy thin films patterned with square lattices of diamond-shaped antidots of variable size and separation. We observe remarkably reproducible mode structures in the low-frequency, hysteretic regime in which various domain wall patterns and unsaturated magnetization textures exist and are strongly affected by the geometry of the antidot lattice. The magnetic field, frequency and angular dependences of the observed modes (some not previously reported) are in good agreement with recently developed micromagnetic simulations. The mode reproducibility is attributed to a reproducible evolution of domain wall textures governed by their pinning to antidot edges. We conclude variations of antidot size, shape and spacing can be used to control magnetic reversal, domain wall pinning and spin wave mode structure.