Magnetic and electrical transport properties of artificial spin ice

This thesis explores the mechanisms of the magnetic reversal of permalloy artificial spin ice arrays. The main research foci include the influence of domain wall propagation on the magnetic reversal of honeycomb artificial spin ice, the low temperature behaviour of honeycomb artificial spin ice and the classification of inverse permalloy opals as three dimensional artificial spin ice. Room temperature imaging of the magnetisation configuration of the nanobars through the magnetic reversal, via scanning transmission X-ray microscopy, photoemission electron microscopy and Lorentz transmission electron microscopy, showed non random domain wall propagation through the frustrated vertices of the honeycomb artificial spin ice arrays. OOMMF simulations suggest that the origin of such non-randomness lies in the domain wall chirality. Boundary conditions necessary for domain wall injection into artificial spin ice arrays were investigated. A reduction of the edge nanobars width of 2/3 was needed to prevent random domain wall nucleation from the array edges. Electrical transport measurements showed evidence of a change in the magnetic reversal, driven by domain wall propagation, of honeycomb permalloy artificial spin ice below 15 K. The transition temperature was found to be proportional to the square of the saturation magnetisation of the ferromagnetic material used. The change in the magnetic reversal was associated with the non-random vertex domain wall positioning below the transition temperature due to the influence of vertex dipole interactions. Room temperature Lorentz transmission electron microscopy images and temperature dependent electrical transport measurements of three dimensional permalloy inverse opals showed the potential of magnetic inverse opals to act as three dimensional artificial spin ice systems.Open Acces

Control of the gyration dynamics of magnetic vortices by the magnetoelastic effect

The influence of a strain-induced uniaxial magnetoelastic anisotropy on the magnetic vortex core dynamics in microstructured magnetostrictive Co$_{40}$Fe$_{40}$B$_{20}$ elements was investigated with time-resolved scanning transmission x-ray microscopy. The measurements revealed a monotonically decreasing eigenfrequency of the vortex core gyration with the increasing magnetoelastic anisotropy, which follows closely the predictions from micromagnetic modeling

Unexpected field-induced dynamics in magnetostrictive microstructured elements under isotropic strain

We investigated the influence of an isotropic strain on the magnetization dynamics of microstructured magnetostrictive Co40Fe40B20 (CoFeB) elements with time-resolved scanning transmission x-ray microscopy. We observed that the application of isotropic strain leads to changes in the behavior of the microstructured magnetostrictive elements that cannot be fully explained by the volume magnetostriction term. Therefore, our results prompt for an alternative explanation to the current models used for the interpretation of the influence of mechanical strain on the dynamical processes of magnetostrictive materials

Collective skyrmion motion under the influence of an additional interfacial spin-transfer torque

Here we study the effect of an additional interfacial spin-transfer torque, as well as the well-established spin-orbit torque, on skyrmion collections - group of skyrmions dense enough that they are not isolated from one another - in ultrathin heavy metal / ferromagnetic multilayers, by comparing modelling with experimental results. Using a skyrmion collection with a range of skyrmion diameters, we study the dependence of the skyrmion Hall angle on diameter and velocity. As for an isolated skyrmion, a nearly-independent skyrmion Hall angle on skyrmion diameter for all skyrmion collection densities is reproduced by the model which includes interfacial spin-transfer torque. On the other hand, the skyrmion Hall angle change with velocity is significantly more abrupt compared to the isolated skyrmion case. This suggests that the effect of disorder on the collective skyrmion behavior is reduced compared to the isolated case. Our results further show the significance of the interfacial spin-transfer torque in ultrathin magnetic multilayers. Due to the good agreement with experiments, we conclude that the interfacial spin-transfer torque should be included in micromagnetic simulations for reproduction of experimental results.Comment: 18 pages, 4 figure

MALTS: A Tool to Simulate Lorentz Transmission Electron Microscopy From Micromagnetic Simulations

Here we describe the development of the MALTS software which is a generalised tool that simulates Lorentz Transmission Electron Microscopy (LTEM) contrast of thin magnetic nanostructures. Complex magnetic nanostructures typically have multiple stable domain structures. MALTS works in conjunction with the open access micromagnetic software Object Oriented Micromagnetic Framework or MuMax. Magnetically stable trial magnetisation states of the object of interest are input into MALTS and simulated LTEM images are output. MALTS computes the magnetic and electric phases accrued by the transmitted electrons via the Aharonov-Bohm expressions. Transfer and envelope functions are used to simulate the progression of the electron wave through the microscope lenses. The final contrast image due to these effects is determined by Fourier Optics. Similar approaches have been used previously for simulations of specific cases of LTEM contrast. The novelty here is the integration with micromagnetic codes via a simple user interface enabling the computation of the contrast from any structure. The output from MALTS is in good agreement with both experimental data and published LTEM simulations. A widely-available generalized code for the analysis of Lorentz contrast addresses is a much needed step towards the use of LTEM as a standardized laboratory technique.Comment: Associated with MALTS program that is available at http://www3.imperial.ac.uk/people/w.branford/researc

Dataset associated with 'Deterministic Field-Free Skyrmion Nucleation at a Nanoengineered Injector Device'

STXM data corresponding to Finizio et al., "Deterministic Field-Free Skyrmion Nucleation at a Nanoengineered Injector Device", Nano Lett. 2019, 19, 10, 7246-725

Probing depth is an independent risk factor for HbA1c levels in diabetic patients under physical training: a cross-sectional pilot-study

Abstract Background This cross-sectional study investigates the potential association between active periodontal disease and high HbA1c levels in type-2-diabetes mellitus subjects under physical training. Methods Women and men with a diagnosis of non-insulin-dependent diabetes mellitus and ongoing physical and an ongoing exercise program were included. Periodontal conditions were assessed according to the CDC-AAP case definitions. Venous blood samples were collected for the quantitative analysis of HbA1c. Associations between the variables were examined with univariate and multivariate regression models. Results Forty-four subjects with a mean age of 63.4 ± 7.0 years were examined. Twenty-nine subjects had no periodontitis, 11 had a moderate and 4 had a severe form of periodontal disease. High fasting serum glucose (p < 0.0001), high BMI scores (p = 0.001), low diastolic blood pressure (p = 0.030) and high probing depth (p = 0.036) were significantly associated with high HbA1c levels. Conclusions Within the limitations of this study HbA1c levels are positively associated with high probing pocket depth in patients with non-insulin-dependent diabetes mellitus under physical exercise training. Control and management of active periodontal diseases in non-insulin-dependent patients with diabetes mellitus is reasonable in order to maximize therapeutic outcome of lifestyle interventions