Skating on spin ice: On the application of Lorentz microscopy and numerical techniques to characterise phase transitions and non-equilibrium phenomena in artificial spin ices

Artificial spin ices are arrays of nano-scale magnetic islands correlated by the interactions of their associated macrospins. They have proven an excellent playground in which to study phase transitions and non-equilibrium phenomena. Originally envisaged as a two-dimensional analogue to the frustrated rare-earth pyrochlores, they are now seen in their own right as promising candidates for a wide range of applications, including nanomagnetic computation and magnonics. At the same time, the capability of tuning their behaviour---whether by means of the constituent material, the fabrication pattern, or the application of external stimuli---enables the realisation of unusual aspects of statistical physics. This thesis comprises a combined numerical and experimental study of artificial spin ice. The aims are twofold. First, it seeks to address how magnetic order and defect textures are influenced by the choice of lattice geometry. Second, it considers one route towards making artificial spin ice configurable via a coupling to a site-specific exchange bias. In the initial segment of this thesis, the recently studied pinwheel form of artificial spin ice is described. This is created by rotating each island in the square lattice about 45 degrees through its centre. The rotation angle of the islands acts as a proxy for a mechanism to vary the interactions between spins. A transition between antiferromagnetism in the square lattice and ferromagnetism in the pinwheel lattice is predicted. The phase diagram and critical exponents of the transition are obtained numerically. The nature of this transition is confirmed experimentally using in-situ Lorentz transmission electron microscopy on thermally annealed cobalt arrays. Varying degrees of thermalisation are observed across the samples, as well as an apparent change in the nature of the defects: from one-dimensional strings in square ice to two-dimensional vortex-like structures for geometries similar to pinwheel. The numerical scaling of these quantities is consistent with that predicted by the Kibble-Zurek mechanism. Finally, a two-dimensional hybrid artificial spin ice is outlined. In this, exchange bias is inserted at specific sites to constrain the magnetisation dynamics of individual islands. By examining correlations, a model for the influence of this pinning is constructed. As the density of constrained spins is varied, different magnetic textures are observed following a simulated field demagnetisation. These simulations show good agreement with results obtained experimentally. In this manner, local control over individual islands provides a route to tuning the global response of the array, thus making the system configurable. This is an essential step towards device-based applications. Taken together, these results illustrate the interplay between topology and magnetic order in artificial spin structures, and enable the exploration of critical phenomena in frozen and glassy systems. The findings presented here demonstrate conclusively that artificial spin ice is an excellent test bed with which to probe out-of-equilibrium dynamics. They will also underpin its potential use in fields which are reliant on adressing specific microstates, such as neuromorphic computing.

Progress towards omnidirectional transformation optics with lenses

We study, theoretically, omni-directional Euclidean transformation-optics (TO) devices comprising planar, light-ray-direction changing, imaging, interfaces. We initially studied such devices in the case when the interfaces are homogeneous, showing that very general transformations between physical and electromagnetic space are possible. We are now studying the case of inhomogeneous interfaces. This case is more complex to analyse, but the inhomogeneous interfaces include ideal thin lenses, which gives rise to the hope that it might be possible to construct practical omni-directional TO devices from lenses alone. Here we report on our progress in this direction

Tuning magnetic order with geometry: thermalisation and defects in two-dimensional artificial spin ices

Artificial spin ices are arrays of correlated nano-scale magnetic islands that prove an excellent playground in which to study the role of topology in critical phenomena. Here, we investigate a continuum of spin ice geometries, parameterised by rotation of the islands. In doing so, we morph from the classic square ice to the recently studied pinwheel geometry, with the rotation angle acting as a proxy for controlling inter-island interactions. We experimentally observe a transition from antiferromagnetic ordering in square ice to a slight preference for ferromagnetic vertices in the weakly-coupled pinwheel ice using Lorentz transmission electron microscopy on thermally annealed cobalt arrays. The rotation angle also affects the relaxation timescales for individual arrays, leading to varying degrees of thermalisation, and an apparent change in the nature of the defects supported: from one-dimensional strings in square ice to two-dimensional vortex-like structures for geometries similar to pinwheel. The numerical scaling of these quantities is consistent with that predicted by the Kibble-Zurek mechanism. Our results show how magnetic order in artificial spin ices can be tuned by changes in geometry and suggest the possibility of realising a truly frustrated ice-rule phase in two-dimensional systems. Furthermore, we demonstrate this system as a testbed to investigate out-of-equilibrium dynamics across phases

Ice-rule made manifold: phase transitions, topological defects and manifold restoration in two-dimensional artificial spin systems

Artificial spin ices are arrays of correlated nano-scale magnetic islands that prove an excellent playground in which to study the role of topology in critical phenomena. Here, we investigate a continuum of spin ice geometries, parameterised by rotation of the islands. In doing so, we morph from the classic square ice to the recently studied pinwheel geometry, with the rotation angle acting as a proxy for controlling inter-island interactions. We experimentally observe a change in ground state magnetic order from antiferromagnetic to ferromagnetic across this class of geometries using Lorentz transmission electron microscopy on thermally annealed cobalt arrays. The change in ordering leads to an apparent change in the nature of the defects supported: from one-dimensional strings in the antiferromagnetic phase to two-dimensional vortex-like structures in the ferromagnetic one, consistent with the scaling predicted by the Kibble-Zurek mechanism. Our results show how magnetic order in artificial spin i ces can be tuned by changes in geometry so that a truly frustrated ice-rule phase is possible in two-dimensional systems. Furthermore, we demonstrate this system as a testbed to investigate out-of-equilibrium dynamics across phases

Parallel mode differential phase contrast in transmission electron microscopy, I: theory and analysis

In Part I of this diptych, we outline the parallel mode of differential phase contrast (TEM-DPC), which uses real-space distortion of Fresnel images arising from electrostatic or magnetostatic fields to quantify the phase gradient of samples with some degree of structural contrast. We present an analysis methodology and the associated software tools for the TEM-DPC method and, using them together with numerical simulations, compare the technique to the widely used method of phase recovery based on the transport-of-intensity equation (TIE), thereby highlighting the relative advantages and limitations of each. The TEM-DPC technique is particularly suitable for in situ studies of samples with significant structural contrast and, as such, complements the TIE method since structural contrast usually hinders the latter, but is an essential feature that enables the former. In Part II of this work, we apply the theory and methodology presented to the analysis of experimental data to gain insight into two-dimensional magnetic phase transitions

'Vernacular Voices: Black British Poetry'

ABSTRACT Black British poetry is the province of experimenting with voice and recording rhythms beyond the iambic pentameter. Not only in performance poetry and through the spoken word, but also on the page, black British poetry constitutes and preserves a sound archive of distinct linguistic varieties. In Slave Song (1984) and Coolie Odyssey (1988), David Dabydeen employs a form of Guyanese Creole in order to linguistically render and thus commemorate the experience of slaves and indentured labourers, respectively, with the earlier collection providing annotated translations into Standard English. James Berry, Louise Bennett, and Valerie Bloom adapt Jamaican Patois to celebrate Jamaican folk culture and at times to represent and record experiences and linguistic interactions in the postcolonial metropolis. Grace Nichols and John Agard use modified forms of Guyanese Creole, with Nichols frequently constructing gendered voices whilst Agard often celebrates linguistic playfulness. The borders between linguistic varieties are by no means absolute or static, as the emergence and marked growth of ‘London Jamaican’ (Mark Sebba) indicates. Asian British writer Daljit Nagra takes liberties with English for different reasons. Rather than having recourse to established Creole languages, and blending them with Standard English, his heteroglot poems frequently emulate ‘Punglish’, the English of migrants whose first language is Punjabi. Whilst it is the language prestige of London Jamaican that has been significantly enhanced since the 1990s, a fact not only confirmed by linguistic research but also by its transethnic uses both in the streets and on the page, Nagra’s substantial success and the mainstream attention he receives also indicate the clout of vernacular voices in poetry. They have the potential to connect with oral traditions and cultural memories, to record linguistic varieties, and to endow ‘street cred’ to authors and texts. In this chapter, these double-voiced poetic languages are also read as signs of resistance against residual monologic ideologies of Englishness. © Book proposal (02/2016): The Cambridge History of Black and Asian British Writing p. 27 of 4

Forging connections: anthologies, arts collectives, and the politics of inclusion

The changing social and political landscape of twentieth-century Britain catalysed a remarkable rise in collaborative activity by artists and activists of black and Asian heritage. Creative communities began to gather in both local and regional contexts, with the aim of sharing resources and securing an audience. This chapter records some of these many activities, tracing the groups’ genesis, manifest objectives, and key contributions. It argues that anthologising should be understood as a specifically motivated activity. Literary anthologies of poetry and fiction served to showcase the diversity of contemporary writing, while also suggesting its coherence. Drawing on the concept of “strategic essentialism” elucidated by Gayatri Chakravorty Spivak, I show that the anthology acts to ensure the visibility of a group, bannered as a unified and singly-titled selection of texts, while also insisting on the differences within: the heterogeneous multiplicity of black and Asian British experiences and creative practices

Parallel Mode Differential Phase Contrast in Transmission Electron Microscopy, I: Theory and Analysis

In Part I of this diptych, we outline the parallel mode of differential phase contrast (TEM-DPC), which uses real-space distortion of Fresnel images arising from electrostatic or magnetostatic fields to quantify the phase gradient of samples with some degree of structural contrast. We present an analysis methodology and the associated software tools for the TEM-DPC method and, using them together with numerical simulations, compare the technique to the widely used method of phase recovery based on the transport-of-intensity equation (TIE), thereby highlighting the relative advantages and limitations of each. The TEM-DPC technique is particularly suitable for in situ studies of samples with significant structural contrast and, as such, complements the TIE method since structural contrast usually hinders the latter, but is an essential feature that enables the former. In Part II of this work, we apply the theory and methodology presented to the analysis of experimental data to gain insight into two-dimensional magnetic phase transitions.ISSN:1431-9276ISSN:1435-811