An Investigation of the Magnetic Structure in Small Regularly Shaped Particles Using Transmission Electron Microscopy

The work described in this thesis is a study of the domain structures in small regularly shaped particles of a soft magnetic material using the techniques of Lorentz microscopy. By using such a material the observed domain structures are primarily determined by the shape of the particle. The basic concepts of ferromagnetism and a discussion of the theoretical framework of micromagnetics is given in chapter 1. Electron microscopy is a powerful tool for the observation of the magnetisation distributions within magnetic materials and in chapter 2 the most important techniques are reviewed in context with the two modes used throughout this work namely the Fresnel and differential phase contrast (DPC) mode. Also briefly discussed in chapter 2 is the theory of image formation in a scanning transmission electron microscope (STEM) for DPC imaging which allows the induction distributions within thin film magnetic objects to be mapped. Previous studies of regularly shaped magnetic particles are reviewed in chapter 3 along with a discussion of the different types of domain walls encountered in magnetic thin films. The shapes to be studied in this project are introduced in this chapter. Fabrication of the particles was performed using the technique of electron beam lithography and the implementation of this process is described in chapter 4. The main body of results in this thesis is presented in chapters 5-7. Chapter 5 deals with preliminary studies of the square and rectangular shapes (PAT1) using the Fresnel mode which was useful in identifying the domain geometries within the particles. The regular domain structures observed in these particles were categorised and the variation of the domain structure with the precise shape of the particle was noted. Further investigation of these structures was made using the DPC mode and described in chapter 6. This mode allowed confirmation of the domain structures observed in chapter 5 to be made as well as providing direct observation of stray fields emanating from particles which did not possess flux closure domain configurations. The techniques used to obtain quantitative information on the domain wall widths are also given in chapter 6. Variation of the particle shape and its influence on the observed domain structures is the subject of chapter 7 where the diamond (PAT2) and triangular (PAT3) shapes are investigated. Chapter 8 contains conclusions drawn from the observations of chapters 5-7 along with suggestions for possible continuation of this work

Resistive switching in ZrO2 films: physical mechanism for filament formation and dissolution

Resistive switching devices, also called memristors, have attracted much attention due to their potential memory, logic and even neuromorphic applications. Multiple physical mechanisms underpin the non-volatile switching process and are ultimately believed to give rise to the formation and dissolution of a discrete conductive filament within the active layer. However, a detailed nanoscopic analysis that fully explains all the contributory events remains to be presented. Here, we present aspects of the switching events that are correlated back to tunable details of the device fabrication process. Transmission electron microscopy and atomically resolved electron energy loss spectroscopy (EELS) studies of electrically stressed devices will then be presented, with a view to understanding the driving forces behind filament formation and dissolution

Electrostatic charging artefacts in Lorentz electron tomography of MFM tip stray fields

Using the technique of differential phase contrast (DPC) Lorentz electron microscopy, the magnetic stray field distribution from magnetic force microscopy (MFM) tips can be calculated in a plane in front of the tip using tomographic reconstruction techniques. Electrostatic charging of the tip during DPC imaging can significantly distort these field reconstructions. Using a simple point charge model, this paper illustrates the effect of electrostatic charging of the sample on the accuracy of tomographic field reconstructions. A procedure for separating electrostatic and magnetic effects is described, and is demonstrated using experimental tomographic data obtained from a modified MFM tip

Compressed sensing electron tomography using adaptive dictionaries: a simulation study

Electron tomography (ET) is an increasingly important technique for examining the three-dimensional morphologies of nanostructures. ET involves the acquisition of a set of 2D projection images to be reconstructed into a volumetric image by solving an inverse problem. However, due to limitations in the acquisition process this inverse problem is considered ill-posed (i.e., no unique solution exists). Furthermore reconstruction usually suffers from missing wedge artifacts (e.g., star, fan, blurring, and elongation artifacts). Compressed sensing (CS) has recently been applied to ET and showed promising results for reducing missing wedge artifacts caused by limited angle sampling. CS uses a nonlinear reconstruction algorithm that employs image sparsity as a priori knowledge to improve the accuracy of density reconstruction from a relatively small number of projections compared to other reconstruction techniques. However, The performance of CS recovery depends heavily on the degree of sparsity of the reconstructed image in the selected transform domain. Prespecified transformations such as spatial gradients provide sparse image representation, while synthesising the sparsifying transform based on the properties of the particular specimen may give even sparser results and can extend the application of CS to specimens that can not be sparsely represented with other transforms such as Total variation (TV). In this work, we show that CS reconstruction in ET can be significantly improved by tailoring the sparsity representation using a sparse dictionary learning principle

Pixelated detectors and improved efficiency for magnetic imaging in STEM differential phase contrast

The application of differential phase contrast imaging to the study of polycrystalline magnetic thin films and nanostructures has been hampered by the strong diffraction contrast resulting from the granular structure of the materials. In this paper we demonstrate how a pixelated detector has been used to detect the bright field disk in aberration corrected scanning transmission electron microscopy (STEM) and subsequent processing of the acquired data allows efficient enhancement of the magnetic contrast in the resulting images. Initial results from a charged coupled device (CCD) camera demonstrate the highly efficient nature of this improvement over previous methods. Further hardware development with the use of a direct radiation detector, the Medipix3, also shows the possibilities where the reduction in collection time is more than an order of magnitude compared to the CCD. We show that this allows subpixel measurement of the beam deflection due to the magnetic induction. While the detection and processing is data intensive we have demonstrated highly efficient DPC imaging whereby pixel by pixel interpretation of the induction variation is realised with great potential for nanomagnetic imaging

Isomerisation and Substitution Reactions of Dimolybdenum Complexes

The work in this thesis deals with quadruply bonded dimolybdenum compounds. Many new complexes have been synthesised and characterised by various spectroscopic techniques. Complexes of general formula Mo2X4(P~P)2 where X = Cl or Br and P~P is a diphosphine ligand, can exist as two geometric isomers. In many cases the a (chelated) isomer isomerises to the IB (bridged) form in dry dichioromethane solution. This transformation has also been observed for Mo2Cl4(dppe)2 in the solid state. Kinetic parameters have been collected for the solution isomerisation of alpha-Mo2Cl4(dppp)2 and for the solid state isomerisation of alpha-Mo2Cl4(dppe)2. The kinetic data are compared with those obtained for other dimolybdenum complexes. The possible mechanisms for the isomerisation reactions are reviewed and discussed in the light of the new data obtained for the solid state reaction. Many quadruply bonded complexes contain weakly bound ligands which can easily be displaced. The complex Mo2(TFMS)4 contains very labile ligands. Reactions of this complex with ligands such as DMF or acetonitrile lead to total ligand substitution. [Mo2(O2CCH3 )2(CH3CN)6]2+ has labile acetonitrile ligands and partial ligand substitution was observed when the cation was reacted with diphosphine or diamine ligands. The product from the reaction of [MO2(O2CCH3)2(CH3CN)6]2+ and dmpe gives [Mo2(O2CCH3)2(dmpe)2]2+, the latter complex has a trans arrangement of acetate ligands while the former has cis. A mechanism for this reaction is presented. Optical activity in quadruply bonded complexes has generated considerable interest in recent years. The configurations and conformations of a variety of complexes of the general formula B-Mo2Cl4. (P~P)2 and [Mo2(O2CCH3)2(L~L)4-2x(CH3CN)6]2+(x = 1 or 2), where P~P is a chiral diphosphine ligand and L~L can be either a chiral diamine or diphosphine ligand, have been studied

Lorentz TEM imaging of stripe structures embedded in a soft magnetic matrix

N\'eel walls in soft magnetic NiFe/NiFeGa hybrid stripe structures surrounded by a NiFe film are investigated by high resolution Lorentz transmission electron microscopic imaging. An anti-parallel orientation of magnetization in 1000 nm wide neighboring unirradiated-irradiated stripes is observed by forming high angle domain walls during magnetization reversal. Upon downscaling the stripe structure size from 1000 nm to 200 nm a transition from a discrete domain pattern to an effective magnetic medium is observed for external magnetic field reversal. This transition is associated with vanishing ability of hosting high angle domain walls between adjacent stripes. The investigation also demonstrated the potentiality of Lorentz microscopy to image periodic stripe structures well under micron length-scale.Comment: 7 pages, 6 figure

Magnetic microscopy of topologically protected homochiral domain walls in an ultrathin perpendicularly magnetized Co film

Next-generation concepts for solid-state memory devices are based on current-driven domain wall propagation, where the wall velocity governs the device performance. It has been shown that the domain wall velocity and the direction of travel is controlled by the nature of the wall and its chirality. This chirality is attributed to effects emerging from the lack of inversion symmetry at the interface between a ferromagnet and a heavy metal, leading to an interfacial Dzyaloshinskii-Moriya interaction that can control the shape and chirality of the magnetic domain wall. Here we present direct imaging of domain walls in Pt/Co/AlO$_x$ films using Lorentz transmission electron microscopy, demonstrating the presence of homochiral, and thus topologically protected, N\'{e}el walls. Such domain walls are good candidates for dense data storage, bringing the bit size down close to the limit of the domain wall width

On the scaling behaviour of cross-tie domain wall structures in patterned NiFe elements

The cross-tie domain wall structure in micrometre and sub-micrometre wide patterned elements of NiFe, and a thickness range of 30 to 70nm, has been studied by Lorentz microscopy. Whilst the basic geometry of the cross-tie repeat units remains unchanged, their density increases when the cross-tie length is constrained to be smaller than the value associated with a continuous film. This occurs when element widths are sufficiently narrow or when the wall is forced to move close to an edge under the action of an applied field. To a very good approximation the cross-tie density scales with the inverse of the distance between the main wall and the element edge. The experiments show that in confined structures, the wall constantly modifies its form and that the need to generate, and subsequently annihilate, extra vortex/anti-vortex pairs constitutes an additional source of hysteresis.Comment: 4 pages, 5 figures, accepted for publication in Europhysics Letters (EPL