Determination of optimal experimental conditions for accurate 3D reconstruction of the magnetization vector via XMCD-PEEM.

This work presents a detailed analysis of the performance of X-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) as a tool for vector reconstruction of magnetization. For this, 360° domain wall ring structures which form in a synthetic antiferromagnet are chosen as the model to conduct the quantitative analysis. An assessment is made of how the quality of the results is affected depending on the number of projections that are involved in the reconstruction process, as well as their angular distribution. For this a self-consistent error metric is developed which allows an estimation of the optimum azimuthal rotation angular range and number of projections. This work thus proposes XMCD-PEEM as a powerful tool for vector imaging of complex 3D magnetic structures.

Revealing 3D magnetization of thin films with soft X-ray tomography: magnetic singularities and topological charges.

The knowledge of how magnetization looks inside a ferromagnet is often hindered by the limitations of the available experimental methods which are sensitive only to the surface regions or limited in spatial resolution. Here we report a vector tomographic reconstruction based on soft X-ray transmission microscopy and magnetic dichroism data, which has allowed visualizing the three-dimensional magnetization in a ferromagnetic thin film heterostructure. Different non-trivial topological textures have been resolved and the determination of their topological charge has allowed us to identify a Bloch point and a meron-like texture. Our method relies only on experimental data and might be of wide application and interest in 3D nanomagnetism.

Effect of annealing on the magnetic states of FEBID-grown cobalt nanopatterns examined by off-axis electron holography.

The growth of cobalt nanopatterns (NPs) using focused electron-beam induced deposition (FEBID) for localised magnetic studies is presented. The initial FEBID products are shown to be polycrystalline and form hetero-structured core-shell NPs through surface oxidation. Off-axis electron holography is performed to reconstruct their morphology, thickness profile and image their individual magnetic vortex domain states. In situ annealing to 400°C promoted migration of the Co-overspray to grow the Co NPs and improved their crystallinity through coarsening, as well as induced diffusion of embedded carbon out of their surface. It is found that the change in their morphology and chemical instability under heating restricts their suitability for examining thermally induced magnetic variations. LAY DESCRIPTION: In this paper, electron microscopy is used to deposit magnetic cobalt nanopatterns and characterise the effect of in-situ heating on their chemistry, structure and magnetic properties. The electron beam of the secondary electron microscope is used to dissociate an injected precursor gas near the SiN membrane substrate of in-situ transmission electron microscopy (TEM) chips and locally deposit the elemental Co in circular patterns ∼ 90 nm in diameter. TEM reveals formation of a Co-oxide shell and embedding of carbon from the precursor gas during growth. The technique of electron holography is used to image the magnetism of the core-shell Co / Co-oxide nanopatterns, which are shown to exhibit magnetic vortex states. In-situ annealing results in migration of the Co overspray to increase their height and carbon diffusion from their surface, as well as change in their original magnetic state through change of orientation. It is found that the change in the morphology and chemistry of Co nanopatterns under heating limits their use for studying the effect of temperature on their magnetism in isolation.

Anomalous Temperature Behavior of the Chiral Spin Helix in CrNb_{3}S_{6} Thin Lamellae.

Using Lorentz transmission electron microscopy and small-angle electron scattering techniques, we investigate the temperature-dependent evolution of a magnetic stripe pattern period in thin-film lamellae of the prototype monoaxial chiral helimagnet CrNb_{3}S_{6}. The sinusoidal stripe pattern appears due to formation of a chiral helimagnetic order (CHM) in this material. We found that as the temperature increases, the CHM period is initially independent of temperature and then starts to shrink above the temperature of about 90 K, which is far below the magnetic phase transition temperature for the bulk material T_{c} (123 K). The stripe order disappears at around 140 K, far above T_{c}. We argue that this cascade of transitions reflects a three-stage hierarchical behavior of melting in two dimensions.

A transmission electron microscope study of Néel skyrmion magnetic textures in multilayer thin film systems with large interfacial chiral interaction.

Skyrmions in ultrathin ferromagnetic metal (FM)/heavy metal (HM) multilayer systems produced by conventional sputtering methods have recently generated huge interest due to their applications in the field of spintronics. The sandwich structure with two correctly-chosen heavy metal layers provides an additive interfacial exchange interaction which promotes domain wall or skyrmion spin textures that are Néel in character and with a fixed chirality. Lorentz transmission electron microscopy (TEM) is a high resolution method ideally suited to quantitatively image such chiral magnetic configurations. When allied with physical and chemical TEM analysis of both planar and cross-sectional samples, key length scales such as grain size and the chiral variation of the magnetisation variation have been identified and measured. We present data showing the importance of the grain size (mostly < 10 nm) measured from direct imaging and its potential role in describing observed behaviour of isolated skyrmions (diameter < 100 nm). In the latter the region in which the magnetization rotates is measured to be around 30 nm. Such quantitative information on the multiscale magnetisation variations in the system is key to understanding and exploiting the behaviour of skyrmions for future applications in information storage and logic devices.

Pinning and hysteresis in the field dependent diameter evolution of skyrmions in Pt/Co/Ir superlattice stacks.

We have imaged Néel skyrmion bubbles in perpendicularly magnetised polycrystalline multilayers patterned into 1 µm diameter dots, using scanning transmission x-ray microscopy. The skyrmion bubbles can be nucleated by the application of an external magnetic field and are stable at zero field with a diameter of 260 nm. Applying an out of plane field that opposes the magnetisation of the skyrmion bubble core moment applies pressure to the bubble and gradually compresses it to a diameter of approximately 100 nm. On removing the field the skyrmion bubble returns to its original diameter via a hysteretic pathway where most of the expansion occurs in a single abrupt step. This contradicts analytical models of homogeneous materials in which the skyrmion compression and expansion are reversible. Micromagnetic simulations incorporating disorder can explain this behaviour using an effective thickness modulation between 10 nm grains.

Chiral Surface Twists and Skyrmion Stability in Nanolayers of Cubic Helimagnets.

Theoretical analysis and Lorentz transmission electron microscopy (LTEM) investigations in an FeGe wedge demonstrate that chiral twists arising near the surfaces of noncentrosymmetric ferromagnets [Meynell et al., Phys. Rev. B 90, 014406 (2014)] provide a stabilization mechanism for magnetic Skyrmion lattices and helicoids in cubic helimagnet nanolayers. The magnetic phase diagram obtained for freestanding cubic helimagnet nanolayers shows that magnetization processes differ fundamentally from those in bulk cubic helimagnets and are characterized by the first-order transitions between modulated phases. LTEM investigations exhibit a series of hysteretic transformation processes among the modulated phases, which results in the formation of the multidomain patterns.

Magnetic microscopy and topological stability of homochiral Néel domain walls in a Pt/Co/AlOx trilayer.

The microscopic magnetization variation in magnetic domain walls in thin films is a crucial property when considering the torques driving their dynamic behaviour. For films possessing out-of-plane anisotropy normally the presence of Néel walls is not favoured due to magnetostatic considerations. However, they have the right structure to respond to the torques exerted by the spin Hall effect. Their existence is an indicator of the interfacial Dzyaloshinskii-Moriya interaction (DMI). Here we present direct imaging of Néel domain walls with a fixed chirality in device-ready Pt/Co/AlOx films using Lorentz transmission electron and Kerr microscopies. It is shown that any independently nucleated pair of walls in our films form winding pairs when they meet that are difficult to annihilate with field, confirming that they all possess the same topological winding number. The latter is enforced by the DMI. The field required to annihilate these winding wall pairs is used to give a measure of the DMI strength. Such domain walls, which are robust against collisions with each other, are good candidates for dense data storage.

Aberration corrected Lorentz scanning transmission electron microscopy.

We present results from an aberration corrected scanning transmission electron microscope which has been customised for high resolution quantitative Lorentz microscopy with the sample located in a magnetic field free or low field environment. We discuss the innovations in microscope instrumentation and additional hardware that underpin the imaging improvements in resolution and detection with a focus on developments in differential phase contrast microscopy. Examples from materials possessing nanometre scale variations in magnetisation illustrate the potential for aberration corrected Lorentz imaging as a tool to further our understanding of magnetism on this lengthscale.

Quantitative Fresnel Lorentz microscopy and the transport of intensity equation.

Imaging of the magnetic structure of thin films by the Fresnel mode of Lorentz microscopy has been re-evaluated in terms of the Ampérian current density within a sample. The conditions for which this imaging can be treated as linear are discussed for quantitative application. Additionally, the consequences for magnetic phase reconstruction using the transport of intensity equation for defocused images are considered. While the range of applicability may initially appear rather limited examples of objects containing different spatial frequency components are used to illustrate the possibilities where relatively large defocus values may be used.

A new design of specimen stage for in situ magnetising experiments in the transmission electron microscope.

A new stage for carrying out in situ magnetising experiments in the transmission electron microscope has been designed, constructed and tested. The principal advantages of the stage are that it delivers horizontal fields with negligible perturbation to the illumination and is suitable for operation in pulsed or continuous field mode. Details of its performance, including field calibration, are given. The paper concludes with a description of where the stage is likely to be of most use.

Instrumentation, techniques, and applications of electron microscopy in the solid state physics group at Glasgow University.

This review discusses some of the work performed by the Solid State Physics Group at Glasgow University. A major aim of the group is to obtain quantitative information with high spatial resolution and to do this reliably requires a thorough understanding of both the instrumentation and the interactions between the electron beam and the specimen. Thus the first part of the review discusses those aspects of instrumentation and techniques that the group has considered in detail while the final part deals with applications which involve the study of a wide range of materials covering metallurgical, semiconductor, organic, and magnetic systems. In all these applications, the results from a range of techniques have been required to provide as complete a picture of the material as possible.