title: Exploring methods for imaging dynamics in transmission electron microscopy
Creators: name: Lamb, Raymond James
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: Transmission electron microscopy, TEM, skyrmions, FeGe, FeRh, Cu2OSeO3, Lorentz imaging, stroboscopic imaging, electron beam lithography, nanomagnetism.

description: Transmission electron microscopy (TEM) has enabled investigation of materials at atomic resolution and magnetic contrast in the nanometre regime. Progress in spherical aberration correction continues to push the limits of spatial resolution for this instrument and provides researchers unique methods of investigation in the area of materials physics. The drive towards realising nano devices requires both the spatial resolution to observe the system and the temporal resolution to understand how the system evolves in response to stimuli. The time resolution of TEM has classically been limited by both the number of electrons used to image and the noise generated by the imaging devices. In this thesis we explore methods of accessing time-resolved information from a JEOL ARM200cF - "MagTEM" through use of the active pixel sensor (APS) detector Medipix3 capable of single electron counting and noise-free readout. Investigations into both single-shot imaging and stroboscopic imaging are performed to assess the potential of time resolved imaging of dynamics processes in TEM. Chapter 1 introduces the magnetic theory used to support the understanding of the magnetic dynamics imaged as part of this thesis. Starting with the individual magnetic energy terms contributing to the formation of magnetic ordering such as ferromagnetism we introduce how the competition between these terms results in the formation of magnetic structure. Magnetic domain walls are discussed with reference to both field-driven and current-driven dynamics. Finally, the anti-symmetric exchange term known as the Dzyaloshinskii-Moriya interaction (DMI) is introduced in order to discuss the magnetic skyrmion, which is a prominent feature of both chapters 3 and 4. In chapter 2 we introduce the instrumentation used to both image and fabricate samples used in the thesis with focus on TEM. The TEM is discussed section by section from the column components and lenses through imaging modes both structural and magnetic finishing with a broad overview of detector technology and the role of the APS Medipix3 within that field. Three primary methods of sample fabrication were used in this thesis; electron beam lithography (EBL), thin film metal deposition and focused ion beam (FIB) lithography which are discussed in turn, specifically their role in producing electron transparent specimens. Chapter 3 presents the results of a detailed study into the magnetic dynamics of Cu2OSeO3 during a field induced phase transition between the helical and skyrmion phase using the Medipix3 camera. This experiment provided a system with dynamics in the low millisecond time-regime that would be difficult to resolve with a charge-coupled device (CCD) camera due to noise, therefore providing an ideal experiment to demonstrate the advantages of the Medipix3 detector and the limits of single-shot imaging. The chapter opens with a presentation of the quality of individual images captured during 10ms exposure imaging and presents a method of displaying the evolution of the magnetic state of the sample in time in a single figure termed a ’time-evolution profile’. Next, the specific dynamics present in the image stack are then investigated chronologically. First, evidence is presented of the appearance of isolated skyrmion-like meta-stable objects ahead of the skyrmion lattice during the phase transition. Second, the resolution of individual skyrmion positions on a frame-by-frame basis is processed into vector maps providing insight into the collective motion of skyrmions within the lattice. Finally, during rotation of the skyrmion lattice we observe formation of multi-domain skyrmion states in which 5-7 defects are present along the domain boundary. Analysis is performed on sequentially recorded frames containing 5-7 defects to investigate the mechanism of domain boundary motion during lattice rotation. Continuing the investigation of single shot imaging of skyrmion lattice dynamics in chapter 4, we investigate high angle skyrmion lattice domain boundaries in the near room temperature helical material FeGe. The chapter starts with an investigation into the nucleation of the lattice boundaries and the effect of magnetic disorder in the helical state prior to a field induced transition to the skyrmion phase. Imaging of the high angle domain boundary reveals the presence of a bi-stable defect moving between two positions. The defect appears to contain a paired skyrmion system changing position within a set of four neighbouring skyrmions along a single lattice vector. 2D Gaussian fitting is performed using the program Atomap to aid in the identification of the defect position and is processed using an Arrhenius law method to give an estimate of the energy barrier between the defect positions. The capability of the Medipix3 to correctly identify between defect positions enables construction of composite images with total exposure durations longer than the mean lifetime of the dynamic process. Finally, the frame-by-frame motion of the skyrmions during transition between defect sites is analysed to determine the effect of reorientation on the surrounding skyrmions. In chapter 5 we investigate the potential of the Medipix3 detector to access submicrosecond resolution stroboscopic imaging through evaluation of the time contribution of physical processes involved in the registering of a single electron count. The hardware of the Medipix3 is discussed with predictions of the temporal responses of key components such as the charge sensitive amplifier (CSA) and comparator followed by an investigation into the effect of charge sharing between pixels on this time response. In order to demonstrate the time resolution we characterise a simulated reproducible dynamic process by oscillating a 60keV electron beam using electrostatic deflection plates. Using this benchmark, the ultimate time resolution of the Medipix3 is determined for both single pixel mode (SPM) and charge summing mode (CSM). The methodology for time resolved stroboscopic imaging developed in chapter 5 is applied to the real magnetic system FeRh during a temperature induced phase transition from anti-ferromagnetic to ferromagnetic in chapter 6. In this experiment the magnetic moment of a FeRh sample is investigated through small angle electron scattering in response to being heated using a DENSsolutions MEMS-based Wildfire heating chip. Initially static temperature images were taken using a Gatan Orius CCD camera before moving onto imaging the dynamic magnetic transition in response to a pulsed voltage source. Imaging is performed both in single-shot (1ms minimum) and stroboscopically (10μm minimum) to demonstrate the applied limits of these methods. This experiment provides a method of investigating the thermal timescales at which in-situ thermal transitions can be driven using resistive heating. Finally, chapter 7 contains a summary of the findings from results chapters 3 to 6 with a focus on the implication for time revolved TEM and the possible future work accessible using the developed methods.