arXiv subjects

D. McGrouther

Publications and source records attributed to D. McGrouther.

17 recordsLinked to original sources

arXiv2024

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

In this work we present a detailed analysis on the performance of X-ray magnetic circular dichroism photo-emission electron microscopy (XMCD-PEEM) as a tool for vector reconstruction of the magnetization. For this, we choose 360$^{\circ}$ domain wall ring structures which form in a synthetic antiferromagnet as our model to conduct the quantitative analysis. We assess 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 we develop a self-consistent error metric, which indicates that the main factor of improvement comes from selecting the projections evenly spread out in space, over having a larger number of these spanning a smaller angular range. This work thus poses XMCD-PEEM as a powerful tool for vector imaging of complex 3D magnetic structures.

cond-mat.mes-hall↗

arXiv2023

Observation and formation mechanism of 360° domain wall rings in Synthetic Anti-Ferromagnets with interlayer chiral interactions

The Interlayer Dzyaloshinskii-Moriya interaction (IL-DMI) chirally couples spins in different ferromagnetic layers of multilayer heterostructures. So far, samples with IL-DMI have been investigated utilizing magnetometry and magnetotransport techniques, where the interaction manifests as a tunable chiral exchange bias field. Here, we investigate the nanoscale configuration of the magnetization vector in a synthetic anti-ferromagnet (SAF) with IL-DMI, after applying demagnetizing field sequences. We add different global magnetic field offsets to the demagnetizing sequence in order to investigate the states that form when the IL-DMI exchange bias field is fully or partially compensated. For magnetic imaging and vector reconstruction of the remanent magnetic states we utilize X-ray magnetic circular dichroism photoemission electron microscopy, evidencing the formation of 360$^{\circ}$ domain wall rings of typically 0.5-3.0 $μm$ in diameter. These spin textures are only observed when the exchange bias field due to the IL-DMI is not perfectly compensated by the magnetic field offset. From a combination of micromagnetic simulations, magnetic charge distribution and topology arguments, we conclude that a non-zero remanent effective field with components both parallel and perpendicular to the anisotropy axis of the SAF is necessary to observe the rings. This work shows how the exchange bias field due to IL-DMI can lead to complex metastable spin states during reversal, important for the development of novel spintronic devices.

cond-mat.mes-hall↗

arXiv2018

Ultrafast Generation and Control of an Electron Vortex Beam via Chiral Plasmonic Near Fields

Vortex-carrying matter waves, such as chiral electron beams, are of significant interest in both applied and fundamental science. Continuous wave electron vortex beams are commonly prepared via passive phase masks imprinting a transverse phase modulation on the electron's wave function. Here, we show that femtosecond chiral plasmonic near fields enable the generation and dynamic control on the ultrafast timescale of an electron vortex beam. The vortex structure of the resulting electron wavepacket is probed in both real and reciprocal space using ultrafast transmission electron microscopy. This method offers a high degree of scalability to small length scales and a highly efficient manipulation of the electron vorticity with attosecond precision. Besides the direct implications in the investigation of nanoscale ultrafast processes in which chirality plays a major role, we further discuss the perspectives of using this technique to shape the wave function of charged composite particles, such as protons, and how it can be used to probe their internal structure.

quant-ph↗

arXiv2018

Laser-Induced Skyrmion Writing and Erasing in an Ultrafast Cryo-Lorentz Transmission Electron Microscopy

We demonstrate that light-induced heat pulses of different duration and energy can write skyrmions in a broad range of temperatures and magnetic field in FeGe. Using a combination of camera-rate and pump-probe cryo-Lorentz Transmission Electron Microscopy, we directly resolve the spatio-temporal evolution of the magnetization ensuing optical excitation. The skyrmion lattice was found to maintain its structural properties during the laser-induced demagnetization, and its recovery to the initial state happened in the sub-μs to μs range, depending on the cooling rate of the system.

cond-mat.str-el↗

arXiv2018

Ultrafast holography enabled by quantum interference of ultrashort electrons

Holography relies on the interference between a known reference and a signal of interest to reconstruct both the amplitude and phase of that signal. Commonly performed with photons and electrons, it finds numerous applications in imaging, cryptography and arts. With electrons, the extension of holography to the ultrafast time domain remains a challenge, although it would yield the highest possible combined spatio-temporal resolution. Here, we show that holograms of local electromagnetic fields can be obtained with combined attosecond/nanometer resolution in an ultrafast transmission electron microscope (UEM). Unlike conventional holography, where the signal and the reference are spatially separated and then recombined to interfere, in our method we use electromagnetic fields to split an electron wave function in a quantum coherent superposition of different energy states. In the image plane, spatial modulation of the electron-energy distribution reflects the phase relation between reference and signal fields, which we map via energy-filtered UEM. Beyond imaging applications, this approach allows implementing optically-controlled and spatially-resolved quantum measurements in parallel, providing an efficient and versatile tool for the exploration of electron quantum optics.

cond-mat.mes-hall↗

arXiv2018

Antiferromagnetic-ferromagnetic phase domain development in nanopatterned FeRh islands

The antiferromagnetic to ferromagnetic phase transition in B2-ordered FeRh is imaged in laterally confined nanopatterned islands using photoemission electron microscopy with x-ray magnetic circular dichroism contrast. The resulting magnetic images directly detail the progression in the shape and size of the FM phase domains during heating and cooling through the transition. In 5 um square islands this domain development during heating is shown to proceed in three distinct modes: nucleation, growth, and merging, each with subsequently greater energy costs. In 0.5 um islands, which are smaller than the typical final domain size, the growth mode is stunted and the transition temperature was found to be reduced by 20 K. The modification to the transition temperature is found by high resolution scanning transmission electron microscopy to be due to a 100 nm chemically disordered edge grain present as a result of ion implantation damage during the patterning. FeRh has unique possibilities for magnetic memory applications; the inevitable changes to its magnetic properties due to subtractive nanofabrication will need to be addressed in future work in order to progress from sheet films to suitable patterned devices.

cond-mat.mtrl-sci↗

arXiv2017

From attosecond to zeptosecond coherent control of free-electron wave functions using semi-infinite light fields

Light-electron interaction in empty space is the seminal ingredient for free-electron lasers and also for controlling electron beams to dynamically investigate materials and molecules. Pushing the coherent control of free electrons by light to unexplored timescales, below the attosecond, would enable unprecedented applications in light-assisted electron quantum circuits and diagnostics at extremely small timescales, such as those governing intramolecular electronic motion and nuclear phenomena. We experimentally demonstrate attosecond coherent manipulation of the electron wave function in a transmission electron microscope, and show that it can be pushed down to the zeptosecond regime with existing technology. We make a relativistic pulsed electron beam interact in free space with an appropriately synthesized semi-infinite light field generated by two femtosecond laser pulses reflected at the surface of a mirror and delayed by fractions of the optical cycle. The amplitude and phase of the resulting coherent oscillations of the electron states in energymomentum space are mapped via momentum-resolved ultrafast electron energy-loss spectroscopy. The experimental results are in full agreement with our theoretical framework for light-electron interaction, which predicts access to the zeptosecond timescale by combining semi-infinite X-ray fields with free electrons.

physics.optics↗

arXiv2017

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 < 10nm) measured from direct imaging and its potential role in describing observed behaviour of isolated skyrmions (diameter < 100nm). 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 device applications.

cond-mat.mtrl-sci↗

arXiv2016

Medipix3 Demonstration and understanding of near ideal detector performance for 60 & 80 keV electrons

In our article we report first quantitative measurements of imaging performance for the current generation of hybrid pixel detector, Medipix3, as direct electron detector. Utilising beam energies of 60 & 80 keV, measurements of modulation transfer function (MTF) and detective quantum efficiency (DQE) have revealed that, in single pixel mode (SPM), energy threshold values can be chosen to maximize either the MTF or DQE, obtaining values near to, or even exceeding, those for an ideal detector. We have demonstrated that the Medipix3 charge summing mode (CSM) can deliver simultaneous, near ideal values of both MTF and DQE. To understand direct detection performance further we have characterized the detector response to single electron events, building an empirical model which can predict detector MTF and DQE performance based on energy threshold. Exemplifying our findings we demonstrate the Medipix3 imaging performance, recording a fully exposed electron diffraction pattern at 24-bit depth and images in SPM and CSM modes. Taken together our findings highlight that for transmission electron microscopy performed at low energies (energies <100 keV) thick hybrid pixel detectors provide an advantageous and alternative architecture for direct electron imaging

physics.ins-det↗

arXiv2016

Internal structure of hexagonal skyrmion lattices in cubic helimagnets

We have utilised a high spatial resolution imaging method, Differential Phase Contrast (DPC) performed in a scanning transmission electron microscope (STEM), for precise measurement of the magnetic induction distribution in skyrmion states in noncentrosymmetric magnetically ordered materials. Applied to investigate the internal structure of hexagonal skyrmion lattice cells, stabilised by an out-plane applied magnetic field in an FeGe nanowedge specimen, mapping of the in-plane component of magnetic induction has yielded "average" skyrmion profiles and observation of internal six-fold symmetry. With increasing field strength, the diameter of "average" skyrmion cores was observed to decrease accompanied by a non-linear variation of the lattice periodicity. Variations in structure for individual skyrmions were studied utilising an advanced DPC detection scheme with a variety of symmetry lowering distortions being observed. Our observations are consistent with a theoretical phenomenological model, which has predicted the structure of hexagonal skyrmion lattice cells and also that twisting states near to the material surfaces provide a basis for energetic stabilisation of the skyrmion lattice over the conical phase. There was good agreement with experiment for predictions of bulk skyrmion structure and their response (core-size & lattice periodicity variation) to an applied field.

cond-mat.mes-hall↗

arXiv2015

Chiral surface twists and skyrmion stability in nanolayers of cubic helimagnets

Lorentz transmission electron microscopy (LTEM) investigations of modulated states in a FeGe wedge and detailed calculations 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 skyrmion lattices and helicoids in cubic helimagnet nanolayers. The calculated magnetic phase diagram for free standing cubic helimagnet nanolayers shows that magnetization processes in these compounds fundamentally differ from those in bulk cubic helimagnets and are characterized by the first-order transitions between modulated phases and the formation of specific multidomain states. The paper reports LTEM observations of multidomain patterns in FeGe free-standing nanolayers.

cond-mat.mes-hall↗

arXiv2015

Direct comparison of domain wall behavior in Permalloy nanowires patterned by electron beam lithography and focused ion beam milling

Nominally identical permalloy nanowires, with widths down to 150 nm, were fabricated onto a single electron transparent Si$_{3}$N$_{4}$ membrane using electron beam lithography (EBL) and focused ion beam (FIB) milling. Transmission electron microscopy (TEM) experiments were performed to compare the nanostructures produced by these two techniques in what we believe is the first direct comparison of fabrication techniques for nominally identical nanowires. Both EBL and FIB methods produced high quality structures with edge roughness being of the order of the mean grain size 5 -10 nm observed in the continuous films. However, significant grain growth was observed along the edges of the FIB patterned nanowires. Lorentz TEM \emph{in situ} imaging was carried out to compare the magnetic behavior of the domain walls in the patterned nanowires with anti-notches present to pin domain walls. The overall process of domain wall pinning and depinning at the anti-notches showed consistent behaviour between nanowires fabricated by the two methods with the FIB structures having slightly lower characteristic fields compared to the EBL wires. However, a significant difference was observed in the formation of a vortex structure inside the anti-notches of the EBL nanowires after depinning of the domain walls. No vortex structure was seen inside the anti-notches of the FIB patterned nanowires. Results from micromagnetic simulations suggest that the vortex structure inside the anti-notch can be suppressed if the saturation magnetization (M$_{s}$) is reduced along the nanowires edges. Whilst the two fabrication methods show that well defined structures can be produced for the dimensions considered here, the differences in the magnetic behavior for nominally identical structures may be an issue if such structures are to be used as conduits for domain walls in potential memory and logic applications.

cond-mat.mes-hall↗

arXiv2015

Reproducible domain wall pinning by linear non-topographic features in a ferromagnetic nanowire

We demonstrate that for multilayered magnetic nanowires, where the thickness and composition of the individual layers has been carefully chosen, domain walls can be pinned at non-topographic sites created purely by ion irradiation in a focused ion beam system. The pinning results from irradiation induced alloying leading to magnetic property modification only in the affected regions. Using Lorentz transmission electron microscopy, we have studied the pinning behavior of domain walls at the irradiation sites. Depending on the irradiation dose, a single line feature not only pinned the domain walls but also acted to control their structure and the strength of their pinning.

cond-mat.mes-hall↗

arXiv2015

Filming the formation and fluctuation of Skyrmion domains by cryo-Lorentz Transmission Electron Microscopy

Magnetic skyrmions are promising candidates as information carriers in logic or storage devices thanks to their robustness, guaranteed by the topological protection, and their nanometric size. Currently, little is known about the influence of parameters such as disorder, defects or external stimuli, on the long-range spatial distribution and temporal evolution of the skyrmion lattice. Here, using a large (7.3x7.3μm$^{2}$) single crystal nano-slice of Cu$_{2}$OSeO$_{3}$, we image up to 70,000 skyrmions, by means of cryo-Lorentz Transmission Electron Microscopy as a function of the applied magnetic field. The emergence of the skyrmion lattice from the helimagnetic phase is monitored, revealing the existence of a glassy skyrmion phase at the phase transition field, where patches of an octagonally distorted skyrmion lattice are also discovered. In the skyrmion phase, dislocations are shown to cause the emergence and switching between domains with different lattice orientations and the temporal fluctuations of these domains is filmed. These results demonstrate the importance of direct-space and real-time imaging of skyrmion domains for addressing both their long-range topology and stability.

cond-mat.mes-hall↗

arXiv2015

Engineering magnetic domain-wall structure in permalloy nanowires

Using Lorentz transmission electron microscopy we investigate the behavior of domain walls pinned at non-topographic defects in Cr(3 nm)/Permalloy(10 nm)/Cr(5 nm) nanowires of width 500 nm. The pinning sites consist of linear defects where magnetic properties are modified by a Ga ion probe with diameter ~ 10 nm using a focused ion beam microscope. We study the detailed change of the modified region (which is on the scale of the focused ion spot) using electron energy loss spectroscopy and differential phase contrast imaging on an aberration (Cs) corrected scanning transmission electron microscope. The signal variation observed indicates that the region modified by the irradiation corresponds to ~ 40-50 nm despite the ion probe size of only 10 nm. Employing the Fresnel mode of Lorentz transmission electron microscopy, we show that it is possible to control the domain wall structure and its depinning strength not only via the irradiation dose but also the line orientation.

cond-mat.mtrl-sci↗

arXiv2014

DMI meter: Measuring the Dzyaloshinskii-Moriya interaction inversion in Pt/Co/Ir/Pt multilayers

We describe a field-driven domain wall creep-based method for the quantification of interfacial Dzyaloshinskii-Moriya interactions (DMI) in perpendicularly magnetized thin films. The use of only magnetic fields to drive wall motion removes the possibility of mixing with current-related effects such as spin Hall effect or Rashba field, as well as the complexity arising from lithographic patterning. We demonstrate this method on sputtered Pt/Co/Ir/Pt multilayers with a variable Ir layer thickness. By inserting an ultrathin layer of Ir at the Co/Pt interface we can reverse the sign of the effective DMI acting on the sandwiched Co layer, and therefore continuously change the domain wall (DW) structure from right- to the left-handed Néel wall. We also show that the DMI shows exquisite sensitivity to the exact details of the atomic structure at the film interfaces by comparison with a symmetric epitaxial Pt/Co/Pt multilayer.

cond-mat.mtrl-sci↗

arXiv2008

Direct observation of domain wall structures in curved permalloy wires containing an anti-notch

The formation and field response of head-to-head domain walls in curved permalloy wires, fabricated to contain a single anti-notch, have been investigated using Lorentz microscopy. High spatial resolution maps of the vector induction distribution in domain walls close to the anti-notch have been derived and compared with micromagnetic simulations. In wires of 10 nm thickness the walls are typically of a modified asymmetric transverse wall type. Their response to applied fields tangential to the wire at the anti-notch location was studied. The way the wall structure changes depends on whether the field moves the wall away from or further into the notch. Higher fields are needed and much more distorted wall structures are observed in the latter case, indicating that the anti-notch acts as an energy barrier for the domain wall.

cond-mat.mtrl-sci↗