D. McGrouther

Use of ZnO thin films as sacrificial templates for metal organic vapor phase epitaxy and chemical lift-off of GaN

Continued development of GaN-based light emitting diodes is being hampered by constraints imposed by current non-native substrates. ZnO is a promising alternative substrate but it decomposes under the conditions used in conventional GaN metal organic vapor phase epitaxy (MOVPE). In this work, GaN was grown on ZnO/c-Al2O3 using low temperature/pressure MOVPE with N2 as a carrier and dimethylhydrazine as a N source. Characterization confirmed the epitaxial growth of GaN. The GaN was lifted-off the c-Al2O3 by chemically etching away the ZnO underlayer. This approach opens up the way for bonding of the GaN onto a support of choice.

Controlled domain wall injection into ferromagnetic nanowires from an optimized pad geometry

The authors present an improved geometry for a micron-scale pad for the injection of vortex domain walls (VDWs) into ferromagnetic nanowires. The pad supports a single vortex magnetization state, the chirality of which can be controlled simply by field saturation along a specific direction. We show, using Lorentz transmission electron microscopy, that utilization of such pads allows the chirality of VDWs injected into the attached wire to be predetermined. Furthermore, the pad vortex state is highly stable and survives repeated injection and depinning of VDWs from an asymmetric notch located some distance along the wire.

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.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 DzyaloshinskiiMoriya interaction that can control the shape and chirality of the magnetic domain wall. Here we present direct imaging of domain walls in Pt/Co/AlOx films using Lorentz transmission electron microscopy, demonstrating the presence of homochiral, and thus topologically protected, Néel 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.

Effect of substrate hardness on splat morphology in high-velocity thermal spray coatings

In this study, Ni-chrome alloy particles were thermally sprayed onto a variety of substrate materials using the high-velocity air fuel (HVAF) technique. Although the various substrate materials were sprayed using identical powder material and thermal spray conditions, the type and variation of splat morphologies were strongly dependent on the substrate material. Predominantly solid splats are observed penetrating deeply into softer substrates, such as aluminum, whereas molten splats were observed on harder substrates, which resisted particle penetration. The observed correlation between molten splats and substrate hardness could be due a dependency of deposition efficiencies of solid and molten splats on the substrate material. However, it was found that conversion of particle kinetic energy into plastic deformation and heat, dependent on substrate hardness, can make a significant contribution towards explaining the observed behavior.

Nanopatterning of a thin ferromagnetic CoFe film by focused-ion-beam irradiation

High-resolution magnetic patterning of a thin CoFe layer has been performed by irradiation using a focused-ion-beam system. Features <50nm wide were formed reproducibly. The irradiated pattern comprised sets of alternating 3.0- and 1.0-μm-long magnetic wires, 100 nm wide. During magnetization reversal, the longer wires reversed at a lower field resulting in the formation of an ordered array of domains with density 10μm−1 supporting antiparallel magnetization. The ability to create domains at predefined locations is important both for fundamental studies and technological applications.

Effect of Ga+ ion irradiation on the structural and magnetic properties of CoFe/IrMn exchange biased bilayers

The effects of focused beam Ga+ ion irradiation on the physical microstructure and magnetic properties of CoFe/IrMn exchange biased bilayers have been investigated by transmission electron microscopy. Only for Ga+ ion doses >1014 ions cm−2 was the microstructure detectably altered with increases in both the bilayer mean grain size and texture being observed. At larger doses (>1015 ions cm−2) radical alterations to the grain morphology and polycrystalline nature of the film were observed including formation of a remarkable needle-like phase at 3×1015 ions cm−2. The magnetic properties and magnetization reversal behavior of the bilayer were studied using the Fresnel mode of Lorentz microscopy. Ga+ ion doses >1013 ions cm−2 were found to progressively reduce the bias field strength and coercivity with the former reaching half original strength at 2×1014 ions cm−2. Higher doses were found to alter the reversal mechanism accompanied by progressive degradation of the magnetic properties. It is likely that the o...

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.

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.

Direct observation of domain wall structures in curved permalloy wires containing an antinotch

The formation and field response of head-to-head domain walls in curved permalloy wires, fabricated to contain a single antinotch, have been investigated using Lorentz microscopy. High spatial resolution maps of the vector induction distribution in domain walls close to the antinotch 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 antinotch 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 antinotch acts as an energy barrier for the domain wall.

Filming the formation and fluctuation of skyrmion domains by cryo-Lorentz transmission electron microscopy

Significance The need for denser storage devices calls for new materials and nanostructures capable of confining single bits of information in a few nanometers. A new topological distribution of spins termed skyrmions is emerging, which promises to robustly confine a small magnetization in a few-nanometers-wide circular domain. A great deal of attention is being devoted to the understanding of these magnetic patterns and their manipulation. We manufactured a large nanoslice supporting over 70,000 skyrmions, and film their evolution in direct-space via cryo-Lorentz transmission electron microscopy. We reveal the octagonal distortion of the skyrmion lattice and show how these distortions and other defects impact its long-range order. These results pave the way to the control of a large two-dimensional array of skyrmions. 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 (7.3×7.3 μm2) single-crystal nanoslice (150 nm thick) of Cu2OSeO3, 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 fluctuation 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.