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.Comment: Accepted for publication in Physical Review Applie

The anatomy and function of Cleland's ligaments

The cutaneous ligaments of the digits have been recognized by anatomists for several centuries, but the best known description is that of John Cleland. Subsequent varying descriptions of their morphology have resulted in the surgical community having an imprecise view of their structure and dynamic function. We micro-dissected 24 fresh frozen fingers to analyze the individual components of Cleland's ligamentous system. Arising from the proximal interphalangeal (PIP) joint, proximal, and sometimes middle phalanx, we found strong ligaments that ran proximally (PIP-P) and distally (PIP-D). On each side of each finger there was a PIP-P ligament present, which passed obliquely from the lateral side of the proximal and sometimes middle phalanx towards its insertion into the skin at the level of the proximal phalanx. The distal (PIP-D) ligaments were found to pass obliquely distally on the radial and ulnar aspects of the digit towards cutaneous insertions around the middle phalanx. A similar arrangement exists more distally with fibres originating from the DIP joint and middle phalanx (the DIP-P pass obliquely proximally, and the DIP-D, distally). Each individual PIP ligament consisted of three different layers originating from fibres overlying the flexor tendon sheath, periosteum or joint capsule, and extensor expansion. Ligaments arising at the DIP joint had two layers equivalent to the anterior two layers of the proximal ligaments. Cleland's ligaments act as skin anchors maintaining the skin in a fixed relationship to the underlying skeleton during motion and functional tasks. They also prevent the skin from 'bagging', protect the neurovascular bundle, and create a gliding path for the lateral slips of the extensor tendon

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-{\mu}s to {\mu}s range, depending on the cooling rate of the system

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 wal

Measuring and tailoring the Dzyaloshinskii-Moriya interaction in perpendicularly magnetized thin films

We investigate the Dzyaloshinskii-Moriya interactions (DMIs) in perpendicularly magnetized thin films of Pt/Co/Pt and Pt/Co/Ir/Pt. To study the effective DMI, arising at either side of the ferromagnet, we use a field-driven domain wall creep-based method. The use of only magnetic field 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. Inserting an ultrathin layer of Ir at the top Co/Pt interface allows us to access the DMI contribution from the top Co/Pt interface. We show that the insertion of a thin Ir layer leads to reversal of the sign of the effective DMI acting on the sandwiched Co layer, and therefore continuously changes the domain wall structure from the right- to the left-handed Néel wall. The use of two DMI-active layers offers an efficient way of DMI tuning and enhancement in thin magnetic films. The comparison with an epitaxial Pt/Co/Pt multilayer sheds more light on the origin of DMI in polycrystalline Pt/Co/Pt films and demonstrates an exquisite sensitivity to the exact details of the atomic structure at the film interfaces

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

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° 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 future spintronic devices