Pinning Site Research Articles

High-frequency vortex dynamics and losses in the mixed state of nanostructured superconductors with columnar defects

We suggest a theoretical model for the microwave response of Abrikosov vortices in nanostructured superconductors with columnar defects, which serve as effective pinning sites for vortices that are considered elastic strings. For this case, we suppose that microwave losses related to the vortex oscillations in a microwave field arise caused by viscous oscillations of vortex kinks, which connect different parts of the vortex line pinned on adjacent columnar defects and vortex segments pinned on these defects. We consider the case of inclined magnetic field H, which creates vortex staircases consisting of pinned vortex segments and vortex kinks inside the superconductor. In this case, rf losses due to viscous kinks oscillations under the microwave current action arise even at T = 0 if the inclination angle of the magnetic field θΗ, concerning the columnar pinning site axis (z axis), exceeds some locking angle value θL.

Dynamic phases and combing effects for elongated particles moving over quenched disorder.

We consider a two-dimensional system of elongated particles driven over a landscape containing randomly placed pinning sites. For varied pinning site density, external drive magnitude, and particle elongation, we find a wide variety of dynamic phases, including random structures, stripe or combed phases with nematic order, and clogged states. The different regimes can be identified by examining nematic ordering, cluster size, number of pinned particles, and transverse diffusion. In some regimes we find that the pinning can enhance the particle alignment, producing a nonmonotonic signature in the nematic ordering with a maximum at a particular combination of pinning density and drive. The optimal nematic occurs when a sufficient number of particles can be pinned, generating a local shear and leading to what we call a combing effect. At high drives, the combing effect is reduced when the number of pinned particles decreases. For stronger pinning, the particles form a heterogeneous clustered or clogged state that depins into a fluctuating state with high diffusion.

Self-supported cobalt oxide electrocatalysts with hierarchical chestnut burr-like nanostructure for efficient overall water splitting

The hierarchical nanostructure catalyst on the self-supported electrode with a rational and large-area distribution is essential for high-efficiency overall water splitting. Herein, a novel strategy based on spatially shaped femtosecond laser ablation pretreatment is proposed for constructing self-supported cobalt oxide electrocatalysts with hierarchical chestnut burr-like nanostructures on a copper substrate. The femtosecond laser ablation can create pinning and attachment sites for the growth of nanowires to form stable self-supported structures. Due to the hierarchical three-dimensional fluffy structures and tight adhesion between active materials and substrates, the prepared self-supported electrocatalysts can provide rapid charge transfer, a large surface area with ample active sites, accelerated electrolyte diffusion, effective catalytic components, and high conductivity during the electrocatalytic process. Naturally, the self-supported electrode demonstrated favorable electrocatalytic properties in alkaline solutions (1 M KOH), presenting low overpotentials of 105 and 235 mV at the current density of 10 mA cm−2 during the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Moreover, the constructed electrolyzer system with the self-supported electrocatalysts can facilitate overall water splitting at the low cell voltage of 1.51 V to achieve a current density of 10 mA cm−2 and exhibits durability for up to 72 h, indicating a high level of activity and stability for water electrolysis. This work presents a novel perspective for the construction of electrocatalysts structures with stable and efficient properties.

Growth and self-jumping of single condensed droplet on nanostructured surfaces: A molecular dynamics simulation

Our molecular dynamics simulation demonstrates that the condensed nanodroplets can achieve self-jumping off the nanostructured surface driven by the Laplace pressure difference. The growth and self-jumping dynamics of nanodroplets condensed on the superhydrophobic nanostructured surface with local hydrophilic pinning site are systematically investigated. We reveal that a curvature difference between the droplet top and bottom is generated due to the confinement of the groove walls, which leads to a Laplace pressure difference. It increases with growing droplet and the droplet detaches from the pinning site and finally jumps off when it reaches the threshold against the pinning force from the hydrophilic pinning site. We find that the characteristics of the hydrophilic pinning site shows competitive effects on the whole process, which can be generally divided into incubation and burst stages. Increasing the surface wettability and size of the pinning site promotes the droplet growth but blocks the droplet self-jumping in the meantime due to the increased pinning force. We also find that the droplet-jumping highly depends on the confined portion of the droplet after detaching from the pinning site and its decrease will diminish the excessive surface energy stored in the droplet, which makes the converted kinetic energy insufficient to support the jump-off. The present work sheds light on the fundamental understandings of passive method for condensate removal, self-cleaning, thermal management, etc.

Local distortions of surface domain walls in cylindrical microwires observed by magneto-optics

Domain wall dynamics in highly magnetostrictive microwires is remarkably sensitive to thermal treatment. One of the reasons is a partial release of the internal stresses that originate during the production process. Another reason is an interaction of the domain wall with local pinning sites, such as surface inhomogeneities, free volumes, or clusters of ferromagnetic atoms, present in as-cast microwires. However, the observation of domain wall shape variations after thermal treatment or following an interaction with a pinning site is challenging. In this paper, we utilize the magneto-optical Kerr effect (MOKE) to locally map the domain wall shape and velocity in cylindrical microwires. The domain wall is trapped in a magnetic potential well, moving back and forth with limited speed. Two complementary techniques are employed in the study: a laser-based setup and time-resolved MOKE microscopy. Different kinds of domain wall propagation are observed. While a planar domain wall propagates synchronously with the potential well, surface pinning gives rise to local jumps and distortions of the domain wall. We relate the first kind of motion to the pinning-free propagation of the domain wall and the second to the propagation of a distorted wall. Finally, local speed and the shapes of the domain wall before and after the introduction of an artificial pinning site are compared.

Deterministic Field-Free Skyrmion Nucleation at a Nanoengineered Injector Device.

Magnetic skyrmions are topological solitons promising for applications as encoders for digital information. A number of different skyrmion-based memory devices have been recently proposed. In order to demonstrate a viable skyrmion-based memory device, it is necessary to reliably and reproducibly nucleate, displace, detect, and delete the magnetic skyrmions, possibly in the absence of external applied magnetic fields, which would needlessly complicate the device design. While the skyrmion displacement and detection have both been thoroughly investigated, much less attention has been dedicated to the study of the skyrmion nucleation process and its sub-nanosecond dynamics. In this study, we investigate the nucleation of magnetic skyrmions from a dedicated nanoengineered injector, demonstrating the reliable magnetic skyrmion nucleation at the remnant state. The sub-nanosecond dynamics of the skyrmion nucleation process were also investigated, allowing us to shine light on the physical processes driving the nucleation.

The Effect of Pain Reduction during Headframe Fixation for Stereotactic Radiosurgery by the Preceded Local Anesthesia under the Needle Cap Guidance.

BackgroundGamma knife radiosurgery (GKRS) has become a major alternative in the neurosurgical field. However, many patients complained of considerable discomfort during the fixation of rigid headframe. This study investigated whether our modified procedure could reduce fixation-related pain.MethodsSixty-six patients who underwent GKRS were enrolled in this study. Thirty-one patients (Group A) underwent the conventional subcutaneous infiltration technique, and 35 patients (Group B) did the modified procedure. In group A, the headframe was held in position by an assistant, and local anesthetics were injected subcutaneously using a 23-gauge spinal needle at pinning sites. Subsequently, pins were applied according to measurements based on spinal needle depth. In group B, with the frame held in position by an assistant, pin sites were marked with a surgical pen under the guidance of needle cap placed on the pin holes. The head frame was then removed, and local anesthetics were injected subcutaneously and periosteally at each marked pin site using a 26-gauge needle. The headframe was then repositioned 5 minutes after local infiltration, and pins were applied according to measurements based on spinal needle depth. To evaluate pain severity during procedures, visual analogue scale (VAS) scores were recorded during local infiltration and frame placement with pins. The pain scores of the two groups were analyzed statistically.ResultsGroup B had a significantly lower VAS score during frame placement than group A (7.26 vs. 3.61; p<0.001), and mean VAS score at local infiltration was also significantly lower in group B (4.74 vs. 3.74; p=0.008).ConclusionPatients in group B experienced significantly less pain than those in group A during pin placement. Pre-fixation time advanced local anesthesia might reduce pain during stereotactic procedures, and the use of a 26-gauge needle appeared in less pain during local infiltration.

Numerical Simulation of Abrikosov Vortex at Columnar Defect in Superconductor

We report a numerical modeling of single vortex depinning and its subsequent dynamics in HTS film with extended linear defects under the influence of the transport current. Numerical simulation of stable pinned vortex state and its escape from a linear defect has been performed. The non-stationary dynamics of vortex escape has been investigated and time-dependent solution for vortex displacement from the defect has been obtained. The delay effect in vortex escape process has been studied and the time delay has been estimated. The impact of processes being studied on electrodynamic properties of a superconductor has also been discussed. The dynamics of vortex escape from columnar pinning site described in the present work is important both for understanding of vortex dynamics and applying high-Tc superconductors with columnar defects.

Micromagnetic simulation of domain wall propagation along meandering magnetic strip with spatially modulated material parameters

Feasibility of two-dimensional propagation of the domain wall (DW) was investigated by micromagnetic simulations. Successful bit-by-bit propagation of the DW was demonstrated in a designed meandering magnetic strip with periodic material parameter modulation, used as DW pinning sites (PSs). The DW was successively shifted along the straight part and around the corner with a spin polarized current pulses with 1 ns-width, 3 ns-interval and same amplitude. A practical current amplitude margin (30 % of mid value) was achieved by analyzing the energy landscape around the meandering corner and optimizing the location of the PSs, which energy barrier height assures a thermal stability criterion (&amp;gt;60 kBT).

Influence of dual-component microcapsules on self-healing efficiency and performance of metal-epoxy composite-lap joints

ABSTRACTDual-component microcapsules were synthesized by solvent evaporation technique using epoxy resin and hardener as core materials and polymethyl methacrylate (PMMA) as shell wall materials. Morphology, core content, and size distribution of microcapsules were monitored by controlling the various processing parameters such as agitation speed, core–shell weight ratio, and concentration of emulsifiers. The molecular structure, morphologies, and thermal characteristics of the microcapsules were examined under Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and thermogravimetric analysis (TGA), respectively. Synthesized dual-component microcapsules were entrenched into the epoxy polymer to introduce the healing features in single lap shear epoxy adhesive joints. Healing efficiency as high as 89% was achieved when 10 wt% dual-component microcapsules were introduced in adhesives. Investigation of the fractured surfaces of the healing enabled adhesives reveals the presence of crack pinning and crack blunting sites represented by characteristic tails at the wake of microcapsules in cohesive zone. Such failure mechanisms responsibly influence the healing efficiency.

Detection of ferromagnetic domain wall pinning and depinning with a semiconductor device

We demonstrate the detection of a ferromagnetic domain wall using a nanoscale Hall cross. A narrow permalloy wire is defined lithographically on top of a Hall cross fabricated from an InAs quantum well. The width of the Hall cross (500 nm–1 μm) is similar to the width of the ferromagnetic wire (200–500 nm), and a geometric pinning site is fabricated in the ferromagnetic wire to trap a domain wall within the area of the Hall cross. The devices provide a signal that is often the same order of magnitude as the offset Hall voltage when a domain wall is located above the Hall cross, and may be useful for memory applications. Different geometries for the Hall cross and ferromagnetic wire are tested, and radiofrequency pulses are sent into the wire to demonstrate current driven domain wall motion. Further changes to the Hall bar geometry with respect to the wire geometry are investigated by numerical computation. A large gain in signal is seen for Hall bars only slightly wider than the ferromagnetic wires as compared to those twice as wide, as well as a larger sensitivity to the exact position of the domain wall with respect to the center of the Hall cross.

Numerical assessment of the influence of foundation pinning, deck resistance, and 3D site geometry on the response of bridge foundations to demands of liquefaction-induced lateral soil deformation

The 2010 Maule earthquake in Chile caused extensive lateral spreading at the site of the Mataquito river bridge. The surface soil deformation patterns and relatively minor structural damage to the bridge observed at this site suggest that foundation pinning effects played a prominent role in the overall response of the bridge abutments to the lateral spreading. 3D finite element analysis of a Mataquito river bridge abutment is used to examine the role of foundation pinning, lateral bridge deck resistance, the deck expansion gap, and the 3D site geometry on the response of the abutment and approach embankment to a simulated lateral spreading event. A parameter study using a second set of 3D finite element models is conducted in order to further investigate the influence of 3D geometric site effects, such as the width of the embankment crest and thickness of the non-liquefiable crust, on the response of deep foundations to kinematic demands consistent with lateral spreading.

An analytical model for vortex core pinning in a micromagnetic disk

A two-parameter analytical model is constructed to describe a thin, magnetically soft, circular disk in the vortex state. The model is capable of describing the change in evolution of net magnetization and of vortex core position when the core interacts with a magnetic pinning site. The basis of the two-parameter model is formed by a piecewise, physically continuous, magnetization distribution constructed with two regions described by different one-parameter models. Benchmarking against numerical simulations of ideal disks with and without pinning sites shows that the model provides accurate predictions of magnetization, hysteretic transitions, and 2-D displacement of the vortex core in the presence of pinning sites. The demonstrated accuracy of the model supports its use as an empirical tool to extract quantitative maps of vortex pinning energies from measurements of magnetization.

Unusual magneto-transport of YBa2Cu3O7−δ films due to the interplay of anisotropy, random disorder and nanoscale periodic pinning

We study the general problem of a manifold of interacting elastic lines whose spatial correlations are strongly affected by the competition between random and ordered pinning. This is done through magneto-transport experiments with YBa2Cu3O7−δ thin films that contain a periodic vortex pinning array created via masked ion irradiation, in addition to the native random pinning. The strong field-matching effects we observe suggest the prevalence of periodic pinning, and indicate that at the matching field each vortex line is bound to an artificial pinning site. However, the vortex-glass transition dimensionality—quasi-two dimensional instead of the usual three dimensional—evidences reduced vortex-glass correlations along the vortex line. This is also supported by an unusual angular dependence of the magneto-resistance, which greatly differs from that of Bose-glass systems. A quantitative analysis of the angular magneto-resistance allows us to link this behaviour to the enhancement of the system anisotropy, a collateral effect of the ion irradiation.

Domain wall pinning by a stray field from NiFe on a Co/Ni nanowire

We have investigated the influence of a stray field on a magnetic domain wall (DW). The depinning fields of a DW in a perpendicularly magnetized Co/Ni nanowire with partly-stacked Ni81Fe19/SiO2 islands on it was measured. The depinning field for the wire with Ni81Fe19/SiO2 was found to be higher than that for the wire without the stack, indicating that the stray field from the Ni81Fe19/SiO2 stack clearly served as an artificial pinning site for the DW. The magnetic force microscopy observation evidenced that the DW was pinned near the Ni81Fe19 stack.

Vortex Domain Wall Formation in Permalloy Nanowires with Geometric Pinning Sites

An optimal geometric pinning site on Permalloy nanowires of varying widths has been investigated and applied in a magnetic memory scenario using micromagnetic simulations. Minimal limits on two key factors; the applied field length and the domain wall formation length are established such that vortex domain walls are reliably formed in the structures to facilitate lower powered domain wall movement using spin-polarised current. The symmetric wires with the nanoconstrictions at both sides have been found to favour the formation of the vortex domain wall compared with the asymmetric wires with the nanoconstrictions at only one side of the wires. The detailed micromagnetic simulations show that the domain wall formation length and the applied field length are optimal to form the vortex domain walls when they are equal to the nanowire width.

Magnetic vortex behavior and its dynamics in nanomagnets in the presence of impurities

Using spin dynamics simulations we study and present a model for pointlike magnetic impurities in nano-magnets based on the exchange coupling between these pointlike impurities and their neighbors. We observed two possible types of impurities, acting as pinning (attractive) or scatter (repulsive) sites. A gaussian profile was observed for the interaction potential energy ranging up to two lattice parameters. Using the known values of the parameters for Permalloy-79 we have calculated the interaction energy of the vortex core with a single defect. We estimated the interaction range as approximately 10 nm. Both results agree quite well with experimental measurements. Also, we study the influence of magnetic impurities in the frequency of the gyrotropic mode in nanodisks of Permalloy-79. We observed fluctuations in the frequency which is in agreement with recent experimental results.

High domain wall magneto-resistance in amorphous TbFeCo wires

By introducing artificial pinning sites to locally modify coercivity in perpendicularly magnetized TbFeCo wires, strip domains were constructed without expense of specimen damage. The construction of strip domains with well-defined shape and controllable number enables a determination of magneto-resistance contributing from singe domain wall. Contrast to crystalline materials, a rather high domain wall magneto-resistance ratio near 20% was found at room temperature, indicating the amorphous TbFeCo is a potential candidate for spintronic devices.

Dynamic Notch Pinning Fields for Domain Walls in Ferromagnetic Nanowires

Artificial defects such as notches and antinotches are often attached to magnetic nanowires to serve as trapping (pinning) sites for domain walls. The magnetic field necessary to release (depin) the trapped domain wall from the notch depends on the type, geometric shape, and dimensions of the defect but is typically quite large. Conversely we show here that for some notches and antinotches there exists a much smaller driving field for which a moving domain wall will travel past the defect without becoming trapped. This dynamic pinning field also depends on the type, geometric shape and defect dimensions. Micromagnetic simulation is used to investigate both the static and dynamic pinning fields and their relation to the topologic structure of the domain wall.

Tailoring laser-induced domain wall pinning

Field-induced domain wall (DW) motion in Permalloy microwires under the influence of thermal gradients is studied using magneto-optical Kerr-microscopy. The DW is found to be pinned at two positions in a laser spot focused onto the wire. By tuning the laser power, the pinning strength can be engineered. On increasing the laser power, the depinning field for the first pinning site in the first half of the laser spot is found to decrease, while for the second pinning site in the spot region located beyond the centre of the spot it increases. Below a threshold power W T , the pinning strength can be tuned reversibly while above W T irreversible increases in the pinning strength due to structural changes occur. This effect can be employed for flexible pinning sites for DW devices.