Defect Pinning Research Articles

Rotary Topological Defects in an Oscillator Lattice Loop and Their Injection Locked Properties

ABSTRACTThe lattice loop formed by adjacent coupling of tunnel diode LC oscillators has been shown to generate topological defects due to broken mirror symmetry, which rotate in the opposite direction to the rotating pulse. Leveraging the injection locking of the rotational dynamics of these topological defects can lead to a subharmonic injection locking scheme with a large division ratio. This paper aims to enhance the design by elucidating the relationship between loop size, the number of topological defects, and the division ratio. Furthermore, we perform bifurcation analysis of the injection locking system, demonstrating that when the external signal strength reaches a certain threshold, defect pinning by the external signal occurs, leading to an extension of the lock range. The dynamics of topological defects are well suited to the so‐called progressive multiphase injection locking. In this paper, we clarify the degree of lock range extension achieved by introducing this technique.

Active nematic ratchet in asymmetric obstacle arrays.

We numerically investigate the effect of an asymmetric periodic obstacle array in a two-dimensional active nematic. We find that activity in conjunction with the asymmetry leads to a ratchet effect or unidirectional flow of the fluid along the asymmetry direction. The directional flow is still present even in the active turbulent phase when the gap between obstacles is sufficiently small. We demonstrate that the dynamics of the topological defects transition from flow mirroring to smectic-like as the gap between obstacles is made smaller, and explain this transition in terms of the pinning of negative winding number defects between obstacles. This also leads to a nonmonotonic ratchet effect magnitude as a function of obstacle size, so that there is an optimal obstacle size for ratcheting at fixed activity.

Identifying and Exploring Synthetic Antiferromagnetic Skyrmions

AbstractSynthetic antiferromagnetic (SAF) skyrmions are emerging as novel information carriers due to their high mobility and lack of a skyrmion Hall effect. However, distinguishing SAF skyrmions from their ferromagnetic counterparts using imaging techniques like magneto‐optical microscopy remains challenging. While the suppressed intrinsic skyrmion Hall effect (SkHE) has been commonly used to identify SAF skyrmions, it is important to note that other factors such as defect pinning and dipolar interaction can also lead to a suppressed SkHE. Therefore, there is an urgent need for a universal identification method that can reliably differentiate SAF skyrmions from ferromagnetic ones. In this study, the generation of a SAF skyrmion within a standard SAF stack is demonstrated and its motion with almost no SkHE is investigated. Furthermore, a universal identification method is proposed wherein the application of an out‐of‐plane field allows the SAF skyrmion to be decoupled into two domains, which can either expand or contract with the application of an electric current. By expediting the development of a reliable means of identifying SAF skyrmions, these findings will accelerate the realization of practical applications based on these unique information carriers.

Non-symmetric pinning of topological defects in living liquid crystals

Topological defects, such as vortices and disclinations, play a crucial role in spatiotemporal organization of equilibrium and non-equilibrium systems. The defect immobilization or pinning is a formidable challenge in the context of the out-of-equilibrium system, like a living liquid crystal, a suspension of swimming bacteria in lyotropic liquid crystal. Here we control the emerged topological defects in a living liquid crystal by arrays of 3D-printed microscopic obstacles (pillars). Our studies show that while −1/2 defects may be easily immobilized by the pillars, +1/2 defects remain motile. Due to attraction between oppositely charged defects, positive defects remain in the vicinity of pinned negative defects, and the diffusivity of positive defects is significantly reduced. Experimental findings are rationalized by computational modeling of living liquid crystals. Our results provide insight into the engineering of active systems via targeted immobilization of topological defects.

Investigating the strain controlled epitaxial growth of Mn3Ge films through thickness modulation

Mn3Ge, a typical member of the Heusler family, has a high spin polarization and large spin Hall angle, making it a potential material for spintronic devices. Regarded as a topological Weyl semi-metal, Mn3Ge could be applied in topological spintronics owing to its special Fermi-arc-type surface states and various spin transport properties. In this study, we grew high-quality perpendicularly magnetized Mn3Gefilms through magnetron sputtering. X-ray diffraction (XRD) showed that hexagonal antiferromagnetic Mn3Ge was mixed with tetragonal ferromagnetic Mn3Ge. Thickness-dependent double-phase Mn3Ge films with large magnetic anisotropy and robust anomalous Hall effect (AHE) were obtained. The triangular spin structure of hexagonal Mn3Ge enhances the AHE; however, it shrinks the coercivity of the tetragonal ferromagnetic property. This manipulation of coexisting multi-phases comes from the strain of the substrate during growth, achieved by controlling the film thickness. Structural, magnetic, and transport measurements demonstrated stress modulation of the defect pinning, magnetic anisotropy, and spin transport properties. The coexistence of antiferromagnetic and ferromagnetic Mn3Ge provides the possibility of a new generation of Mn3X-based devices for applications in spin-torque memories.

Magnetic Evaluation of Heat-Resistant Martensitic Steel Subjected to Microstructure Degradation.

The present paper investigates the use of the magnetic hysteresis loop technique to nondestructively evaluate microstructural degradation in heat-resistant martensitic (HRM) steels. The degradation impairs the safe operation of thermal power plants and it is thus essential to periodically assess it using nondestructive evaluation (NDE) techniques. In this contribution, HRM steels are thermally aged up to 16,000 h at 675 °C to simulate the microstructural degradation, then the changes in the magnetic coercivity, hardness, and microstructure are systematically characterized and the relations between them are determined. Both coercivity and hardness decrease with thermal aging duration, which can be interpreted in terms of the microstructure parameters’ evolution based on the pinning of crystal defects on domain walls and dislocations. Coercivity and hardness share the same softening trend with aging time, and good linear relations between coercivity, hardness, and microstructure parameters are found. These results provide a key to understanding the magnetic parameter evolution in HRM steels and suggest the possibility of using magnetic technologies for the NDE of microstructure degradation in thermal power plants.

Thermally Induced Domain Reconfiguration in Ferroelectric Alkaline Niobate

AbstractSuppressing losses that originate from domain‐wall motion in ferroelectrics is essential to high‐power piezoelectric applications. Conventional models of the hardening effect assume that domain‐wall mobility can be greatly impeded due to the pinning of lattice defects or charge carriers. However, these models cannot satisfactorily explain the dramatical increase of mechanical quality factor from ≈500 to ≈2500 in ferroelectric sodium lithium niobate induced by annealing. In the present study, the specific hardening effect is systematically investigated from multiple perspectives, including kinetics, phase transition, and domain configuration. The enhancement is proposed to be associated with the thermally induced domain reconfiguration at the nanoscale approaching the ferroelectric‐paraelectric phase transition temperature. The comprehensive experimental observations reveal a unique mechanism of the hardening effect in ferroelectrics, shedding light on the enhancement of the performance of high‐power piezoelectric applications.

Simultaneously achieving high performance of energy storage and transparency via A-site non-stoichiometric defect engineering in KNN-based ceramics

Ceramic-based transparent dielectric materials are regarded as the best candidates for advanced energy storage and conversion materials because of their outstanding optical and electric properties. Nevertheless, due to the presence of low density, low band gap energy and large grain size, it is difficult to simultaneously obtain high energy storage density and high optical transmittance in lead-free based ceramics, which limiting their further development of practical applications. In this work, the relaxor ferroelectric ceramics of x mol% Er3+-doped 0.91 K0.5Na0.5NbO3-0.09BamSrnTiO3 (xEr-SrmBan) were constructed via A-site non-stoichiometric defect engineering. It is worth noting that excessive addition of Sr and Ba, especially Sr, can significantly refine the grain size and domain size on account of vacancy-related defect pinning. Finally, high energy storage density (W = 6.39 J/cm3, Wrec = 3.42 J/cm3) together with high optical transmittance (∼72% at 900 nm) can be achieved simultaneously in 0.25Er-Sr1Ba0.5 due to the exist of dense structure, ultrafine grain size (<100 nm), small-sized nano-microdomains (i.e., PNRs, < 50 nm) and large bandgap energy (∼3.10 eV). Furthermore, a fast discharge time of 42 ns can also be realized. The above results indicate that the present study helps to promote the development of advanced transparent energy storage and conversion materials.

Cold sintering of the ceramic potassium sodium niobate, (K0.5Na0.5)NbO3, and influences on piezoelectric properties

Cold Sintering was applied to densify a Potassium-Sodium Niobate solid solution composition, 0.5KNbO3-0.5NaNbO3 (KNN); the process uses a transient chemical sintering aid, moderate pressure (400 MPa), and temperatures between 230–300 °C to obtain ceramics of ∼92 to 96 % theoretical density. Typically, sintering temperatures between ∼1000−1050 °C are required to density KNN using conventional methods. In this paper, the densification was investigated during heating, particularly the shrinkage in the first 60 min of the cold sintering process. The low-field dielectric and electrical properties of the resulting ceramics were found to be comparable to conventionally sintered KNN. Electric fields up to 80 kV/cm could be applied, however the ceramics showed pinched hysteresis loops, even after poling over a wide range of temperatures and electric fields. A Rayleigh analysis was used to investigate domain dynamics and high reversible permittivity. The irreversible behavior was an order of magnitude lower than in conventionally sintered KNN, likely associated with defect pinning of ferroelectric domains. A Transmission Electron Microscopy (TEM) study revealed a high density of line defects in most grains; dislocations in the grains limit poling and domain wall movement, thus suppressing both the piezoelectric properties and the hysteresis. Furthermore, TEM observations indicated crystalline grain boundaries that were faceted with terrace kink ledges. These observations point to the importance of the initial powder optimization and grain boundary diffusion when using cold sintering to prepare ceramics that are intended to show bulk cooperative properties such as ferroelectricity. The impact of limited high temperature homogenization of bulk diffusional processes is discussed.

Dehydrogenation-induced crystal defects for significant enhancement of critical current density in polycrystalline H-doped MgB2

The present study discovers the significant enhancement of critical current density by the pinning of borohydride and crystal defects in the hydrogen-treated MgB2 bulks. Based on the concept of gas doping, the nanosized borohydride Mg(BH4)2 is formed by synthesizing H-doped MgB2 bulks in an H2 atmosphere at 300 °C and 350 °C, and the critical current density was enhanced over the entire field. The H-doped MgB2 bulks are then experienced dehydrogenation at 300 °C and 350 °C, respectively, and the decomposition of Mg(BH4)2 induced nanosized pits on the surface of the MgB2 grains, leading to a further enhancement of critical current density, 1.5 × 104 A cm−2 at 20 K and 2.5 T, which is three times larger than that of the un-doped MgB2 sample, 4.8 × 103 A cm−2. The hydrogenation and dehydrogenation hardly changed the superconducting transition temperature or the pinning mechanism of the MgB2 samples. The enhancement of the critical current density is possibly attributed to the pinning effects of the crystal defects, and the reduction of MgO.

Size-dependent anomalous Hall effect in noncollinear antiferromagnetic Mn3Sn films

The coercive field of ferromagnets generally increases with decreasing the sample size to hundreds of nanometers mainly because of the (edge) defect pinning. We investigate size-dependent anomalous Hall effect (AHE) in (112¯0)-oriented noncollinear antiferromagnetic Mn3Sn films. The switching field (coercive field) of the AHE decreases abruptly when the width of the Hall bar decreases to hundreds of nanometers, giving rise to the reduced coercive field from 445 to 30 mT for Hall bar with width from 2 μm to 100 nm. This observation is in contrast to the ferromagnetic counterpart. The transition from a multidomain to single domain-like mode and the reduction of Néel temperature are proposed to explain the coercivity variation. Our finding provides a promising candidate for the device miniaturization and adds a different dimension to antiferromagnetic spintronics.

Elinvar property of cold-rolled NiTi alloy

We tailor the temperature dependence of elastic modulus (TDEM) of polycrystalline NiTi plates by cold rolling up to 60% thickness reduction. In-plane Elinvar property is obtained with a low elastic modulus temperature coefficient of 6.0 × 10−5 K−1 over a temperature range of 192 K. The microstructure mainly consists of approximately equal volume fractions of {111}<uvw>-oriented B2 and {102}<uvw>-oriented B19′ nanocrystals of high thermal stability due to defect pinning and grain refinement. The opposite TDEMs of the two phases can compensate each other and cause the Elinvar behavior. Such mechanism can also be used to obtain Elinvar property for other shape memory alloys.

Charging effects and anomalous resistive features of superconducting boron doped diamond films

Anomalous resistive peaks below the superconducting transition temperature in heavily boron doped nanocrystalline diamond films could have potential application in switching devices, however it’s exact origin is still under study. We establish a temperature dependence of this resistive phase similar to what has been reported for in Josephson junction arrays and other granular superconductors where the charge duel of the Berezinskii-Kosterlitz-Thouless (BKT) transition has been observed. Non-linear magnetic field dependence of the resistance with a temperature dependent peak feature below the critical field are also presented. Pronounced temperature dependent hysteresis in the current voltage sweeps at temperatures below the determined BKT critical point are related to pinning of charge defects. It is shown that these collective features allude to a Charge-BKT transition between charge and anti-charge analogues.

Doping induced dielectric anomaly below the Curie temperature in molecular ferroelectric diisopropylammonium bromide.

A dielectric anomaly induced by doping has been observed at about 340 K in chlorine-doped diisopropylammonium bromide. The dielectric anomaly has a switchable behaviour, which indicates potential applications on switches and sensors. Temperature-dependent Raman spectrum, X-ray diffraction and differential scanning calorimetry do not show any anomaly around the dielectric anomaly temperature, which prove that the dielectric anomaly does not come from structure phase transition and has no specific heat variety. It is assumed that this dielectric anomaly can be attributed to the freezing of ferroelectric domain walls induced by the pinning of point defects.

Phase diagram and flux pinning in Bi-doping BaPbO3 compounds

BaPb1−xBixO3-δ with x = 0.1–0.3 were prepared by a solid-state reaction method. X-ray diffraction patterns show that the cell parameters increase with the increasing x. From the result of transport measurement, we have drawn a BaPb1−xBixO3-δ phase diagram, which is divided into three regions: metal, semiconducting and superconducting. In the superconducting region, the doping x dependence of critical temperature can be fitted well by a parabolic equation, 1-Tc/Tcmax = A(x-xc)2 with A = 120.68 and xc = 0.223. Compared with the phase diagram of high–Tc cuprates, we think that x > 0.223 should be underdoped region and x < 0.223 is overdoped region. The field dependence of critical temperature for all samples displays a power law, Hc = Hc(0) (1-T/Tc)n and the n-value depends on the doping level. The effective pinning energy is enhanced by Bi-doping. In low field, a possible strong collective flux pinning of 2D defects may be produced by stripe-like nanoscale structure and/or Bi3+ and Bi5+ ions induced dislocations. Above crossover field, a weak pinning of linear-like defects and/or point defects dominated in higher field. The magnetization measurement confirms the flux pinning mechanism.

Effective magnetic pinning schemes for enhanced superconducting property in high temperature superconductor YBa2Cu3O7−x: a review

Enhanced superconducting properties under magnetic field in high temperature superconductors are critical for their technological applications and can be enhanced by both defect pinning and magnetic pinning. Different from defect pinning introduced by nonmagnetic pinning centers, magnetic pinning has some advantages over defect pinning, as it pins the magnetic flux rather than the normal core vortices. Various magnetic materials and different designed architectures have been demonstrated to provide magnetic pinning effect. Four major pinning schemes including metal/YBCO, oxide/YBCO, nanocomposite/YBCO and nanoparticle embedded YBCO have been reviewed. Representative literatures for each magnetic pinning scheme are discussed in detail to explore the pinning enhancement for each scheme. In addition, combined magnetic pinning and defect pinning schemes are proposed to further improve superconducting properties.

A Structural Optimization of Ferrite/YBCO Bilayers

Magnetic particles can be used to take advantage of both defect and magnetic pinning in high-temperature superconductors. We combined layers of ferrite+cerium oxide with YBCO to see their effect both on the structure of the YBCO and on flux pinning. The deposition temperature of the ferrite greatly affects its structure and also the saturation magnetization. Combined with YBCO, the magnetic layer distorts the structure of YBCO, although, for example, the critical temperature remains rather unchanged. According to the structural results achieved here, the magnetic pinning layer is best positioned on the top of YBCO. This way, an improvement in J c (B) is seen in all the field range, and also, a clear improvement of the accommodation field was measured. However, it does not play a huge role whether the ferrite is CoFe 2 O 4 , NiFe 2 O 4 , or ZnFe 2 O 4 , all mixed with CeO 2 .

Pin potential effect on vortex pinning in YBa2Cu3O7-δ films containing nanorods: Pin size effect and mixed pinning

The pin size effect and mixed pinning of nanorods and matrix defects are discussed for YBa2Cu3O7-δ films containing nanorods. BaSnO3 nanorods with a diameter of 11 nm and BaHfO3 nanorods with a diameter of 7 nm were prepared, and critical current density (Jc) and resistivity were measured in the films. When the coherence length was larger than the nanorod size at high temperatures near the critical temperature, the trapping angle and activation energy of the vortex flow depended on the nanorod diameter. At a moderate temperature of 65−77 K, the pin size effect on Jc disappeared since the coherence length became smaller than the nanorod size. At a low temperature of 20 K, the contribution from matrix pinning became comparable to that of nanorods in a high magnetic field due to the small coherence length. Thus, the temperature-dependent coherence length caused the pin potential situation to vary significantly, namely, the pin size effect and mixed pinning, which strongly affected vortex pinning in YBa2Cu3O7-δ containing nanorods.

Enhanced superconducting properties of YBa2Cu3O7−δ thin film with magnetic nanolayer additions

Vertically aligned nanocomposite (VAN) (La0.7Sr0.3MnO3)0.5(CeO2)0.5 and pure La0.7Sr0.3MnO3 layers were incorporated into YBa2Cu3O7−δ (YBCO) thin films as bilayer stacks for magnetic flux pinning enhancement. The films show high epitaxial quality, suggested by XRD and TEM study. The critical temperature Tc of the bilayers is about 90K, which is close to that of pure YBCO films, while both the self-field Jcsf and in-field critical current density Jcin-field are largely enhanced. Among all samples, the film with VAN cap layer shows the highest Jc values in all field ranges. This study demonstrates an effective way towards the tunable pinning effect for YBCO coated conductors by both defect and magnetic pinning.

Enhanced Flux Pinning Properties in $\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7-\delta}$/ $(\hbox{CoFe}_{2}\hbox{O}_{4})_{0.3}(\hbox{CeO}_{2})_{0.7} $ Multilayer Thin Films

A combined defect and magnetic pinning effect has been demonstrated in this work for improving critical current density $(J_{c})$ of $\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7-\delta} $ (YBCO) thin films. Most of the previous work has focused on either defect pinning or magnetic pinning. In this work, we introduced a unique nanocomposite with two phases, e.g., ferrimagnetic $\hbox{CoFe}_{2}\hbox{O}_{4}$ and non-magnetic $\hbox{CeO}_{2}$ . As a demonstration, the composition of $(\hbox{CoFe}_{2}\hbox{O}_{4})_{0.3}(\hbox{CeO}_{2})_{0.7} $ was chosen for this study. The nanocomposite was incorporated into YBCO films as multilayers, i.e., 1-, 2-, and 4-interlayers. A detailed study on the microstructural and superconducting properties was conducted. The results show that the critical temperature remains at $\sim$ 90 K even after introducing interlayers. More importantly, the films with interlayers show enhanced self-field and in-field $J_{c}$ , especially the 2-interlayer sample with $J_{c}^{sf}$ of 6.36 $\hbox{MA/cm}^{2}$ at 77 K. The enhanced $J_{c}$ properties are attributed to both magnetic pinning from $\hbox{CoFe}_{2}\hbox{O}_{4}$ and defect pinning from $\hbox{CeO}_{2}$ , as well as the well-designed pinning landscapes.