Pinning Mechanism Research Articles

Synergistic improvement of epoxy composites with multi-walled carbon nanotubes and hyperbranched polymers

This paper presented the preparation of composites that consist of multi-walled carbon nanotubes (MWCNTs), hyperbranched polymers (HBPs), epoxy resins (EP) and curing agent. The influences of HBPs and MWCNTs content on the curing process of epoxy systems and mechanical performances of MWCNTs/HBPs/EP composites were discussed. Results from differential scanning calorimeter (DSC), tensile, impact and toughness tests are provided. The tensile and impact fracture surface were observed by field-emission scanning electron microscopy. The addition of HBPs resulted in accelerating the curing process of epoxy systems. MWCNTs/HBPs/EP composites were capable of increasing tensile strength by up to 38%, giving a tensile modulus of 25%, impact strength of 68%, fracture toughness of 66%, and a glass transition temperature (Tg) of 46% in comparison with unmodified epoxy thermosets. Field emission-scanning electron micrographs showed that these increases were associated with a synergistic effect of plastic deformation mechanism and crack pinning mechanism, which were induced by the present of HBPs and MWCNTs, respectively.

Improved Critical Current Density and Magnetic Properties of Pr6O11-doped MgB2 Bulks Synthesized by Mg-diffusion Method

The effects of the addition of Pr6O11 nanopowder on the critical current density (Jc), irreversible field (Hirr) and upper critical field (Hc2) of the MgB2 bulks prepared by a Mg-diffusion method were investigated. The results show that the Pr6O11 nanopowder doping can significantly enhance the Jc, Hirr and Hc2 of the MgB2 bulks, but has little effect on the superconducting transition temperature (Tc). At 20 K in a self-field, the Jc value of the 1 wt% Pr6O11-doped MgB2 sample is 3.61×105 A/cm2, which is nearly 5 times larger than that of the undoped sample. In addition, at 10 K, the Hc2 and Hirr of the bulk increase by 1.9 and 2.6 T, respectively. The effect of Pr6O11-doping on superconductivity and flux pinning mechanism was also discussed.

Thermally activated flux flow, vortex-glass phase transition and the mixed-state Hall effect in 112-type iron pnictide superconductors

The transport properties in the mixed state of high-quality Ca0.8La0.2Fe0.98Co0.02As2 single crystal, a newly discovered 112-type iron pnictide superconductor, are comprehensively studied by magneto-resistivity measurement. The field-dependent activation energy, U0, is derived in the framework of thermally activated flux flow (TAFF) theory, yielding a power law dependence U0~Hα with a crossover at a magnetic field around 2 T in both H⊥ab and H//ab, which is ascribed to the different pinning mechanisms. Moreover, we have clearly observed the vortex phase transition from vortex-glass to vortex-liquid according to the vortex-glass model, and vortex phase diagrams are constructed for both H⊥ab and H//ab. Finally, the results of mixed-state Hall effect show that no sign reversal of transverse resistivity ρxy(H) is detected, indicating that the Hall component arising from the vortex flow is also negative based on the time dependent Ginzburg-Landau (TDGL) theory. Meanwhile, the transverse resistivity ρxy(H) and the longitudinal resistivity ρxx(H) follow the relation |ρxy(H)|=Aρxx(H)β well with an exponent β~2.0, which is in line with the results in theories or experiments previously reported on some high-Tc cuprates.

Multifold interface and multilevel crack propagation mechanisms of graphene oxide/polyurethane/epoxy membranes interlaminar-toughened carbon fiber-reinforced polymer composites

Graphene oxide/polyurethane/epoxy (GO/PU/EP) membranes were directly fabricated by functionalization of graphene oxide with epoxy-grafted polyurethane (GO-UE), and the interface correlation and crack propagation mechanisms in GO/PU/EP membranes interlaminar-toughened carbon fiber-reinforced polymer composites were investigated. The functionalized GO-UE with corrugation and scrolling nature of graphene sheets was evenly dispersed in GO/PU/EP membranes below 0.50 wt% loading. Mode I fracture toughness, flexural properties and interlaminar shear strength of GO/PU/EP membranes-toughened composites were enhanced in comparison with untoughened composites and PU/EP membranes-toughened composites, which was ascribed to the multifold interface bonding between the GO-UE layers, epoxy matrix and carbon fiber. Schematic models of multilevel crack propagations were proposed based on different crack extension directions to GO-UE and the morphology evolutions of GO-UE in the interlaminar region and at the carbon fiber interface in toughened composites, which highlighted the toughening mechanisms of crack pinning, crack deflection and separation between GO-UE layers.

Effect of Cr atoms in vortex dynamics of NbSe2 superconductor and study of second magnetization peak effect

We report on the high quality single crystal of CrxNbSe2 growth to study the effect of Cr impurity on the vortex dynamics of NbSe2 superconductor using M–H curves with different relaxation rates. The field dependence nature of critical current density (Jc) is well investigated using Larkin-Ovchinnikov collective pinning model and Anderson-Kim’s plastic creep model. We have observed significant improvement in the field dependence nature of Jc with increasing Cr impurities in NbSe2. This reveals the crossover of vortex lattice from elastic (E) to plastic (P) creep region causing second magnetization peak (SMP) or fishtail effect at highest Cr concentration, which has not been yet observed. Also addition of Cr impurity enhances the Jc up to 4 × 105 A/cm2. However, the interaction between core region of vortices and pinning centres ensure the δl pinning in Cr0.0005NbSe2 and significantly improved with increment of Cr impurity, which allowing both δl and δTc mechanism. Both, δl and δTc core pinning strongly enhance the coupling between vortices in pinning potential of nonstatistical distribution of Cr impurity, which makes 3D magnetic fluxes into 2D nature. Furthermore, the point pining and surface pinning mechanisms are responsible for pinning at low magnetic field region in Cr0.0005NbSe2. The appearance of prominent peak in pinning force density near upper critical field (Hc2) indicates the lattice softening at higher Cr impurity. These studies not only provide fundamental study of vortex dynamic but also open the door for further study in low Tc superconducting materials.

Role of Bi/Tm substitution in Bi-2212 system on crystal structure quality, pair wave function and polaronic states

This comprehensive study finds strongly out the crucial variations in the dc electrical resistivity, superconducting, crystal structural and flux pinning mechanisms with the partial replacement of homovalent Tm+3 inclusions by Bi+3 impurity in the active layers of Bi-2212 superconducting material. Materials of type Bi2.1-xTmxSr2.0Ca1.1Cu2.0Oy with molar ratio changes of 0.00 ≤ x ≤ 0.30 are prepared by conventional solid-state reaction route in atmospheric pressure and the characterizations are exerted by the bulk density, dc electrical resistivity (ρ-T), powder X-ray diffraction (XRD), critical current density (Jc), scanning electron microscopy (SEM) and electron dispersive X-ray (EDX) experimental measurements. The combination of experimental results evaluated from the bulk density, dc electrical resistivity, XRD and EDX measurements points out that the Tm foreign impurities mostly incorporate successfully into the Bi-2212 crystal lattice. In fact, the EDX investigations verify that the thulium impurities may mostly be substituted for the bismuth sites in the crystal structure. All the experimental results declare that the characteristic features improve regularly with the increment in the Tm impurity level up to x = 0.07 beyond which the properties degrade dramatically. In this respect, the sample with x = 0.07 exhibits highest electrical conductive/metallic characteristics as a consequence of the refinement in the crystal structure quality and connectivity between the superconducting grains, being favored by bulk density and related relative degrees of granularity surveys. Likewise, the material presents the maximum offset-onset critical transition temperature values of 85.61 K–85.85 K due to the increment in the formation of effective and strong electron-phonon coupling probabilities and optimization of mobile hole carrier concentrations in the Cu-O2 consecutively stacked layers. In other words, the optimum content level leading to transit inherently over-doped nature into optimally doped state strengthens the amplitude of pair wave function for the Bi-2212 material. In more sophisticated interpretations, the presence of optimum dopant in the crystal structure changes the vibrational mode intensities of O (1)CuA1g, B1g phonons and O (2)SrA1g phonon so that the formation possibility of bipolaron out of two polarons increases strongly in a polarizable lattice (polaronic effect). Additionally, the highest self-field Jc of 95 A/cm2 confirms the fact that the optimum dopant augments the effective nucleation centers along the intragrain and inter-grain boundaries in the crystal system. Similarly, the material prepared with x = 0.07 presents the smoothest, densest, largest average crystalline distribution, lowest porosity and most uniform surface appearance with the finest connection between the superconducting grains. The XRD results (the increased high phase, c-axis length and average grain size but decreased a-axis length) also show the optimum dopant level of x = 0.07 for Bi-2212 crystal system. All in all, the paper developing a strong methodology about why the characteristic properties improve with the presence of Tm impurity in the Bi-2212 system may be a pioneering research to construct newly, novel and feasible market areas for the Bi-2212 superconducting ceramics in the universe economy.

Effects of Zr Doping on Magnetic and Structural Properties of DyBa2Cu3O <inline-formula> <tex-math notation="LaTeX">$_{7-\delta }$</tex-math> </inline-formula> Thin Films

DyBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> -Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-δ</sub> thin films (with x = 0.00, 0.02, 0.04, and 0.06) were synthetized using the chemical solution deposition method on single crystalline LaAlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrates. Effects resulting from Zr doping were investigated by means of magnetic and structural measurements. X-ray diffraction analysis demonstrated a strong c-axis orientation in the DyBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-δ</sub> thin films, with only minor reflections due to the presence of Dy <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and BaZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> phases. Narrow transitions with ΔTc ranging from 1 to 2 K for samples with x = 0.00 and 0.06, respectively, were observed in ac magnetic susceptibility curves, where the nondoped sample demonstrated the highest T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">onset</sup> of 91.7 K compared to the Zr-doped samples. Critical current densities J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the thin films were obtained from magnetization loop measurements with applied magnetic fields up to 6 T and by employing the Bean model. The sampled doped with 4% Zr exhibited the highest J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> value (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> = 3.5 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , self-field, 77 K) in the low field range and we, in general, observed that Zr-doped films demonstrated higher values compared to the pure sample. Pinning force plots (F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> versus B) reveal a significant improvement over the magnetic field range investigated of the maximum pinning force for Zr-doped samples. We found F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Max</sup> = 1.7 GN/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and 3.8 GN/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> for pure and 6% Zr samples, respectively. Analysis based on the Dew-Hughes model shows that normal surface pinning is the dominating pinning mechanism.

Distinct stochasticities between ferromagnetic domain-wall motions driven by magnetic field and electric current

We report the experimental observation of distinct stochasticities between domain-wall motions driven by either a magnetic field or an electric current. In Pt/Co/Ta trilayer films, the arrival time of the current-induced domain-wall motion has a much broader dispersion compared to that of the field-induced motion. Images of the magneto-optical Kerr effect microscope reveal that the current-induced motion experiences significantly stronger domain-wall pinning, contrary to the field-induced domain-wall motion, even though both types of motion take place in the same area of the film. Consequently, the former exhibits stepwise propagation in time, whereas the behavior of the latter is smooth and monotonic. A simple analytic model based on depinning statistics is proposed to explain the relationship between the wide dispersion and the strong pinning mechanisms in current-induced domain-wall motion.

Effect of magnetic field on the flux pinning mechanisms in Al and SiC co-doped MgB2 superconductor

MgB2 superconductor samples co-doped with 0.02 wt. Al2O3 and 0–0.05 wt. SiC were studied by magnetization – magnetic field (M-H) loop measurements at different temperatures. The critical current density has been calculated by the Bean model, and the irreversibility field, Hirr, has been obtained by the Kramer method. The pinning mechanism of the co-doped sample with 2% Al and 5% SiC was investigated in particular due to its having the highest Hirr. The normalized volume pinning force f = F/Fmax as a function of reduced magnetic field h = H/Hirr has been obtained, and the pinning mechanism was studied by the Dew-Houghes model. It was found that the normal point pinning (NPP), the normal surface pinning (NSP), and the normal volume pinning (NVP) mechanisms play the main roles. The magnetic field and temperature dependence of contributions of the NPP, NSP, and NVP pinning mechanisms were obtained. The results show that the contributions of the pinning mechanisms depend on the temperature and magnetic field. From the temperature dependence of the critical current density within the collective pinning theory, it was found that both the δl pinning due to spatial fluctuations of the charge-carrier mean free path and the δTc pinning due to randomly distributed spatial variations in the transition temperature coexist at zero magnetic field in co-doped samples. Yet, the charge-carrier mean-free-path fluctuation pinning (δl) is the only important pinning mechanism at non-zero magnetic fields.

Pressure assisted enhancement in superconducting properties of Fe substituted NbSe2 single crystal

The impact of hydrostatic pressure (P) up to 1 GPa on Tc, Jc and the nature of the pinning mechanism in FexNbSe2 single crystals have been investigated within the framework of the collective theory. We found that the pressure can induce a transition from the regime where pinning is controlled by spatial variation in the critical transition temperature (δTc) to the regime controlled by spatial variation in the mean free path (δℓ). Furthermore, Tc and low field Jc are slightly induced, although the Jc drops more rapidly at high fields than at ambient P. The pressure effect enhances the anisotropy and reduces the coherence length, resulting in weak interaction of the vortex cores with the pinning centers. Moreover, the P can induce the density of states, which, in turn, leads to enhance in Tc with increasing P. P enhances the Tc with the rates of dTc/dP of 0.86, 1.35 and 1.47 K/GPa for FexNbSe2, respectively. The magnetization data are used to establish a vortex phase diagram. The nature of the vortices has been determined from the scaling behaviour of the pinning force density extracted from the Jc–H isotherms and demonstrates the point pinning mechanism.

Direct observation of in-plane anisotropy of the superconducting critical current density in Ba(Fe1−xCox)2As2 crystals

The phase diagram of iron-based superconductors exhibits structural transitions, electronic nematicity, and magnetic ordering, which are often accompanied by an electronic in-plane anisotropy and a sharp maximum of the superconducting critical current density (${J}_{\text{c}}$) near the phase boundary of the tetragonal and the antiferromagnetic-orthorhombic phase. We utilized scanning Hall-probe microscopy to visualize the ${J}_{\text{c}}$ of twinned and detwinned $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ ($x=5%--8%$) crystals to compare the electronic normal state properties with superconducting properties. We find that the electronic in-plane anisotropy continues into the superconducting state. The observed correlation between the electronic and the ${J}_{\text{c}}$ anisotropy agrees qualitatively with basic models, however, the ${J}_{\text{c}}$ anisotropy is larger than predicted from the resistivity data. Furthermore, our measurements show that the maximum of ${J}_{\text{c}}$ at the phase boundary does not vanish when the crystals are detwinned. This shows that twin boundaries are not responsible for the large ${J}_{\text{c}}$, suggesting an exotic pinning mechanism.

Hydrostatic pressure-induced huge enhancement of critical current density and flux pinning in Fe1−xCoxSe0.5Te0.5 single crystals

We performed a systematic study of the hydrostatic pressure (HP) effect on the supercon-ducting transition temperature (Tc), critical current density (Jc), irreversibility field (Hirr), upper critical field (Hc2), and flux pinning mechanism in un-doped and 3 at.% Co-doped FeSe0.5Te0.5 crystals. We found that Tc is increased from 11.5 to 17 K as HP increases from 0 to 1.2 GPa. Remarkably, the Jc is significantly enhanced by a factor of 3 to 100 for low and high temperature and field, and the Hirr line is shifted to higher fields by HP up to 1.2 GPa. Based on the collective pinning model, the δl pinning associated with charge-carrier mean free path fluctuation is responsible for the pinning mechanism of Fe1−xCoxSe0.5Te0.5 samples with or without pressure. A comprehensive vortex phase diagram in the mixed state is constructed and analysed for the 3 at.% Co-doped sample.

INVESTIGATION OF ANGULAR AND TEMPERATURE DEPENDENCE OF SPIN-WAVE RESONANCE SPECTRA IN MULTILAYER MAGNETIC FILMS

In this letter we study the dependences of the resonance field Hn and the linewidth 2ΔHn of the spin-wave resonance (SWR) modes in two-layer ferrite-garnet films with dissipative and mixed spinpinning mechanisms. It is established that for films with a dissipative mechanism of spin pinning, the increase in the linewidth at ΘH = 35° is associated with the maximum misorientation between the external magnetic field and the direction of the equilibrium magnetization of the sample. For films with a mixed spin-pinning mechanism, an increase in the linewidth 2ΔHn with increasing angle ΘH is associated with an increase in the depth of penetration of the spin wave into the pinning layer for such films. On the temperature dependence of the linewidth 2ΔHn for samples with a mixed fixation mechanism at a perpendicular orientation, an increase in the width of the absorption peak lines is observed. And with an increase in the peak number, the maximum of the linewidth shifts to the low-temperature region. With parallel orientation, the linewidth 2ΔHn in a wide temperature range from 20°C to 200°C remains practically unchanged.

Concept development and prediction of VCR engine performance using slider crank pin mechanism based on quasi-dimensional combustion modeling

Energy, economy and pollution factor of internal combustion engine profoundly influence the development of the society. It is well known that efficiency of an internal combustion engine increases by compression ratio, and is limited due to knocking and high thermal stress development in the combustion chamber of an engine. On another side, an efficiency of internal combustion engine decreases toward the more top engine speed. Considering this, a concept is proposed which can change its compression ratio with speed. Variation in compression ratio is achieved by a change in stroke length through moving crank pin within the crank. In this paper the mechanism of operation and prediction of performance characteristics of variable compression ratio spark ignition engine with varying speed using quasi-dimensional combustion simulation mode is presented. It was observed that proposed mechanism provides better fuel efficiency at higher engine speed. Further, it is inferred that 3 mm increase in crank length results in 7% increment in thermal efficiency and 8% decrement in brake specific fuel consumption for 13 compression ratio at 6000 rpm.

Comparison study of the flux pinning enhancement of YBCO superconductor with BZO and BZO + Y2O3 mixed phase additions

Adding nanophase defects to YBa2Cu3O7-δ (YBCO) superconductor thin films is well-known to enhance flux pinning, resulting in an increase in current density (Jct). While many previous studies focused on single phase additions, the addition of several phases simultaneously shows promise in improving current density by combining different pinning mechanisms. This paper compares the effect of the addition of two insulating, nonreactive phases of barium zirconium oxide (BZO) and yttrium oxide Y2O3, both as a single addition of BZO and as a double addition in conjunction with Y2O3. Processing parameters vary the target composition volume percent of BZO from 2-6 vol. %, while maintaining 3 vol. % Y2O3, and the remaining vol. % YBCO. Pulsed laser deposition produced thin films on LaAlO3 (LAO) and SrTiO3 (STO) substrates at various deposition temperatures. Comparison of strong and weak flux pinning mechanisms, current densities, critical temperatures, and microstructures of the resulting films will be presented. Films produced from the 2 vol. % BZO + 3 vol. Y2O3 doped YBCO target at a deposition temperature of 825 °C attained the highest current density. The addition of second phase Y2O3 impacted the film microstructure, resulting in more isotropic behavior when compared to the YBCO films doped with only BZO.

Point defect induced giant enhancement of flux pinning in Co-doped FeSe0.5Te0.5 superconducting single crystals

Point defect pinning centers are the key factors responsible for the flux pinning and critical current density in type II superconductors. The introduction of the point defects and increasing their density without any changes to the superconducting transition temperature Tc, irreversibility field Hirr, and upper critical field Hc2, would be ideal to gain insight into the intrinsic point-defect-induced pinning mechanism. In this work, we present our investigations on the critical current density Jc, Hc2, Hirr, the activation energy U0, and the flux pinning mechanism in Fe1-xCoxSe0.5Te0.5 (x = 0, 0.03 and 0.05) single crystals. Remarkably, we observe that the Jc and U0 are significantly enhanced by up to 12 times and 4 times for the 3at.% Co-doped sample, whereas, there is little change in Tc, Hirr, and Hc2. Furthermore, charge-carrier mean free path fluctuation, δl pinning, is responsible for the pinning mechanism in Fe1-xCoxSe0.5Te0.5.

Study of the second magnetization peak and the pinning behaviour in Ba(Fe0.935Co0.065)2As2 pnictide superconductor

Isothermal magnetic field dependence of magnetization and magnetic relaxation measurements were performed for the axis on a single crystal of Ba(Fe0.935 Co0.065)2As2 pnictide superconductor having Tc = 21.7 K. The second magnetization peak (SMP) for each isothermal M(H) was observed in a wide temperature range from Tc to the lowest temperature of measurement (2 K). The magnetic field dependence of relaxation rate R(H), showed a peak (Hspt) between Hon (onset of SMP in M(H)) and Hp (peak field of SMP in M(H)), which is likely to be related to a vortex-lattice structural phase transition, as suggested in the literature for a similar sample. In addition, the magnetic relaxation measured for magnetic fields near Hspt showed some noise, which might be the signature of the structural phase transition of the vortex lattice. Analysis of the magnetic relaxation data using Maley’s criterion and the collective pinning theory suggested that the SMP in the sample was due to the collective (elastic) to plastic creep crossover, which was also accompanied by a rhombic to square vortex lattice phase transition. Analysis of the pinning force density suggested a single dominating pinning mechanism in the sample, which did not showing the usual and nature of pinning. The critical current density (Jc), estimated using the Bean critical state model, was found to be 5 × 105 A cm−2 at 2 K in the zero magnetic field limit. Surprisingly, the maximum of the pinning force density was not responsible for the maximum value of the critical current density in the sample.

Improvement of flux pinning in GdBa2Cu3O7−δ thin film by nanoscale ferromagnetic La0.67Sr0.33MnO3 pretreatment of substrate surface

The present paper presents the effects of ferromagnetic La0.67Sr0.33MnO3 (LSMO) nanoparticles on the pinning characteristics of an epitaxial GdBa2Cu3O7−δ ((Gd) BCO) film deposited on top. LSMO nanoparticles with the size between 10 and 20nm were obtained on a (001) STO substrate by RF sputtering method. The analyses of magnetic measurements revealed that the presence of a complex vortex pinning mechanism within the (Gd) BCO film deposited on the undecorated substrate. With respect to a reference (Gd) BCO film, two additional pinning effects in LSMO decorated (Gd) BCO film were observed. One is the effect of the threading dislocations due to LSMO nanoparticles, and the other one is the LSMO nanoparticles itself. Like other nanosized particles, the LSMO nanoparticles on substrate will induce threading dislocations passing through the (Gd) BCO film. These threading dislocations serve as correlated pinning centers along the c-axis, which lead to the improvement of flux pinning properties in LSMO decorated (Gd) BCO film. According to the ferromagnetic property of LSMO material, a magnetic pinning mechanism was proposed to explain the significant increase of Jc and Fp values. The flux pinning abilities of LSMO nanoparticles were evaluated. The enhanced pinning characteristics in the high field region and high temperature together with the movement of H Fpmax (the corresponding field of Fpmax) also suggest a magnetic pinning mechanism for LSMO nanoparticles existed in LSMO decorated (Gd) BCO film.

Quasistatic remanence in Dzyaloshinskii-Moriya interaction driven weak ferromagnets and piezomagnets

We explore remanent magnetization ($\mu$) as a function of time and temperature, in a variety of rhombohedral antiferromagnets (AFM) which are also weak ferromagnets (WFM) and piezomagnets (PzM). These measurements, across samples with length scales ranging from nano to bulk, firmly establish the presence of a remanence that is quasi static in nature and exhibits a counter-intuitive magnetic field dependence. These observations unravel an ultra-slow magnetization relaxation phenomenon related to this quasi static remanence. This feature is also observed in a defect free single crystal of $\alpha$-Fe$_2$O$_3$, which is a canonical WFM and PzM. Notably, $\alpha$-Fe$_2$O$_3$ is not a typical geometrically frustrated AFM and in single crystal form, it is also devoid of any size or interface effects, which are the usual suspects for a slow magnetization relaxation phenomenon. The underlying pinning mechanism appears exclusive to those AFM which are either symmetry allowed WFM, driven by Dzyaloshinskii-Moriya Interaction (DMI) or can generate this trait by tuning of size and interface. The qualitative features of the quasi static remanence indicate that such WFM are potential piezomagnets, in which magnetization can be tuned by \textit{stress} alone.

RAS: a robotic assembly system for steel structure erection and assembly

This research focuses on a long-standing, yet critical problem in the erection of steel structures. In the current state of practice, steel workers must stand on an unfinished structure to assist with the assembly of structural elements manually. They must pull on the wire hanging under the rigging elements to align the bolting holes of the moving and fixed elements. This work is often performed in high places, which can be very risky. Therefore, we have developed a robotic assembly system (RAS) for steel beam erection and assembly to prevent workers from having to work in a high place. The RAS consists of four methods: rotation, alignment, bolting, and unloading. The rotation method involves a flywheel installed on top of the rigging beam, which aims to rotate the beam to the assembly angle. The alignment method includes both vertical and horizontal alignment. The vertical alignment relies on a camera and a marker on the column to align the beam altitude. The horizontal alignment relies on a specially-designed beam, which allows for it to be smoothly guided into the right position. The bolting method is used to connect the beam to a fixed element. We designed an additional guide hole above each bolt hole. The bolt can be inserted in the guide hole and slid to the bolt hole. The unloading method is used to unload the crane cable and the RAS. We use a pin mechanism for the beam-hook connection so it can easily be unplugged by a motor. The system is built in a scaled experimental construction site to validate its feasibility. The results show that the RAS can operate the assembly process without humans working at risky heights, and can complete faster than the traditional method. In conclusion, we have developed a robotics assembly system that can help reduce the frequency of accidental falls during the steel beam assembly process. The RAS adheres to the process of the current erection method and can be broadly introduced to existing construction sites.