Change In Critical Current Density Research Articles

Enhanced spin orbit torque efficiency induced by large skew scattering in perpendicular Pt/Co/Ta multilayers with superlattice/alloying Nb (Ir) insertion

Perpendicular magnetization switching driven by spin–orbit torque (SOT) facilitates great potential applications in high-efficient memory and logic. However, SOT-based devices suffer from a relatively low SOT efficiency and ultrahigh current density in the conventional heavy metal/ferromagnet bilayer structure. Here, we report that the SOT behavior can be effectively tuned by inserting the ultrathin Nb superlattice into heavy metal Pt layer compared with Ir insertion or the PtNb alloying layer. A slight change of critical current density (Jc) can be found in the multilayers with Ir insertion. The Jc value for the Pt/Co/Ta multilayer with [Pt/Nb]7 insertion is decreased to 1.4 × 107 A/cm2, approximately 60% lower than that in pure Pt/Co/Ta multilayers. Furthermore, the SOT efficiency is significantly enhanced with increasing the period number due to the tunable spin Hall angle (θSH). Compared with pure Pt layer, the θSH value is increased 47% for the sample with [Pt/Nb]5, which is also significantly larger than that in the one with the PtNb alloying layer. Enhanced skew scattering induced by Nb superlattice plays the main role in these tunable SOT properties. Our findings provide a feasible scheme to engineer high-efficiency SOT-based logic-in-memory.

Investigation of the Correlation between Initial Microstructure and Critical Current Density of Nb-46.5 wt%Ti Superconducting Material

We have investigated the effect of initial microstructures on the change in critical current density (Jc) of Nb-46.5 wt%Ti (NbTi) superconducting material. It is well known that α-Ti phases distributed in NbTi material act as a flux pinning center, resulting in an improvement in critical current density. Therefore, it is crucial to obtain the grain-refined microstructure, which is strongly related with precipitation of uniformly distributed fine α-Ti phases and higher volume faction of α-Ti phases, as α-Ti phases are precipitated at the grain boundaries and triple points during heat treatments. Therefore, in order to characterize the effect of initial microstructure of NbTi on critical current density, different initial microstructures were obtained by applying equal channel angular pressing (ECAP) and hot rolling with different strains. It was revealed experimentally that hot rolling with a higher strain is efficient for obtaining the initial microstructure, which has equiaxed fine grains of β-NbTi with the aid of dynamic recrystallization, and which is helpful for precipitating fine α-Ti phases during intermediate heat treatment. Furthermore, it was confirmed that critical current density can be enhanced by obtaining a smaller α-Ti phase, a higher aspect ratio of α-Ti phase, a higher volume fraction of α-Ti phase and a ribbon-like folded α-Ti phase.

Induced change of critical current density profile in Nb/Al-AlOx/Nb Josephson junctions

A technique to induce spatial modulation of critical current density in niobium based Josephson devices by using a selective thermal annealing is reported. By depositing a carbon film onto selected region of the Josephson element it is possible to induce a localized heating, with a spatial resolution less than 1 μm, exploiting the much higher absorbance coefficient of carbon than the niobium one. The effectiveness of such technique is demonstrated by experimental measurement of the critical current vs. magnetic field, measured at T = 4.2 K, showing that the change of critical current density occurs only in the region corresponding to the absorber film area. Furthermore, the theoretical behaviour, by modelling a suitable step-like junction barrier shape, has been carried out to fit the experimental data in order to verify the selective modulation of critical current. This technique can be very useful in view of quantum computing experiments, Majorana fermions detection and superconducting magnetic sensors.

Effects of Reaction Temperature and Alloying on Performance of Restack-Rod-Process ${\rm Nb}_{3}{\rm Sn}$

Final reactions of 48 or 50 hours were given in a range of temperature from 635 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C to 695 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C to high-J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> Ta- and Ti-alloyed Restack-Rod-Process Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands to investigate the changes in critical current density and the superconducting properties. J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> is the current density in the non-copper region of the wire. Measurements of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> were made at 4.2 K temperature and from 8 to 11.5 T field at BNL and from 12 to 16 T at OST, and these data were fitted to the Summers expression for J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> to extrapolate the scaling field . B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">*</sup> ldr All three(Nb, Ta) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand designs investigated displayed peak J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> values of ~3000 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 12 T and ~at 1500 A/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 15 T for 665 and 680 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C reactions. B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">*</sup> increased monotonically with increasing reaction temperature, a (Nb, Ti) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , By comparison, at each reaction temperature, a strand under development had a B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">*</sup> higher than any of the Ta-alloyed strands. This suggests that Ti-alloyed strands could improve high-field performance if further development can bring their J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> values closer to those of the Ta-alloyed strands. Other implications are also discussed.

Change in critical current density of Bi-2212 single crystals due to high-energy heavy-ion irradiation followed by thermal annealing

Change in critical current density of Bi 2Sr 2CaCu 2O 8+ x (Bi-2212) single crystals due to high-energy heavy-ion (200 MeV Au or 200 MeV Ag) irradiation followed by thermal annealing treatment was investigated. After irradiation at the fluence of 5×10 11 cm −2 for both Au ions and Ag ions, the specimens were annealed in air. Different annealing time dependencies of critical current density ( J c) were observed between the measurement temperatures of 20 and 40 K for both Au- and Ag-irradiated specimens. J c gradually decreased with the increase of annealing time at 20 K. On the other hand, it gradually increased at 40 K. Such different dependencies of J c are considered to be attributed to the different kind of defects that contribute dominantly to the pinning effect at each temperature.

Tensile strain/transverse compressive stress dependence of critical current in Bi(2212) superconducting tapes with Zr-reinforced Ag sheath

The tensile mechanical property and the axial tensile strain/transverse compressive stress dependence of the critical current I c in mono- and 19-core Bi(2212) superconducting tapes with Zr-reinforced Ag sheaths were evaluated at a temperature of 4.2 K and a magnetic field of 14 T. Replacement of Ag sheath by Ag-Zr alloy sheath increased the 0.2% proof stress and the onset tensile strain for I c degradation although I c decreased markedly. Double layer sheath with Ag inner layer and Ag-Zr alloy outer layer improved both the stress-strain relation and the strain characteristics of I c without remarkable degradation in I c . The increase in the number of core from 1–19 degraded the tensile strain vs. I c characteristics of the tape, although no marked change in critical current density is observed. The alloying of sheath also improved the transverse compressive stress vs. I c characteristics. The effects of Zr alloying of Ag sheath on the mechanical behavior and the stress/strain dependence of I c of the superconducting composites are briefly discussed with emphasis on texturing of the oxide superconductor.

Flux pinning and superconductivity in in situ prepared Al- and Zn-doped Cu-Nb composite wires

The effects of Al and Zn on the critical current density, upper critical field and pinning force of in situ prepared Cu-Nb wires have been studied. Doping this system by 1 wt% of these impurities results in a change in critical current density and pinning force. The critical current density increases at lower fields for both additives but decreases at high fields. The upper critical field HC2 shows a decrease for both additives. We have modified the Kramer flux pinning law for these two additives. It is found that the value of n, which is a constant in the scaling law of Fietz and Webb with a value of 2.5, varies in the case of these additives and does not remain constant.

Properties and Defects of Type II Superconductors

Three major superconducting parameters can characterize a type II superconductor: critical transition temperature, Tc, upper critical magnetic field, Hc2, and critical current density, Jc. Because of the variety of crystal structures, chemical stoichiometrics, and microstructures of the materials, these superconducting parameters differ greatly from system to system.It has been found that the critical transition temperature is closely related to the crystal structure and stoichiometry. Previous studies have shown that compounds with a high degree of symmetry tend to be more favorable to superconductivity. Compounds with a cubic structure, such as a bcc structure, usually have higher Tc values than compounds with a hexagonal lattice. Among conventional superconductors, the so-called A-15 structure is most favorable for high transition temperatures. It is well known that many high Tc superconductors such as YBa2Cu3Ox and Bi2Sr2CaCu2Ox have an orthorhombic structure with a high degree of symmetry.The upper critical field, Hc2, is more complicated in terms of its relationship to structural characteristics. Experimental results have indicated that Hc2 is more closely related to the chemical stoichiometry and crystal structure than to the microstructure.However, the critical current density, Jc, can vary tremendously in a compound with a given crystal structure. The change in critical current density is associated with the so-called flux pinning that arises from the interaction between the flux lines and crystal defects. Thus, Jc is determined mostly by the microstructure of the materials.

Critical Current Density Enhancement and Moisture Destruction Studies of the Compound Superconductor Y-Ba-Cu-Ag-O

ABSTRACTWe have fabricated a series of samples of YBa2(Cu1−x Agx)3Oy compounds where 0 ≤ × ≤ 1, with the intention to study the changes in critical current density and the effect of moisture treatment on normal state resistance R in these high transition-temperature superconductors. The samples exhibited a decrease of the transition-temperature Tc vs the Ag concentration x. Tc is about 93 K for YBa2Cu3Oy and 20 KCfor YBa2Ag3Oy, but it is almost unaffected by the presence or Ag for × &lt; 0.2. An enhancement of the critical current density Jc by nearly an order of magnitude for the samples with 0 &lt; × ≤ 0.1 has been observed. Both Jc and R are found to be sensitive to the ambient moisture conditions.

Change of critical current density of superconducting vanadium induced by neutron irradiation

The effects of neutron irradiation on the critical current density J c of vanadium single crystals have been studied. It is shown that the radiation enhances J c up to a fluence of about 4 × 10 18 n/cm 2. The value of the difference in J c 2 before and after irradiation divided by J c 2 before irradiation, [ J c(A.I.) 2− J c(B.I.) 2]/ J c(B.I.) 2, increases in proportion to the neutron fluence, which is tentatively explained by a model in which J c depends on the interaction between flexible flux lines and randomly distributed pinning centers.