Low Critical Current Density Research Articles

ChemInform Abstract: Spin‐Transfer Magnetization Switching in Ordered Alloy‐Based Nanopillar Devices

AbstractReview: 55 refs.

Microstructure and Superconductive Property of MgB2/Al Composite Materials

MgB2has the higher critical temperature of superconducting transition (TC: 39K) among the intermetallic compound superconductive materials, however, MgB2is hard for practical use because of its unworkable and lower critical current density (JC) in a high magnetic field than Nb-based superconductive materials. We have developed the original method of three-dimensional penetration casting (3DPC) to fabricate the MgB2/Al composite materials. In the composite material we made, MgB2particles dispersed to the matrix uniformly. Thus, these composite materials can be processed by machining, extrusion and rolling. TheTCwas determined by electrical resistivity and magnetization to be about 37～39K. In this work, we made composite material with ground MgB2particle with the purpose of extruding thinner wires of composite material, successfully produced φ1mm wire and changed the matrix from pure Al to Al-In alloy.JCof composite materials with the matrix of Al-In alloy was calculated from the width of the magnetic hysteresis based on the extended Bean model. The result was better than that of MgB2/Al composite material without Indium. Microstructures of these samples had been confirmed by SEM observation.

Nanoelliptic Ring-Shaped Magnetic Tunnel Junction and Its Application in MRAM Design With Spin-Polarized Current Switching

Nanoring magnetic tunnel junction (NR-MTJs) with outer diameter of 90 nm, and nanoelliptical ring (NER) -shaped MTJ with major outer diameter of 120 nm, and minor outer diameter of 70 nm were fabricated. For both geometrical structures, the ring width was kept at around 30 nm. The magnetic field driven and current-induced magnetization switching of the NR-MTJs and NER-MTJs were investigated. Compared with the NR-MTJs, the NER-MTJs show a sharp magnetization switching and lower coercivity under the sweep of an external magnetic field, which indicates the higher uniaxial anisotropy of NER-MTJs. In addition, the current-induced magnetization switching was achieved in both MTJs which indicated a lower critical current density in sandwiched-type and spin-valve-type NER-MTJs. This reduction of critical current could be resulting from different distribution of current-induced Oersted field in the NR-MTJs and NER-MTJs, which play an assisted role in spin transfer torque switching. This work proposes the NER-MTJs with the low current density, and small stray field, as data storage bits of magnetic random access memory (MRAM), could be a promising candidate for further reducing the power consumption, scaling memory cell size and increasing density of MRAM.

Spin-transfer magnetization switching in ordered alloy-based nanopillar devices

This paper reviews spin-transfer magnetization switching in ordered alloy-based nanopillar devices. L10-ordered FePt was used for one of the earliest demonstrations of spin-transfer switching in perpendicularly magnetized systems. The behaviour of magnetization switching deviates from the predictions based on a macro-spin model, suggesting incoherent magnetization switching in the system with a large perpendicular magnetic anisotropy. The effect of a 90° spin injector on spin-transfer switching was also examined using L10-ordered FePt. Full-Heusler alloys are in another fascinating material class for spin-transfer switching because of their high-spin polarization of conduction electrons and possible small magnetization damping. A B2-ordered Co2FeAl0.5Si0.5-based device showed a low intrinsic critical current density of 9.3 × 106 A cm−2 for spin-transfer switching as well as a relatively large current-perpendicular-to-plane giant-magnetoresistance (CPP-GMR) up to ∼9%. The specific physical properties of ordered alloys may be useful for fundamental studies and applications in spin-transfer switching.

Quantum crossover in moderately damped epitaxial NbN/MgO/NbN junctions with low critical current density

High quality epitaxial NbN/MgO/NbN Josephson junctions have been realized with MgO barriers up to a thickness of d = 1 nm. The junction properties coherently scale with the size of barrier, and low critical current densities down to 3 A/cm2 have been achieved for larger barriers. In this limit, junctions exhibit macroscopic quantum phenomena for temperatures lower than 90 mK. Measurements and junction parameters support the notion of a possible use of these devices for multiphoton quantum experiments, taking advantage of the fast non equilibrium electron-phonon relaxation times of NbN.

Effect of ZnO Nano-Oxide Addition on the Superconducting Properties of the (Bi.Pb)2223 Phase

The major limitation of Bi-system superconductor applications is the intergrain weak links and weak flux pinning capability producing low critical current density of the Bibased phases. In order to enhance these characteristics and other superconducting properties, effective flux pinning centers are introduced into high temperature superconductors. In this work, different weight percentages of ZnO nano oxide were introduced at the final stage of the Bi1.8Pb0.4Sr2Ca2Cu3O10-y superconductor preparation process. Phase characterization was completed by X-ray diffraction (XRD). Exact constitution of the samples was determined using particle induced X-ray emission (PIXE). Granular and microstructure were investigated using scanning electron microscopy (SEM). Electrical resistivity as function of the temperature was carried to evaluate the relative performance of samples, and finally, E-J characteristic curves were obtained at 77K. Using 0.4 ZnO weight percentage, the electrical and granular properties were greatly enhanced, indicating more efficient pinning mechanisms. A critical current density of 949 A/cm2 was obtained which represents more than twice the value obtained for the pure sample (Jc= 445 A/cm2).

Measurement of Epitaxial NbN/AlN/NbN Tunnel Junctions With a Low Critical Current Density at Low Temperature

Superconducting qubit circuits require high quality low critical current density Josephson tunnel junctions. We have developed high quality epitaxial NbN/AlN/NbN tunnel junctions with critical current density, Jc, ranging from as low as a few A/cm2 to a few tens of kA/cm2. In this work, we measured current-voltage characteristics for junctions with a Jc ~ 32 A/cm2 in the temperature range from 17 mK to above 6.7 K. All the junctions are in good quality and have a high gap voltage (>; 5.5 mV). We found that the junctions' sub-gap resistances were temperature dependent above 2.5 K and saturated at temperatures below 2.5 K. A junction sub-gap leakage current Ir of about 23 pA was measured. The voltage noise power spectral density for 3 junctions with different sizes has been measured at the base temperature of 17 mK with the junctions biased above the gap voltage. The spectral densities clearly show 1/f behavior at low frequency and are proportional to the junction's linear dimension.

Estimation of Critical Current Densities in Polycrystalline ${\rm Sr}_{0.6}{\rm K}_{0.4}{\rm Fe}_{2}{\rm As}_{2}$ Superconductors

The superconducting Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.6</sub> K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.4</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (122 type) polycrystalline iron-based pnictides with Pb or Ag addition were prepared by a normal PIT method. The critical current densities were investigated by ac inductive (Campbell's) method. It was found that a fairly large critical current density over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> A/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> even at the relatively high temperatures flows locally with the perimeter size similar to the average grain size of the bulk samples, while an extremely low global critical current density of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> A/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> flows through the whole sample. Furthermore, a unique history dependence of global critical current density was observed, i.e., it shows a smaller critical current density in the increasing-field process than that in the decreasing-field process. Basing on the experimental results of the penetrating ac magnetic flux profile and the characteristic of ac magnetic field vs. penetration depth, the inter- and intra-granular super-current, the weak-link and the history effect in these materials were discussed.

PATTERNED NANOSCALE MAGNETIC TUNNEL JUNCTIONS WITH DIFFERENT GEOMETRICAL STRUCTURES

In this paper, patterned nanoscale magnetic tunnel junctions (MTJs) with different geometrical structures, including nanodisk (ND), nanoellipse (NE), nanoring (NR) and nanoelliptical ring (NER) with the scale of around 100nm and ring width of around 30nm, were fabricated, respectively. The geometrical-shape dependence of magnetic field-driven and current-induced magnetization switching (CIMS) were studied in the nanoscale magnetic tunnel junctions (MTJs). The NER-MTJs showed robust magnetization switching and low critical current density of CIMS, comparing with other geometrical-shaped MTJs. This may be due to the different distribution of current-induced Oersted field in different geometrical structures, which plays an assisted role in CIMS. The present experiments indicate that NER-MTJs may be one of the promising candidates for the cells of high-density and low-consumption spintronic devices.

Coarse ${\rm Nb}_{3}{\rm Sn}$ Grain Formation and Phase Evolution During the Reaction of a High Sn Content Internal Tin Strand

The formation of coarse Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn grains in Internal Tin (IT) strands has been studied at the example of a prototype strand with high Sn content. Metallographic examination revealed that the comparatively low critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) of this strand is partly due to the formation of a significant fraction of coarse grained Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn at the periphery of the individual filaments within the subelements. The phase evolution during the reaction heat treatment has been determined in situ by high energy synchrotron X-ray diffraction as well as ex situ by Energy Dispersive X-ray Spectroscopy in a Scanning Electron Microscope (SEM) in order to identify the conditions under which the coarse grains form. Similar to what is observed in the tubular type strands, Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn coarse grain formation occurs in the filament areas that had first been transformed into NbSn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> , prior to Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn formation, and it accounts for an estimated <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Jc</i> reduction of roughly 20%. The amount of Cu-Nb-Sn and NbSn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> that is formed during the heat treatment can be reduced by increasing the temperature ramp rate, while the amount of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> formed appears to be hardly influenced by the different heat treatments that have been tested.

Direct observation of nanometer-scale amorphous layers and oxide crystallites at grain boundaries in polycrystalline Sr1−xKxFe2As2 superconductors

We report here an atomic resolution study of the structure and composition of the grain boundaries in polycrystalline Sr0.6K0.4Fe2As2 superconductor. A large fraction of grain boundaries contain amorphous layers larger than the coherence length, while some others contain nanometer-scale crystallites sandwiched in between amorphous layers. We also find that there is significant oxygen enrichment at the grain boundaries. Such results explain the relatively low transport critical current density (Jc) of polycrystalline samples with respect to that of bicrystal films.

Progress of 275 kV–3 kA YBCO HTS cable

A 275kV–3kA high temperature superconducting (HTS) cable has been developed in the Materials & Power Applications of Coated Conductors (M-PACC) project.AC loss reduction of a two-layer HTS conductor was undertaken by removing the edges of YBCO tapes with low critical current density. The HTS conductor using these tapes was fabricated, and low loss of 0.235W/m at 3kArms was achieved.The 275kV–3kA cable was designed, and the 2m model cables were fabricated. This cable had 325mm2 copper stranded former inside the HTS conductor and a 310mm2 copper shield layer on the HTS shield layer for over-current protection. These cables withstood an over-current of 63.0kA for 0.6s, which is the worst situation for 275kV systems.The partial discharge (PD) and V–t characteristics of a liquid nitrogen (LN2)/polypropylene (PP) laminated paper composite insulation system have been integrated into the design of the insulation for the 275kV cable. The results revealed that the PD inception stress (PDIE) did not depend on the insulation thickness, and that lifetime indices of V–t characteristics at PD inception were as high as about 80–100.

1/f noise in NbN/AlN/NbN Josephson junction with a low critical current density

Fabricating high quality Josephson junction is essential in superconducting qubit circuits. In this letter, high quality tunnel junctions with a low critical current density of ∼32A/cm2 were fabricated. The 1/f noise parameter η was obtained from junctions’ low frequency voltage noise power spectral density Sv(f)=1fηA(IbRD)2. Our findings showed that the noise spectrum is junction linear dimension dependent, indicating the junction’s side leakage current induced by defects in the amorphous oxide insulator dominates the origin of the 1/f noise in our epitaxial NbN/AlN/NbN tunnel junctions.

Low spin current-driven dynamic excitations and metastability in spin-valve nanocontacts with unpinned artificial antiferromagnet

This work investigates the spin-torque-related dynamics of nonuniform magnetic vortexlike states in spin-valve nanocontacts, employing an unpinned artificial antiferromagnet as polarizer and amorphous CoFeB as free layer. Subgigahertz spectra are obtained for contacts of 150–200 nm in diameter. Low critical current density and reversibility of the dynamic spectra with respect to the current are obtained. The spectral power and linewidth depend on the in-plane magnetic field, assuming maximum, respectively minimum, values within the free layer’s magnetization switching. For certain field and current windows metastable dynamic states are clearly demonstrated.

Giant electric-field-induced reversible and permanent magnetization reorientation on magnetoelectric Ni/(011) [Pb(Mg1/3Nb2/3)O3](1−x)–[PbTiO3]x heterostructure

We report here an atomic resolution study of the structure and composition of the grain boundaries in polycrystalline Sr0.6K0.4Fe2As2 superconductor. A large fraction of grain boundaries contain amorphous layers larger than the coherence length, while some others contain nanometer-scale particles sandwiched in between amorphous layers. We also find that there is significant oxygen enrichment at the grain boundaries. Such results explain the relatively low transport critical current density (Jc) of polycrystalline samples with respect to that of bicrystal films.

Recent Progress in Material Technology on RE-Ba-Cu-O Bulk Superconductors

The current status of large-grained RE-Ba-Cu-O (RE: Y or rare earth elements) bulk superconductors with excellent superconducting properties is described. Gd-Ba-Cu-O bulk superconductors can trap a very high magnetic field even if they are melt-processed in air. Although the electromagnetic force caused by the trapped field is larger for a larger sample and may break the sample, a large sample of Gd-Ba-Cu-O 46 mm in diameter has the potential of trapped magnetic fields greater than 10 T at around 40 K. In addition, single-grained bulk superconductors as large as 150 mm can be obtained using the RE compositional gradient method. Dy-Ba-Cu-O is an ideal material for current leads because it has low thermal conductivity and high critical current density at 77 K in high magnetic fields. Eu-Ba-Cu-O has low magnetic permeability, and is therefore suitable for bulk NMR applications. Progress in machining technology has made possible various bulk superconductors with complicated shapes such as coils, leading to small and strong electromagnets by stacking several coil-shaped bulk superconductors together.

Numerical Simulation of Permanent Magnet Method: Applicability to $j_{\rm C}$-Measurement in HTS Thick Film

Applicability of the permanent magnet method to the measurement of the critical current density in a high-temperature superconducting (HTS) thick film has been investigated numerically. A numerical code has been developed for analyzing the shielding current density in a HTS sample. By using the code, the permanent magnet method is numerically simulated. The results of computations show that the maximum repulsive force is roughly proportional to the critical current density for the case with a low critical current density. On the other hand, the proportional relation does not hold for the case with a high critical current density. In other words, there exists an upper limit of the measurable critical current density. Furthermore, it is found that the limit increases with the strength of the permanent magnet. This means that applicability of the permanent magnet method to a thick film can be considerably improved by means of a strong magnet.

단층 초전도케이블 샘플에서 교류손실의 수치해석에 대한 연구

AC loss is one of the important factors for commercialization of a high temperature superconducting (HTS) cable from an economic point of view. But AC loss characteristics of the HTS-cable are not elucidated completely because of its complex structure. As an earlier stage of analyzing the AC loss in the 22.9 kV/50 MVA, 100m HTS-cable system of Korea Electric Power Corporation (KEPCO) which is now in collaboration with us, a two-dimensional (2D) numerical model, which takes into account the nonlinear conductivity properties of a high temperature superconductor, has been developed. In order to examine our 2D model, we have prepared several single-layer cable samples whose AC losses are sufficiently reliable due to their simple structure. The AC losses of the samples were experimentally investigated and then compared with our 2D model. The results show that the numerically calculated AC losses are not in good agreement with the measured ones for the cylindrical cable and deca-cable samples with low critical current density. However, the numerically calculated and measured AC losses are relatively in good agreement for the deca-cable and hex-cable samples with high critical current density, although the difference between these two loss data in the deca-cable sample tends to increase in the low current region.

Three-dimensional numerical analysis of magnetic and thermal fields during pulsed field magnetization of bulk superconductors with inhomogeneous superconducting properties

Pulsed field magnetization (PFM) is an effective and practical magnetization method for bulk superconductor. When pulsed magnetic field is applied, however, large shielding current is induced in the direction to prevent magnetic fluxes from penetrating into the bulk superconductor. Trapped magnetic flux characteristics are sensitive to this magnetic shielding effect. In addition, inhomogeneity of superconducting properties influences a lot on the PFM characteristics, because the flux motion induced heat generation, the temperature rise, and the reduction of critical current density are strongly interrelated to one another. Such a complicated problem can be analyzed by three-dimensional finite element method. We have studied magnetic flux motion in bulk superconductor with inhomoneneous critical current distribution during PFM. The results indicate that magnetic flux penetration through the lower critical current density region and the sensitivity of this phenomenon to the assumed analysis model.

超伝導体における磁束ピンニング[7

The flux pinning properties of RE-123 bulk superconductors, RE-123 coated conductors, Bi-2223 tapes and MgB2 superconductors are reviewed. In bulk superconductors, the pinning mechanism of the lower Tc region, such as the Ba sites substituted by RE elements or twin boundaries with oxygen deficiency, is considered to be kinetic energy interaction under a proximity effect with the superconducting matrix. The dependence of the irreversibility field on the superconducting layer thickness is very complicated for RE-123 coated conductors. This is partly attributed to the thickness dependence of the critical current density that arises from the deterioration of the superconducting layer structure in thicker films and partly to the thickness dependence of the flux creep. The reason for the dramatic improvement in the critical current properties is also discussed for Bi-2223 tapes fabricated using the over-pressure sintering technique. Theoretical analysis using the percolation theory clarifies that the low critical current density in conventional MgB2 in situ wires is attributed to low electrical connectivity, which is due to the voids and wetting insulating layers between grains. It is clarified that the flux pinning strength of the grain boundaries in MgB2 is significantly strong, and there is a room for a drastic increase in the flux pinning strength by dirtying the MgB2 through C-doping. Finally, the flux pinning mechanism of columnar defects nucleated by heavy ion irradiation is discussed. It is shown that the columnar defects larger than the coherence length are desirable to increase the probability of flux lines being easily captured by the defects.