P. Mikheenko

Inductance measurements of HTSC films with high critical currents

Abstract Non-destructive inductance measurements of critical currents of thin superconducting YBa 2 Cu 3 O x films were carried out. In the framework of the critical state model, an exact analytical solution for current and field patterns on thin superconducting disks possessing pinning and oriented perpendicular to the external magnetic field was found. The problem was solved for both constant and periodically varying external fields. The solution allows one to describe the states of the film in weak field when there are wide regions free of vortices. The theoretical results were used to restore the temperature dependence of critical current density of HTSC films from their diamagnetic response.

Introduction of pinning centres into Bi - (Pb) - Sr - Ca - Cu - O superconductors

High critical current density is essential for most large-scale applications of high-temperature superconductors (HTSs). In addition to the weak link problem, weak flux pinning is a major cause of the rapid decline of with magnetic field at high temperatures. Through intensive research in the past 10 years the weak pinning problem has been partially overcome and has reached a level approaching the requirement for some commercial applications. In this article, a number of techniques by which effective pinning centres can be introduced to improve flux pinning in Bi - (Pb) - Sr - Ca - Cu - O HTSs are reviewed. These include surface pinning through a spiral growth, fine precipitates produced through phase transformation or phase formation - decomposition, inclusions through chemical doping or addition, cascades and columnar defects created by fast-neutron and heavy ion beam irradiation and various defects induced by mechanical deformation. In particular, the results of Bi-2223 formation - decomposition and cold deformation versus hot deformation have a significant implication for HTS wire fabrication. Comparative studies with comprehensive electrical, magnetic and microstructural characterizations have been carried out to assess the effectiveness of these techniques.

Minute pinning and doping additions for strong, 20K, in-field critical current improvement in MgB2

Minute additions of a combination of Dy2O3 and B4C have been used to enhance both pinning and upper critical field in MgB2. A delicate balance of Dy2O3 and B4C additions is required to improve properties. The Dy2O3 nanoparticles react with B to form 10–15nm DyB4 nanoparticles, while B4C supplies carbon into the MgB2 crystal lattice and increases the upper critical field. The optimum level of Dy2O3 and B4C additions is ∼0.5wt% of Dy2O3 and 0.04wt% of B4C, yielding a Jc (20K) of 105Acm−2 at 2.7T.

Effect of spin-polarized injection on the mixed state of YBa2Cu3O7−δ

We report measurements of the magnetic moment of YBa2Cu3O7−δ films when subjected to injection of short pulses of spin-polarized current from the colossal magnetoresistance (CMR) manganites La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3, and unpolarized current from the nonmagnetic perovskite LaNiO3(LNO). We study relaxation from the Bean critical state, with and without current pulses. For pulses longer than 100 ms, the effect of injection is similar for all samples and is due to heating. Below 1 ms, we see a clear difference between the CMR and LNO samples and attribute this to the spin polarization of the current injected from the CMR materials.

Nonlocal electrodynamics of normal and superconducting films

Electrically conducting films in a time-varying transverse applied magnetic field are considered. Their behavior is strongly influenced by the self-field of the induced currents, making the electrodynamics nonlocal, and consequently difficult to analyze both numerically and analytically. We present a formalism which allows many phenomena related to superconducting and Ohmic films to be modeled and analyzed. The formalism is based on the Maxwell equations and a material current–voltage characteristics, linear for normal metals and nonlinear for superconductors, plus a careful account of the boundary conditions. For Ohmic films, we consider the response to a delta function source-field turned on instantly. As one of few problems in nonlocal electrodynamics, this has an analytical solution, which we obtain in both Fourier and real space. Next, the dynamical behavior of a square superconductor film during ramping up of the field, and subsequently returning to zero, is treated numerically. Then, this remanent state is used as initial condition for triggering thermomagnetic avalanches. The avalanches tend to invade the central part where the density of trapped flux is largest, forming dendritic patterns in excellent agreement with magneto-optical images. Detailed profiles of current and flux density are presented and discussed. Finally, the formalism is extended to multiply connected samples, and numerical results for a patterned superconducting film, a ring with a square lattice of antidots, are presented and discussed.

Artificial pinning centres in YBa2Cu3O7−δ thin films by Gd2Ba4CuWOy nanophase inclusions

Artificial pinning centres produced by Gd2Ba4CuWOy (2411W) nanophase inclusions in YBa2Cu3Oy (YBCO) thin films grown by pulsed laser deposition (PLD) using a YBCO target containing 1 mol% 2411W have been investigated. For comparison, YBCO thin films have been grown under the same conditions using a pure (commercial) YBCO target. The resulting films were characterized by AC susceptibility (superconducting transition) and SQUID magnetometry (critical current density, Jc), and by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy and atomic force microscopy (AFM) (structure, morphology and chemical composition). Jc of the film grown from the composite target is practically the same as that measured for the reference sample at 5 K over the whole range of applied field up to 4.5 T. For temperatures higher than 50 K, however, the presence of the nano-inclusions leads to increased Jc. The relative enhancement in Jc increases with further increase in temperature. Jc of the film containing nanophase inclusions is between two and three times higher than that of the reference sample at 77.3 K and 1.5 T. This can be explained by considering the dimensions of nanophase inclusions compared with the temperature-dependent coherence length. Susceptibility measurements reveal that, apart from a large and sharp transition at about 91 K, there is a secondary, smaller and broader transition between 88 and 84 K in the YBCO film containing nanophase inclusions. This can be explained by substitutions of compatible atoms between YBCO and the 2411W phase (Gd with Y), which might also create additional pinning centres of δTc-type.

Superconductivity in thin-film YBa2Cu3O7−δ/La0.7Ca0.3MnO3 bilayers

We report the influence of the magnetic state of thin films of La0.7Ca0.3MnO3 (LCMO) on the superconducting order parameter in LCMO/YBa2Cu3O7−δ (YBCO) thin-film bilayers. We find that the number density of Cooper pairs is enhanced at the coercive field of the LCMO layer relative to that where the magnetic moment is saturated. This effect is at most ∼10% and is only observable within a few degrees of the superconducting transition temperature. Experiments with thin SrTiO3 layers between the LCMO and YBCO layers provide strong evidence that the effect is a consequence of the spin-polarized nature of the electrons at the interface, rather than a direct magnetic effect.

Integrated nanotechnology of pinning centers in YBa2Cu3Ox films

A controlled pinning change from the ab-plane dominant to the c-axis dominant has been achieved in a novel method of nanostructured YBa2Cu3Ox (YBCO) growth. The method is a synchronous self-assembly of BaZrO3 (BZO) and Ag-assisted YBCO nanothreads. The formation of entangled nanothreads increases the critical current density while keeping the critical temperature close to that in pure YBCO films. The nanothreads extend through the whole thickness of thick films, making the method suitable for increasing total critical current density per centimeter of width (Ic − w). Two growth mechanisms, the formation of BZO nanorods and YBCO nanocolumns, complement each other, form a coherent structure and produce samples with strong correlated pinning. In addition to the increase in Ic − w, correlated pinning leads to an increase in vortex melting temperature in a wide range of magnetic fields. The films grown by this method have high Ic − w both in low magnetic fields along the c axis and high magnetic fields in the ab plane of YBCO. Such a superconductor would be suitable for both cable and magnet applications.

Surface pinning as origin of high critical current in superconducting films

We give theoretical and experimental evidence that high critical currents in laser ablated Y Ba2Cu3Ox films can be due to the pinning of vortices by surface roughness. The theory, which takes into account the bending of the vortex core and its orthogonality to the film surface, predicts that any surface elevation causes an increase in the core length and therefore produces an energy barrier whose height is proportional to the film thickness and can exceed bulk pinning barriers. Comparison with experiments gives satisfactory agreement with the value of critical current density and shows that the latter can be explained by the pinning at a steplike surface relief.

Bi-2223 bar current leads fabricated by the combination of cold isostatic pressing and hot-pressing

Abstract A new method using a combination of cold isostatic pressing (CIP) and hot-pressing (HP) was applied to fabricate Bi-2223 bar current leads. The critical current density (Jc) achieved by this method reached as high as 1000 A/cm2 at 77 K and self-generated magnetic field. This value of Jc presented here is much higher than the best Jc reported for rod current leads which is 570 A/cm2 achieved by CIP technique [Y. Yamada, Bi-based bulk current leads and their applications, in: H. Meada, K. Togano (Eds.), Bismuth-based High-temperature Superconductors, 1996, p. 277.]. The phases and microstructures were analysed by XRD and SEM. The texture and weak link were studied by pole figures and AC susceptibility, respectively. The results show that the grain connectivity, matrix density and texture of the samples were improved significantly by this method.