Olga V Shcherbakova

The effect of nanoscale Fe doping on the superconducting properties of MgB2

Iron is an important sheath material for fabrication of MgB2 wires. However, the effect of Fe doping on the superconducting properties of MgB2 remains controversial. In this work, we present results on nanoscale Fe particle doping in MgB2. The Fe doping experiments were performed using both bulk and thin film forms. Neither free Fe particles nor FeB compound was detected at 1% Fe doping by means of either transmission electron microscopy (TEM) or x-ray diffraction (XRD), suggesting that Fe substituted for Mg in the lattice. The level of Fe substitution for Mg is estimated to occur at a level lower than 1% of Mg, and this substitution is proposed to be responsible for the decrease in transition temperature with Fe doping. Because of the high reactivity of nanoscale Fe particles, Fe doping is largely in the form of FeB at a Fe doping level of 2% while Fe2B was detected at 10% Fe doping by means of both XRD and TEM. The Jc(H) performance was severely depressed at above a 2% doping level. The detrimental effect of nanoscale Fe doping on Jc(H) is attributable to both the Fe substitution for B in the lattice structure and the inclusions of Fe and FeB which act as weak links at grain boundaries.

Sugar as an optimal carbon source for the enhanced performance of MgB2 superconductors at high magnetic fields

In this paper we report the results of an extended study of the effect of sugar doping on the structural and electromagnetic properties of MgB2 superconductors. High values of the upper critical field (Bc2) of 36 T and the irreversibility field (Birr) of 27 T have been estimated at the temperature of 5 K in a bulk MgB2 sample with the addition of 10 wt% of sugar. The critical current density (Jc(Ba)) of sugar-doped samples has been significantly improved in the high field region. The value of transport Jc has reached as high as 108 A m−2 at 10 T and 5 K for Fe-sheathed sugar-doped MgB2 wire. The analysis of the pinning mechanism in the samples investigated indicated that dominant vortex pinning occurs on the surface type of pinning defects, such as grain boundaries, dislocations, stacking faults etc, for both pure and doped MgB2. In sugar-doped samples, pinning is governed by numerous crystal lattice defects, which appear in MgB2 grains as a result of crystal lattice distortion caused by carbon substitution for boron and nano-inclusions. The drastically improved superconducting properties of sugar-doped samples are also attributed to the highly homogeneous distribution and enhanced reactivity of this dopant with host Mg and B powders. The results of this work suggest that sugar is the optimal source of carbon for doping MgB2 superconductor, especially for application at high magnetic fields.

Magnetic field processing to enhance critical current densities of MgB2 superconductors

A magnetic field of up to 12T was applied during the sintering process of pure MgB2 and carbon nanotube (CNT) doped MgB2 wires. The authors have demonstrated that magnetic field processing results in grain refinement, homogeneity, and enhancement in Jc(H) and Hirr. The extent of improvement in Jc increases with increasing field. The Jc for a 10T field processed CNT doped sample increases by a factor of 3 at 10K and 8T and at 20K and 5T, respectively. Hirr for the 10T field processed CNT doped sample reached 9T at 20K, which exceeded the best value of SiC doped MgB2 at 20K. Magnetic field processing reduces the resistivity in CNT doped MgB2, straightens the entangled CNTs, and improves the adherence between CNTs and the MgB2 matrix.

Vortex-glass phase transition and enhanced flux pinning in C4+-irradiated BaFe1.9Ni0.1As2 superconducting single crystals

We report the effects of C4+-irradiation on the superconducting properties of BaFe1.9Ni0.1As2 single crystal. The BaFe1.9Ni0.1As2 single crystal before and after C4+-irradiation was characterized by magnetic, magneto-transport and magneto-optical techniques over a wide range of magnetic fields (0–13 T) and temperatures (2–200 K). We demonstrate that the C4+-irradiation significantly enhances the in-field critical current density (by a factor of up to 1.5 at 5 K) and induces enhanced flux jumping at 2 K, with only a small degradation (by 0.5 K) of the critical temperature, Tc. The vortex phase diagram describing the evolution of the vortex-glass transition temperature with magnetic field and the upper critical field has been resolved for the C4+-irradiated sample. For temperatures below Tc, the resistivity curves and the pinning potential were found to show good scaling, using a modified model for vortex-liquid resistivity. The vortex state is three dimensional at temperatures lower than a characteristic temperature. Good agreement between the thermally activated flux flow model, which is usually employed to account for the resistivity in the vortex-liquid region, and the modified vortex-liquid model, has been observed.

Influence of the cooling rate on the main factors affecting current-carrying ability in pure and SiC-doped MgB2 superconductors

We have systematically studied and compared the effect of cooling rate on microstructure, critical current density, upper critical field and irreversibility field in pure and 10 wt% SiC-added MgB2 superconductors. The sintering process was carried out on the samples at a temperature of 750 °C for 1 h followed by quenching or cooling to room temperature in 0.3 h (2433 °C h−1), 14 h (52 °C h−1) and 25 h (30 °C h−1). Changes in the microstructure due to variations in cooling rate have been studied with the help of scanning and transmission electron microscopy. Correlations between microstructure and superconducting properties have been observed, identified and explained for both pure and SiC-added MgB2 samples. Modifications to the pinning environment and grain boundary transparency are considered to be responsible for variations in the current-carrying ability. The dominant pinning on grain boundaries in the pure MgB2 samples and on nano-inclusions (inducing accompanying defects) in the SiC-doped samples is clearly distinguished. On the basis of our experimental results, we have concluded that the cooling rate can be an important parameter influencing the superconducting properties of MgB2 samples.

The effect of doping level and sintering temperature on Jc(H) performance in nano-SiC doped and pure MgB2 wires

Nanoscale SiC doped Fe∕MgB2 wire samples were prepared by an in situ reaction technique using SiC doping levels of 0, 5, 10, and 15 wt %. Samples were heat treated at different temperatures using different temperature profiles. The effects of doping level and sintering temperature on superconducting properties of wire samples were investigated. The important finding of this study was that the enhancement in Jc(H) by nano-SiC doping can be achieved at different field regions by appropriate compromising of the doping level and sintering temperature.

Rectifying differences in transport, dynamic, and quasi-equilibrium measurements of critical current density

The dependence of the critical current density (Jc) on electric field criteria (Ecr) is studied for high-quality YBCO (YBa2Cu3O7) thin films over the entire applied magnetic field (Ba) range. The quantitative model describing the Jc(Ba) dependence is compared and explained for the critical current densities obtained by different measurement techniques. Transport current and quasi-equilibrium magnetization measurement data can successfully be fitted by the model with appropriate electric field criteria. The dependence of the irreversibility field on the Ecr criterion can be obtained within the model. At the same time, the dynamic magnetization measurements of the Jc(Ba) curves strongly depend on instrumentally defined parameters, introducing inconsistencies in the experimental results. Therefore, the model calculations are able to explain the Jc(Ba) curves only if the instrumental vibrations affecting vortex behaviour are taken into account. However, the nature of the observed dependence on the vibration o...

Large, Controllable Spikes of Magnetoresistance in La2/3Ca1/3MnO3/SrTiO3 Superlattices

We have investigated superlattices consisting of up to 30 epitaxial nanomultilayers (3-7 nm thick) of ferromagnetic La(2/3)Ca(1/3)MnO(3) (LCMO) and insulating SrTiO(3) (STO) hybrids. The superlattices demonstrate dramatic shifts of Curie temperature, indicating the possibility of its tunability. The metal-insulator transition (MIT) has been observed around 140 K. Below the MIT temperature, the superlattices have shown sharp drops of resistivity, facilitating the largest and sharpest magnetoresistance peaks (>2000%) ever observed in LCMO films and superlattices at low temperatures. The observed experimental results can be explained in the frame of the phase separation model in manganites with well-organized structures. The results of magnetic and transport measurements of such hybrid structures are discussed, indicating a magnetodielectric effect in STO interlayers. The magnetic and transport properties of the superlattices are shown to be technology-dependent, experiencing dimensional transitions, which enables the creation of structures with prescribed magnetoresistance characteristics for a broad range of applications.

An all-field-range description of the critical current density in superconducting YBCO films

A new critical current density (Jc) model for high-quality YBCO (YBa2Cu3O7) thin films has been proposed, combining thermally activated flux creep with a vortex pinning potential for columnar defects. The pinning for thermally activated vortices has been described as strong pinning on chains of individual edge dislocations that form low-angle domain boundaries in high-quality YBCO thin films. The model yields an adequate description of the Jc behaviour over the whole applied field range, as verified by direct measurements of Jc in YBCO thin films grown by pulsed-laser deposition. It also indicates that the effective pinning landscape changes under the influence of the external conditions. Remarkably, the pinning potential obtained from the model is consistent with the values obtained for columnar defects, which confirms the validity of the overall approach.

Step-Edge Josephson Junctions on Multilayered High Temperature Superconducting Thin Film

Application of Josephson effect in sensors, electronic and metrological devices working at temperature of 77 K requires fabrication of efficient and reproducible high temperature superconducting (HTS) Josephson junctions. In order to enhance the performance of HTS junctions, we introduce multilayered approach to YBCO-based step-edge Josephson junction manufacture. Thin YBCO and multilayered YBCO/NdBCO/YBCO films of a similar thickness ( ~ 210 nm) were grown by a pulsed-laser deposition technique on MgO (100) substrates with artificially created step edge. Large discrepancies of Ic and Rn values were found between these two types of junction. Results of their structural and electrical properties are reported.