G. K. Perkins

Vortex dynamics in superconducting MgB2 and prospects for applications.

The recently discovered superconductor magnesium diboride, MgB2, has a transition temperature, Tc, approaching 40 K, placing it intermediate between the families of low- and high-temperature superconductors. In practical applications, superconductors are permeated by quantized vortices of magnetic flux. When a supercurrent flows, there is dissipation of energy unless these vortices are ‘pinned’ in some way, and so inhibited from moving under the influence of the Lorentz force. Such vortex motion ultimately determines the critical current density, Jc, which the superconductor can support. Vortex behaviour has proved to be more complicated in high-temperature superconductors than in low-temperature superconductors and, although this has stimulated extensive theoretical and experimental research, it has also impeded applications. Here we describe the vortex behaviour in MgB2, as reflected in Jc and in the vortex creep rate, S, the latter being a measure of how fast the ‘persistent’ supercurrents decay. Our results show that naturally occurring grain boundaries are highly transparent to supercurrents, a desirable property which contrasts with the behaviour of the high-temperature superconductors. On the other hand, we observe a steep, practically deleterious decline in Jc with increasing magnetic field, which is likely to reflect the high degree of crystalline perfection in our samples, and hence a low vortex pinning energy.

First order magnetic transition in doped CeFe2 alloys: phase coexistence and metastability.

First order ferromagnetic (FM) to antiferromagnetic (AFM) phase transition in doped CeFe2 alloys is studied with the micro-Hall probe technique. Clear visual evidence of magnetic phase coexistence on micrometer scales and the evolution of this phase coexistence as a function of temperature, magnetic field, and time across the first order FM-AFM transition is presented. Such phase coexistence and metastability arise as a natural consequence of an intrinsic disorder-influenced first order transition. The generality of these phenomena involving other classes of materials is discussed.

Reducing the operational magnetic field in the prototype magnetocaloric system Gd5Ge4 by approaching the single cluster size limit

We studied polycrystalline samples of the prototype magnetocaloric system Gd5Ge4 using scanning Hall imaging and global magnetometry. The magnetic field required to complete the phase transition that is critical for magnetocaloric application is reduced by up to 20% when small fragments, each consisting of several randomly oriented crystallites, are removed from the bulk. The removal of competing strain fields from neighboring grains when the fragment is embedded in the bulk underlies the observation. We support the results by showing local Hall imaging of phase separation in the bulk. When the bulk is ground into a fine powder, the effect vanishes.

Pinning mechanisms in Ag-sheathed Bi(Pb)SrCaCuO tapes

Abstract The pinning properties of Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3O10+y (2223) wires have been studied in comparison with the normal powder, and the powder scraped from the 2223 tape by the measurements of irreversibility lines (IL). The IL for both the tape and the powder scraped from the tape are nearly equally shifted to higher temperatures in comparison with that for the normal powder. This indicates that the enhanced IL is attributable to the defect pinning. TEM studies reveal that the grains within Ag-clad 2223 tape contain high concentrations of dislocations (1010/cm2) and stacking faults that may act as effective pinning sites. The results on the transport and magnetisation Jc suggest that the critical current is controlled by flux pinning at high temperatures, while the weak links limit the critical current at low temperatures.

Topological hysteresis in the intermediate state of type-I superconductors

Samples had thickness between d = 0.1 and 1.5 mm and were about 1.5 × 1.5 mm 2 in planar dimension. The topological features described here were thickness independent above d � 0.5mm, which indicates that they are not due to surface-related effects. We show data for samples which had demagnetization factors of about N = 0.5 (determined both from initial magnetisation and direct calculations [20].) Quantum Design MPMS magnetometer was used for DC magnetization measurements. MO imaging was performed in a pumped flow-type optical 4 He cryostat using Faraday rotation of polarized light in Bi-doped iron-garnet films with in-plane magnetization [15]. In all images the bright regions correspond to the normal state and dark regions to the superconducting state.

Enhancement of critical current density of (Pb,Sn)-doped Bi-2212 superconductors at high temperature

Abstract The magnetic properties of ceramic samples of Bi 2−2 x (PbSn) x Sr 2 CaCu 2 O y ( x =0,0.1,0.2,0.3 and 0.4) superconductors have been investigated using a vibrating sample magnetometer from 20 to 50 K and up to 8 T. Phase analysis shows that most of the (Pb,Sn) dopant enters the Bi-2212 lattice and occupies Bi sites. The current-carrying scale-length analysis has been determined from the reverse leg of the magnetization hysteresis loops and compared to the actual dimensions of the sample. Magnetization hysteresis loops of the (PbSn)-doped sample exhibit strong pinning behaviour in the higher temperature range. The effect of (Pb,Sn) doping on intragranular critical current J c was found to be enhanced by one order of magnitude at 50 K. We discuss the experimental results within a model of (PbSn) cluster pinning in blocking layers. Doping by (PbSn) clusters into the Bi–O layers provides effective pinning centres, as well as a large reduction of the resistivity anisotropy.

Superconducting critical fields and anisotropy of a MgB2 single crystal

Using a double axis vibrating sample magnetometer, we have made detailed magnetic measurements of the lower critical field Hc1 for fields parallel to the two crystallographic directions of MgB2 single crystals. Additionally, using a novel Hall probe magnetometer we have measured high precision magnetization loops, from which we directly determine the upper critical field Hc2 for both field orientations. Our results suggest that Hc1 is much larger than most previous estimates and that consequently the Ginzburg–Landau parameter κ is very low (less than 5). We find the anisotropy parameter γ ~ 2, independent of temperature over the measured range.

Structure of the superconducting gap in MgB2 from point-contact spectroscopy

We have studied the structure of the superconducting gap in MgB2 thin films by means of point-contact spectroscopy using a gold tip. The films were produced by depositing pure boron on a sapphire substrate, using e-beam evaporation, followed by reaction with magnesium vapour. The films have a Tc of 38.6 ? 0.3 K and resistivity of about 20 ?? cm at 40 K. The point-contact spectra prove directly the existence of a multi-valued order parameter in MgB2, with two distinct values of the gap, ?1 = 2.3 ? 0.3 meV and ?2 = 6.2 ? 0.7 meV at 4.2 K. Analysis of the spectra in terms of the Blonder?Tinkham?Klapwijk model reveals that both gaps close simultaneously at the Tc of the film. Possible mechanisms that can explain the intrinsic coexistence of two values of the gap are discussed.

Fishtails, scales and magnetic fields

Abstract The granularity of high Tc single crystals and the cause of the secondary peak in m - H loops (known as the ‘fishtail’ effect) are still areas of active debate and are crucial to our understanding of the nature of the superconductivity and pinning in high Tc materials. This paper demonstrates the use of the length scale technique to rule out large scale granularity as a cause of the fishtail effect. Magnetic moment and flux creep measurements are discussed as a function of field on single crystals of YBCO; these results exclude several of the other models previously proposed to explain the origin of the fishtail feature, but support an explanation in terms of microstructural regions that are turned normal in an increasing field, and so act as additional pinning centres.

High field scanning Hall probe imaging of high temperature superconductors

Power applications of high temperature superconductors (HTS) are emerging rapidly towards the market place. While first generation Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/ (BSCCO) powder in tube (PIT) conductors are already used in the industrial sector (magnet inserts, transformers and power transmission cables), second generation Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) coated conductors are being developed which promise even greater performance, particularly for applications which require high magnetic fields. Along with the development of these materials, advances in characterization techniques have enabled detailed studies of the associated loss mechanisms. In particular, magnetic imaging techniques (both scanning Hall probe and magneto optics (MO)) have been able to establish the pattern of current flow within the samples, allowing study of grain boundary efficiency, current homogeneity and filament interconnection in multifilamentary PIT tapes. However, as the technology moves towards high field applications, high-field magnetic imaging is required. Presently MO is inherently limited to low fields (<0.1 T) and for this reason we have recently focused on the development of a high resolution scanning Hall probe device to work in high magnetic fields. We present data showing bow the current pattern evolves in the presence of high magnetic field for various samples and discuss the implications for the future development of HTS conductors.