The Circulation Radius and Critical Current Density in Type II Superconductors

Abstract

A method is proposed for estimating the length scale of currents circulating in superconductors. The estimated circulation radius is used to determine the critical current density from magnetic measurements. The obtained formulas are applicable to samples with negligibly small demagnetizing factors and to polycrystalline superconductors. The proposed method has been verified using experimental magnetization loops measured for polycrystalline YBa 2 Cu 3 O 7-δ and Bi 1.8 Pb 0.3 Sr 1.9 Ca 2 Cu 3 O x superconductors. The development of cryogenic technology and considerable progress achieved in manufacturing superconducting tapes and single-crystalline samples [1] are disclosing ways to implement superconductors in microelectronics, power engineering, and transport engineering. At the same time, a significant amount of the research devoted to the influence of material structure and/or composition on the properties of superconductors is still performed on polycrystalline samples. This circumstance is related to the relative simplicity of the methods of synthesis and modification of polycrystalline superconductors as compared to single crystals.In single crystals of high-temperature superconductors (HTSCs), critical current density Jc can reach up to ~10 12 A/m 2 , which is close to the values of depairing current density [2,3]. Due to these high values, J c is usually determined by indirect methods based on magnetic measurements, rather than by direct charge transport measurement techniques. In polycrystalline HTSCs, transport measurements can be used for determining the density of the intergrain critical current, which is several orders of magnitude lower than J c for single-crystalline samples [2]. Therefore, the intragrain critical current density of polycrystalline HTSCs is always determined using magnetic measurements. The determination and comparison of parameters of various promising superconductors by indirect techniques requires correct data interpretation and taking the particular granular structure into account.According to the critical state model [4], the magnetization of a type-II superconductor is determined by critical current density J c and size of the sample. The corresponding expression (Bean's formula) is widely used to find J c from the results of magnetic measurements as

Superconducting YBCO Foams as Trapped Field Magnets

“…A simple estimation of the overall transport current density in the foam sample can be done using the size of the trapped field peak (Btrap) with the relation [26] JcR∝Btrap,max/μ0 where R is the radius of the magnetized area [38]. With a trapped field of 250 G and R= 10 mm, we obtain Jc= 199 A/cm 2 ; and for the maximum trapped field of 400 G, Jc= 318 A/cm 2 .…”

“…Bending down of the curves at high H and high T is due to thermal activation. The open symbols show an intragranular current density (60 K) obtained using the same M(H) data, but the procedure for the determination of the current length scale as described in Reference [38]. This procedure yields an average current-carrying length of ∼6 μm.…”

Influence of intragranular Meissner currents and magnetic flux trapped in granules on the effective field in the intergranular medium and the magnetoresistance hysteresis of a granular HTSC

“…We also note that the M(H) dependences shown in Fig. 2, b are typical for granular HTS in the area of sufficiently high temperatures [48][49][50], and the asymmetry of the magnetization hysteresis loops relative to the abscissa axis is explained by the weakening of the pinning of Abrikosov vortices in the near-surface the layer of granules [48][49][50].…”

Microstructure and Flux Pinning of Reacted-and-Pressed, Polycrystalline Ba0.6K0.4Fe2As2 Powders

“…The field sweep rate was always 0.36 T/min. From the magnetization data, the critical current densities, jc, were evaluated using the extended Bean approach for rectangular samples [27] and the extensions described in [28,29,30]. The irreversibility lines were determined using a criterion of 100 A/cm 2 from the obtained measurements, except the literature data [16], where extrapolation from the jc(H) graphs was employed [31].…”