ac susceptibility studies of intra- and intergrain properties of high- Jc Bi-2212 wires

Variation of effective filament diameter, irreversibility field, anisotropy, and pinning efficiency in Bi-2212 round wires

“…Taking into account the demagnetization factor for samples of cylindrical shape, in the top panel we assumed that the relevant diameter is the single Bi-2212 filament. As already demonstrated in [21], this assumption is, however, incorrect leading to an unphysical χ fil value at low temperature of about −1.4. This can be explained only by screening current being induced on volumes larger than the filaments, an effect caused by the significant filament bridging in Bi-2212 wires.…”

“…All magnetic characterisations were performed with field perpendicular to the wire axis, which is the most relevant to investigating properties related to the transport performance, using samples about 4 mm in length. The superconducting transitions were obtained by DC magnetization in a Quantum Design (QD) MPMS-5 SQUID magnetometer applying 1 mT to estimate the average bundle size [21]. Hysteresis loops were evaluated in a QD 16 T PPMS using a vibrating sample magnetometer (VSM), with a ramp rate of 0.462 T min −1 , in a temperature range between 4.2 and 45 K to evaluate the effective filament diameter d eff at 4.2 K and to obtain the Kramer plots and their extrapolations, H K .…”

“…This characterization allows us to separate the intra and INTER-grain contributions and, analysing the AC amplitude and frequency dependences of the first harmonic χ ′ ′ 1 peaks, to determine the corresponding irreversibility lines and pinning energies. The details of this approach are described in [21] and references within.…”

“…As mentioned above, AC susceptibility can be used to determine sign of granularity or weak connectivity in superconductors [27,28] and we have recently demonstrated that this approach can be used also to evaluate the properties of Bi-2212 round wires (see [21] for detailed explanations about the analysis procedure and contribution identifications). Figure 5 shows an example of AC susceptibility measurements, performed with an AC field of 1 mT at 73 Hz, on one of the Bi-2212 wires investigated here.…”

“…Answers to these questions are vital to understanding what really controls current flow and thus the macroscopic J c . In our previous study [21] we demonstrated that detailed magnetic characterizations can provide useful information for answering some of these questions. For instance, using AC susceptibility we demonstrated that in moderate and low-J c samples (∼5200-4200 A mm −2 at 5 T and 4.2 K, obtained by varying the HT cooling rate) there was evidence of separate intra-and INTER-grain currents with different irreversibility lines and evidence for variable vortex pinning performance.…”