Electromagnetic properties of bulk Bi-Pb-Sr-Ca-Cu-O superconductor at low magnetic fields

Abstract

Magnetization and critical current density measurements have been performed on a Bi_1.7Pb_0.3Sr₂Ca₂Cu₅O _y bulk material as the functions of magnetic field and temperature. The diamagnetic shielding effect has been discussed from the initial magnetization curve. The lower critical field of bulk granular superconductor (H _g) and the lower critical field of superconducting grains (H _c1) are estimated from magnetization curves. They are both linearly decreasing functions of temperature: (dH _g/dT) and (dH _c1/dT) are −0.4 and −1.8 G/K respectively. The transport critical current density drops drastically by a factor of 4 at a magnetic field of about 20G. The magnetization J _c of superconducting grains derived from the remanent magnetization is about 10⁶ A/cm² at 77 K, zero field, much greater than the transport J _c. The experimental results reveal that the transport J _c is dominated by weaklink current between grains. The magnetization J _c versus temperature has been obtained from the remanent flux at zero magnetic field and is a linearly decreasing function of temperature within experimental error.