Lower critical magnetic field Hc1 and intergranular coupling of Bi-2223/Ag concentric tapes

Abstract

Magnetic properties of concentric Bi-2223/Ag tapes with critical current density Jc = 30 kA/cm2 at 77 K in self magnetic field were measured using SQUID magnetometer. Lower critical magnetic field Hc1 | ab was determined from deviation of the virgin magnetization curve from linearity. A technique of transformation of M(H) curve to B(H) curve was adopted to increase the precision of Hc1 determination. This method allowed detection of about 1000 flux lines. The measured temperature dependence of Hc1 can be fitted by the Ginzburg-Landau theory using the two-fluid Gorter-Casimir relation for the temperature dependence of the London penetration depth. This was proved by direct dc susceptibility measurements and fitting the experimental data with model calculations for superconducting slab in Meissner state placed in parallel field. The obtained results imply strong intergranular coupling of Bi-2223 phase of the tapes.