Effects of Ag2O doping on the electromagnetic properties of a BiPbSrCaCuO superconductor

Electromagnetic properties of an Ag2O doped and an undoped BiPbSrCaCuO superconductor were evaluated to investigate the effect of the pinning center on the magnetic shielding and suspension/levitation phenomena. The residual magnetization M = M+-M- increased with the dopant concentration, a maximum for 2% doping, wherein a fine uniform dispersion of Ag particles was observed. The fine Ag particles form a cluster with increasing dopant as the particles condense with each other and grow, consequently does the number of flux passing through decreases, so the magnetization M decreases. This result indicates that M is proportional to the number of magnetic flux lines passing through the sample, because the smaller the particle size the larger the ratio of the surface area to the volume. Magnetic shielding was evaluated by measuring the induced voltage in the secondary coil by placing the sample in between the primary coil. The voltage was initially set to 0.5 V, and decreased to 0.17 and 0.28 V, respectively, for the undoped and 2% Ag2O doped samples. The much less change in the induced voltage for the 2% doped sample is attributed to increased flux shielding by shielding the vortex current. Simultaneous stable levitation and suspension of 2% Ag2O doped disk samples weighing 0.3 g were observed, respectively, above (3 mm) and beneath (2 mm) a toroidal permanent magnet under a field cooled condition. The role of flux pinning is discussed to account for the phenomena by considering the hysteretic force function. (C) 2001 American Institute of Physics.