Ac magnetic hysteresis loops of Bi-Sr-Ca-Cu-O 110-K-phase superconductors and the effect of microstructural alteration.

Abstract

The field variations of ac M-H loops are recorded at 77 K and 33 Hz using a lock-in flat-band detection method in sintered and press sintered 110-K-phase samples of the Bi-Sr-Ca-Cu-O system. From the slope of the M vs ${\mathit{H}}_{\mathit{m}}$ curve the flux profiles and the effective relative permeability (${\mathrm{\ensuremath{\mu}}}_{\mathrm{cer}}$) of the samples are determined. Using this value of ${\mathrm{\ensuremath{\mu}}}_{\mathrm{cer}}$ the intergrain and intragrain contributions to the magnetization are separated out. The intergranular loops of the sintered sample could be simulated very well using Kim's model, while for the press-sintered sample the exponential model gives a better fit. Our results show that the loop closure exhibited by the intergranular loop can be simulated to the critical state models without subtracting the wing portion in contrast to the Dersch-Blatter approach. The intragranular loops of sintered samples show quite a good fit to the exponential model after introducing the surface barrier modification. The intergranular critical current density (${\mathit{J}}_{\mathit{ci}}$) and the apparent lower critical field (${\mathit{H}}_{\mathit{c}1\mathit{g}}$) of the grains are found to be enhanced due to press sintering. The field variations of the various physical quantities obtained from the loops are analyzed to see the effect of microstructural alterations introduced by the press-sintering method. By doing a fast Fourier transform on the M vs t curve the harmonic components are separated out and their variation with the ac field amplitude is studied. \textcopyright{} 1996 The American Physical Society.