The coupling between grains was studied through low-field (<100 Oe) initial magnetization M(H) on bulk samples of Bi-Pb-Sr-Ca-Cu-O with a single high-${\mathit{T}}_{\mathit{c}}$ (110 K) phase prepared under different sintering times ${\mathit{t}}_{\mathit{s}}$. The M(H) shows a variation with changing ${\mathit{t}}_{\mathit{s}}$ due to the weak links between grains and its behavior is characterized by three threshold fields: ${\mathit{H}}_1^{\mathit{B}}$, ${\mathit{H}}_2^{\mathit{B}}$, and ${\mathit{H}}_3^{\mathit{B}}$. The first flux entry into the sample along the grain boundaries due to flux creep occurs at ${\mathit{H}}_1^{\mathit{B}}$, much lower than ${\mathit{H}}_{\mathit{c}1}^{\mathrm{\perp }}$ (∼7 Oe at 77 K, ∼15 Oe at 0 K) for H⊥ c axis determined from the c-axis-oriented tape with ${\mathit{J}}_{\mathit{c}}$=5400 A/${\mathrm{cm}}^2$ (77 K, H=0). Between ${\mathit{H}}_1^{\mathit{B}}$ and ${\mathit{H}}_2^{\mathit{B}}$, M(H) deviates from a straight line due to shielding of the sample volume; between ${\mathit{H}}_2^{\mathit{B}}$ and ${\mathit{H}}_3^{\mathit{B}}$ it falls on the Meissner line, an effect ascribed to electrically decoupled grains. The first flux entry into the grains themselves occurs at ${\mathit{H}}_3^{\mathit{B}}$, above which M(H) is governed by the intragrain magnetic properties of an Abrikosov lattice. Prolonged sintering improves the weak links between the grains and suppresses both the flux creep at the grain boundaries and the electrical decoupling between grains, which opens transport current paths and leads to the rise in ${\mathit{T}}_{\mathit{c}}$(ρ=0).

• Received 11 July 1990

DOI:https://doi.org/10.1103/PhysRevB.43.10455