Quasi-2D characteristics in the ac susceptibility response of polycrystalline (Bi,Pb)SrCaCuO/2223 multiphase systems

Fundamental and third harmonic ac susceptibility measurements as a function of temperature T, ac magnetic field, and dc magnetic field have been performed on Bi(Pb)SrCaCuO/2223 bulk samples consisting of both Bi2Sr2Ca2Cu3O10+δ (2223) and Bi2Sr2CaCu2O8+δ (2212) phases. Since the χ1″(T) peak corresponding to the losses in the weak links between the grains shifts rapidly to lower T and its amplitude is strongly suppressed with increasing the dc magnetic field Hdc, the other peaks related to the losses in the grains become visible. For the samples consisting of single phase 2223, these intragranular peaks are situated near 38 K, 58 K, and 80–88 K (depending on the sample), whereas for the samples consisting of single phase 2212, they are situated near 20 K, 43 K, and 77 K. For Hdc ⩾ 1 kOe, the temperature positions of the intragranular peaks are weakly field dependent. This shows that the transition line determined from the loss peaks behaves like the melting line in two-dimensional (2D) systems. Also, as the field increases, the amplitude of the loss peaks can decrease up to their disappearance, suggesting a layer decoupling process. In this way, we show that, above a crossover field of ∼1 kOe, the vortex lattice in the superconducting grains of polycrystalline samples reproduces the behaviour of the vortex lattice in a quasi-2D object consisting of a set of separated “thin films” with one-, two-, and three-effective-layer thicknesses. For the samples consisting of both 2223 and 2212 phases, three additional peaks were observed, situated near 28 K, 50 K and 75–80 K (depending on the sample). These peaks were attributed to the regions in which the 2212 and 2223 layers alternate.