Experimental evidence for a dimensional crossover of the vortex ensemble in BSCCO (Bi1.8Pb0.4Sr1.8Ba0.2Ca2Cu3Ox) by multi-harmonic ac susceptibility measurements

We have studied a portion of H-T phase diagram of the anisotropic Bi2223 bulk system using multi-harmonic ac susceptibility measurements (χn). Based on the behavior of the third harmonic modulus, ∣χ3∣, the irreversibility line (IL) was obtained from the onset of ∣χ3∣ and an anomalous peak effect (PE) was observed. It has been shown that the anomalous peak is due to the crossover between a three-dimensional (3D) and a quasi-two dimensional (quasi-2D) peculiarity of vortex dynamics, the crossover magnetic field being Hcr ≈ 0.1 T. The obtained portion of Birr(T) is well described by the melting line of the flux lattice. The 3D flux fluctuation part (low field and high temperature) is described by the nearly parabolic temperature dependence B = B0(Tc/T − 1)n with n = 1.58 (sample 1) and n = 1.48 (sample 2). The other region (high field and low temperature) is well described by the temperature dependence of the quasi-2D flux fluctuations, Tm(H)=Tm2D[1+b/(lnB/Bcr)1/ν] corresponding to the weak interaction between pancake vortices from adjacent planes. The 2D limit temperature was determined as Tm2D=36.7K (sample 1) and Tm2D=27.3K (sample 2).