Precise magnetization measurements of single crystalline Bi2Sr2CaCu2O8+δ

Precise dc-magnetization measurements, using a superconducting quantum interference device magnetometer (SQUID), have been performed on single crystalline Bi2Sr2CaCu2O8+delta in magnetic fields both parallel to the c axis (H parallel to c) and tilted away from the c axis towards the ab plane to investigate the vortex state at various temperatures between 40 K and T-c=83.0 K. The magnetization curves at a fixed temperature as a function of magnetic field (H parallel to c) show a clear jump at H-M, which corresponds to the flux-line-lattice-melting transition (FLLMT) as observed previously. In the vicinity of T-c= 83.0 K, it is shown that the H-M(T) line changes its character near T*=79.5 K, below which it can be described well by the FLLMT and above which the change of magnetization, Delta M, and the corresponding change of the entropy, Delta S, at H-M, falls sharply as T-c is approached. A simultaneous decoupling of the Josephson coupling and the melting of the flux-line lattice may be an appropriate picture below T*, whereas above T* an additional degree of freedom makes a contribution to the FLLMT due, perhaps, to vortex-antivortex creation and annihilation processes. In a tilted magnetic field, it is found that the angular dependence of H-M, as well as Delta M and Delta S, obeys a scaling law up to theta less than or equal to 80 degrees. The deduced anisotropy parameter gamma(s) similar or equal to 9 is found to be much lower than the value of gamma(l)=150-200 found in the liquid state. We interpret such a discrepancy as due to the difference of the dimensionality of the vortex pancake state above and below H-M. Furthermore, from the scaling behavior of Delta S in tilted fields, it is inferred that the FLLMT is predominantly ruled by the number of pancake vortices, n, in the superconducting CuO2 layers and that Josephson vortices do not play an important role for the FLLMT.