Entropy, vortex interactions, and the phase diagram of heavy-ion-irradiated Bi2Sr2CaCu2O8+delta

Dynamic and thermodynamic magnetization experiments on heavy-ion-irradiated single-crystalline Bi2Sr2CaCu2OB8+delta are correlated in order to clarify the nature of the mixed-state phase diagram. It is shown that whereas the entropy contribution to the free energy in the London regime plays a minor role in unirradiated crystals and irradiated crystals at fields close to or above the matching field B-phi, it becomes very important at low fields in irradiated crystals with high B-phi. The direct determination of the entro py contribution to the free energy from the reversible magnetization allows one to determine not only the correct values of the pinning energy, but al so to extract quite detailed information on pancake vortex alignment. The c haracteristic field H(int)similar to 1/6B(phi) at which intervortex repulsi on begins to determine the vortex arrangement and the reversible magnetizat ion is shown to coincide with a sharp increase in the irreversibility field H-irr(T) and with the recoupling transition found in Josephson plasma reso nance. Above H-int, the repulsive interaction between vortices causes both the vortex mobility to decrease and pancake alignment to increase. At highe r fields greater than or similar to 1/3B(phi)much greater than B-c1, free v ortices outnumber those that are trapped on a columnar defect. This causes the decrease of the c-axis resistivity and a second crossover of the irreve rsibility held, to a regime where it is determined by plastic creep.