INTERPLANE AND INTRAPLANE SOFTENING OF THE VORTEX STRUCTURE IN BI2.1SR1.9CA0.9CU2O8+DELTA - A 2-STEP TRANSITION

High Q mechanical oscillator and AC susceptibility techniques have been used to study vortex dynamics in high quality single crystals of Bi2.1Sr1.9Ca0.9Cu2O8+delta over a wide range of magnetic fields and different relative orientations between the magnetic field and the crystalline c-axis. Our results confirm the existence of two transitions in the vortex response. We show that the transition at lower temperatures is associated to currents flowing across the Cu-O planes and the other one to currents in the planes. This means that the reversible region of the phase diagram is reached in two steps when increasing temperature.