INFLUENCE OF THE INPLANE ANISOTROPY ON THE VORTEX PHASE-DIAGRAM IN YBA2CU3O-DELTA

Magnetic studies have been carried out in high quality detwinned YBa2C u3Odelta Single crystals. We report for the first time that the maximu m in magnetisation shifts to higher fields when the oxygen concentrati on of the samples increases. Also, the magnetisation peak or ''fishtai l'' peak as commonly referred at, shifts to higher field values as the temperature raises, contrary to previous experimental results. In add ition the peak effect which appears in the magnetic hysteresis loops i s a two step process, being separated at high temperatures into two pe aks, one of which consistently correlates to the multicritical point. Furthermore, the position of these two peaks in the phase diagram is d isorder independent. Our findings strongly suggest the importance of i n intrinsic parameter unique in YBa2Cu3Odelta, namely the in-plane ani sotropy.