Transformations of the peak-effect near the multicritical point in YBa2Cu3Oy single crystals

We investigated magnetisation curves in magnetic fields up to 23T for four YBa2Cu3Oy single crystals with various oxygen contents using a highly sensi tive magnetic torque rig. The latter together with the high quality of the studied samples allowed us to find several new and previously unreported fe atures. We have observed a very narrow peak with a width even smaller than the one seen in conventional superconductors. At low temperatures the peak broadens and transforms into the conventional fishtail peak. With increasin g temperature, above the multicritical point, the sharp peak converts into a magnetisation jump due to melting of the vortex lattice. The BT vortex ph ase diagram is obtained.