Irreversibility line of overdoped Bi2+xSr2-(x+y)Cu1+yO6 +/-delta at ultralow temperatures and high magnetic fields

The irreversible magnetization of the layered high-T-c superconductor Bi2+x Sr2-(x+y)Cu1 divided by yO6+/-delta (Bi-2201) has been measured by means of a capacitive torquemeter up to B-a=28 T and down to T=60 mK. The deduced i rreversibility field B-irr is in excellent agreement with the analytical fo rm of the melting line of a three-dimensional (3D) anisotropic flux lattice as calculated from the Lindemann criterion. For a comparison, the applicab ility of alternative models based on quantum melting, 2D melting, and flux- creep models is discussed. Rescaling the magnetic-held dependence of the pi nning-force with the irreversibility field reveals a similar magnetic-field dependence for different temperatures.