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

The irreversible magnetization of the layered high-T-c superconductor Bi2+xSr2-(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 irreversibility field B-irr is in excellent agreement with the analytical form of the melting line of a three-dimensional (3D) anisotropic flux lattice as calculated from the Lindemann criterion. For a comparison, the applicability of alternative models based on quantum melting, 2D melting, and flux-creep models is discussed. Rescaling the magnetic-field dependence of the pinning-force with the irreversibility field reveals a similar magnetic-field dependence for different temperatures.