Magneto-optical studies of flux penetration in super-hard Nb wire

We present a study of the magnetic response of type-II superconductivity in the extreme pinning limit, where screening currents within an order of magnitude of the Ginzburg-Landau depairing critical current density develop upon the application of a magnetic field. We show that this 'super-hard' limit is realized in highly disordered, cold drawn, Nb wire whose magnetization response is characterized by a cascade of Meissner-like phases, each terminated by a catastrophic collapse of the magnetization. Direct magneto-optic measurements of the flux penetration depth in the virgin magnetization branch are in excellent agreement with the exponential model in which Jc(B) = Jcoexp(-B/B0), where Jco∼5 x 106 A cm-2 for Nb. The implications for the fundamental limiting hardness of a superconductor are discussed