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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) = Jco exp(-B / Bo). where Jco ∼ 5 × 106 A cm-2 for Nb. The implications for the fundamental limiting hardness of a superconductor are discussed. © 2005 IOP Publishing Ltd.

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Superconductor Science and Technology

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