Magnetic field profiles in type-II superconductors with pinning using a new ac technique

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Abstract
A new ac technique is described which allows the experimental determination of the magnetic field profile near the sample surface in type‐II superconductors with pinning. The technique, like a previous method of Campbell, depends only on the validity of the critical‐state model and can separate the position and magnetic field dependence of the flux‐pinning forces. The entire field profile is obtained from a measurement of the waveform of the response of the sample to a small ac magnetic field superimposed on the dc field. Profiles obtained in cold‐worked Nb and Nb–Ti alloys are reported and the results compared with previous measurements. We have found that the critical‐state model does not adequately describe the results obtained on the Nb–Ti alloys when the magnetic field is less than 0.5Hc2.