A ferromagnetic sheath around a superconducting wire results in an unusual transport JC(H). For the field perpendicular to the current, there was a plateau in JC(H) at high temperatures and intermediate fields. This plateau develops into a peak at lower temperatures—resembling a peak effect. A model based on cancellation of the self-field of the current and the external field within the iron sheath was proposed for the explanation of the plateau in JC(H). This model was tested using 3 experiments. Firstly, it was shown that the form of JC(H) for round MgB2/Fe wires was strongly temperature-dependent. This was in contradiction with the model, because the properties of the iron sheath did not change in the measured temperature range. However, the temperature dependence of JC might still account for the change of JC(H). Secondly, the model requires a substantial component of the self-field to be parallel to the external field. The measurements of JC(H) for a field parallel to the current exhibited a peak in JC(H) at high temperatures and a pronounced plateau at low temperatures. The model could not explain this because the self-field and external field were perpendicular in the experiment. Thirdly, the iron sheath was made thinner on one side of the wire, which should have produced an asymmetry in JC(H) in the model for 2 different orientations of the external field. Such asymmetry was not observed. These experiments showed that the effect of the self-field was of much lower importance than an as yet unknown effect that results in the observed plateau and peak in JC(H). Such an effect was likely to be based on a specific interaction between the superconductor and ferromagnet; perhaps similar to the over-critical state effect.

The Relevance of the Self-Field for the 'Peak Effect' in the Transport JC(H) of Iron-Sheathed MgB2 Wires. J.Horvat, S.Soltanian, W.K.Yeoh: Superconductor Science and Technology, 2005, 18, 682-8