Critical Current Anisotropy, Pinning Properties and Relaxation Rate of “Ex‐situ” MgB2/Fe Tapes

Abstract

Hysteresis and magnetic relaxation measurements have been performed on MgB2/Fe tapes in parallel and perpendicular magnetic field in order to investigate the correlation between the anisotropy of the transport current density and the texturing recently analysed by Lezza et al.. Volume pinning forces FP have been obtained from the current densities Jc as a function of the dc magnetic field up to 9 T in the temperature range 5–35 K. The reduced pinning forces do not scale with temperature in the parallel field configuration. It has been shown that this could be explained considering that the texturing occurs mainly at the surface of the sample. The scaling is recovered in perpendicular field for temperature higher than 20 K and the scaled pinning curves have been compared to theoretical predictions. Furthermore, the anisotropy of the pinning properties has been investigated using the present relaxation measurements. The magnetization was found to decrease linearly with ln(t). This allows calculation of the pinning potential energy U0 by using the relationship U0 = kT/{dM/[M0 dln(t)]}. The resulting variation of U0 in field and temperature is not related to the field orientation. The relaxation rate of MgB2 has been compared to other superconducting compounds, i.e. Bi‐2223 and Nb3Sn.