The alternating-current magnetic susceptibility of granular material was investigated as a function of temperature between 15 and 55K, using alternating-current field amplitudes of up to 3.64mT and frequencies of up to 3200Hz. In this range of alternating-current field there were no induced vortices in the grains, as indicated by the independence of χ’(T) and BAC. The alternating-current loss peak that was intrinsic to MgB2 grains neither broadened nor shifted to lower temperature, nor increased in height with increasing BAC. This behavior of the alternating-current loss peak had not previously been observed in MgB2. The χ(T), however, exhibited a strong dependence upon frequency. The dependence upon the frequency of the alternating-current loss was attributed to the frequency dependence of the penetration depth.

AC Loss Intrinsic to MgB2 at Low Magnetic Fields. R.V.Sarmago, M.P.Olbinado: Superconductor Science and Technology, 2005, 18, 307-10