AC loss intrinsic to MgB2 at low magnetic fields

The AC magnetic susceptibility of granular MgB2 has been investigated as a function of temperature in the range between 15 and 55 K, AC field amplitude up to 3.64 mT and frequency up to 3200 Hz. In this range of AC field there are no induced vortices in the grains, as indicated by the independence of on BAC. The AC loss peak that is intrinsic to MgB2 grains neither broadened nor shifted to lower temperature, nor increased in height with increasing BAC. This behaviour of the AC loss peak has not previously been observed in MgB2. chi(T), however, showed strong dependence on the frequency. The dependence on the frequency of the AC loss is attributed to the frequency dependence of the penetration depth lambda.