Temperature and frequency dependence of the dissipated energy density in MgB2 bulk

This paper reports on an ac susceptometric analysis to evaluate the ac losses as a function of frequency and temperature at different external dc applied fields in MgB₂ twin bulk samples. The two investigated samples come from the same preparation batch. In agreement with partial results in literature, it turns out that the ac losses exhibit, as a function of frequency, a nonmonotonic behavior that is strongly temperature-dependent. In the lower range of analyzed temperatures the results are significantly affected by high-energy heavy ion irradiation. These last findings put forward puzzling questions about the creep dynamics in MgB₂. Finally the issue of a comparison with the ac losses in Ag sheathed/Bi2223 tapes is addressed. The comparison is made by means of measurements once performed with the same apparatus. It turns out that, taking into account the expected differences due to different ac applied fields, the absolute values of the dissipated energy density in MgB₂ and Ag/Bi2223 monofilamentary tape are of the same magnitude order and exhibit a similar behavior.