Fluxon dynamics in Li-Al codoped MgB2 by microwave surface resistance measurements

The magnetic-field-induced variations of the microwave surface resistance, Rs, have been investigated in ceramic Mg1−x(LiAl)xB2, with x in the range 0.1-0.4. The measurements have been performed on increasing and decreasing the DC magnetic field, H0, at fixed temperatures. At low temperatures, we have observed a magnetic hysteresis in the Rs(H0) curves in all the investigated samples. On increasing the temperature, the range of H0 in which the hysteretic behavior is visible shrinks; however, in the sample with x=0.1 it is present up to temperatures close to Tc. We show that the field dependence of Rs can be quantitatively justified taking into account the critical-state effects on the fluxon lattice only in the sample with x=0.4. On the contrary, in the samples with x<0.4 the hysteresis exhibits an unusual shape, similar to that observed in others two-gap MgB2 samples, which cannot be justified in the framework of the critical-state models.