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

The magnetic-field-induced variations of the microwave surface resistance, R-s, have been investigated in ceramic Mg1-x(LiAl)(x)B-2, with x in the range 0.1-0.4. The measurements have been performed on increasing and decreasing the DC magnetic field, H-0, at fixed temperatures. At low temperatures, we have observed a magnetic hysteresis in the R-s(H-0) curves in all the investigated samples. On increasing the temperature, the range of H-0 in which the hysteretic behavior is visible shrinks; however, in the sample with x = 0.1 it is present up to temperatures close to T-c. We show that the field dependence of R-s 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. (C) 2010 Elsevier B.V. All rights reserved.