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

TL;DR

This study investigates how fluxon dynamics affect microwave surface resistance in Li-Al codoped MgB2, revealing hysteresis behaviors and flux pinning effects that vary with doping levels and temperature.

Contribution

It provides new insights into fluxon behavior in doped MgB2, highlighting deviations from traditional models at certain doping levels and temperatures.

Findings

Hysteresis observed in R_s(H_0) curves at low temperatures.

Hysteresis range decreases with increasing temperature.

Critical-state effects explain fluxon behavior only in highly doped samples.

Abstract

The magnetic-field-induced variations of the microwave surface resistance, R_s, have been investigated in ceramic Mg_{1-x}(LiAl)_xB_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 MgB_2 samples, which cannot be justified in the framework of the critical-state models.