Diamagnetism and irreversibility lines in EuBa2-xKxCu3O7 compounds

By increasing the K concentration x, the authors observe first a small increase of Tc(x) for x 0.4 and a structural change for x=0.06. Through AC susceptibility, measurements of chi ', chi ", they observe in zero static field a broadening of chi ', as a function of x related to modifications of the intergranular situation by K+-doping. For finite static fields H, a larger broadening of chi ' occurs, increasing also with x. High temperature measurements of the irreversibility lines, related to chi ", indicate that the inter- and intragrain pinning is reduced by x; this phenomenon may be explained by the atomic disorder induced by x and the thermal fluctuations, two effects which may tend to melt the vortex lattice. But at 4.2 K, the low field hysteresis, negligible for low x values, becomes larger for x>0.4, indicating a higher pinning.