Superconducting fluctuations in the reversible magnetization of the iron-pnictide Ba1-xKxFe2As2

We report on isofield magnetization curves obtained as a function of temperature in two single crystals of Ba1-xKxFe2As2 with superconducting transition temperature T-c=28 and 32.7 K. Results obtained for fields above 20 kOe show a well-defined rounding effect on the reversible region extending 1-3 K above T-c(H) masking the transition. This rounding appears to be due to three-dimensional critical fluctuations, as the higher field curves obey a well known scaling law for this type of critical fluctuations. We also analyzed the asymptotic behavior of M vs T curves in the reversible region which probes the shape of the gap near T-c(H). Results of the analysis suggests that phase fluctuations are important in Ba1-xKxFe2As2 which is consistent with nodes in the gap.