rf-SQUID Effect in Bulk Quaternary Borocarbide Superconductors

This paper reports observation of rf-SQUID effect due to natural grain boundary junctions in YNi2B2C (Tc ∼ 15.5 K), ErNi2B2C (Tc ∼ 11.5 K, TN ∼ 6.5 K), and DyNi2B2C (Tc ∼ 6.5 K, TN ∼ 11K), LuNi2B2C (Tc ∼ 16.5 K) and YPd5B3C0.35 (Tc ∼ 23 K) bulk borocarbide superconductors. The observation of rf-SQUID effect due to natural grain boundary junctions in all the five borocarbide superconductors clearly indicates that natural grain boundary junctions in these superconductors behave as Josephson junctions, and this behavior of natural grain boundary junctions in the quaternary borocarbides appear to be universal in this class of superconductors. Observation of rf-SQUID effect in magnetic borocarbide superconductors (ErNi2B2C and DyNi2B2C) at 4.2 K also indicates that antiferromagnetic ordering does not destroy SQUID effect, as TN > 4.2 K in both cases. Rapid increase in flux noise due to thermally activated flux hopping has been observed in all the SQUIDs as temperature approaches Tc.