Radio-frequency SQUID effect due to natural grain-boundary junctions in ErNi2B2C and DyNi2B2C magnetic borocarbide superconductors

This letter reports the observation of rf superconducting quantum interfere nce device (SQUID) effects due to natural grain-boundary junctions in ErNi2 B2C (T(C)approximate to 11.5 K, T(N)approximate to 6.5 K) and DyNi2B2C (T(C )approximate to 6.5 K, T(N)approximate to 11 K) bulk borocarbide magnetic s uperconductors. Voltage-flux SQUID modulations have been observed up to 10 and 5.7 K in ErNi2B2C and DyNi2B2C, respectively. The amplitude of voltage- flux modulations in ErNi2B2C shows a sharp change around 6 K, which has bee n attributed to the occurrence of magnetic ordering in ErNi2B2C. Such a cha nge is not observed in DyNi2B2C rf SQUID since, in this case, T-C< T-N. The flux noise density at 4.2 K of these SQUIDs shows a frequency-independent white-noise region down to 1 Hz. A rapid increase in flux noise has been ob served in both cases as temperature approaches T-C. (C) 1999 American Insti tute of Physics. [S0003-6951(99)02938-1].