Abstract
The observation of voltage-flux oscillations due to the radio-frequency–superconducting quantum interference device (rf-SQUID) effect in polycrystalline bulk MgB2 superconductor is reported. This provides a direct evidence of the presence of Josephson junction-type weak-link grain boundaries in MgB2. Analysis of the SQUID modulation indicates that only a few grain boundaries are of weak-link type whereas others are strong links. Generation of even and odd harmonics are observed when the MgB2 sample is subjected to both ac and dc fields. This also indicates the presence of weak-link grain boundaries. Flux noise in the bulk rf-SQUID in MgB2 is studied and compared with the flux noise in bulk high-Tc rf-SQUID.
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