All focused ion beam fabricated MgB₂ inter-grain nanobridge dc SQUIDs

We have fabricated MgB₂ dc SQUIDs (superconducting quantum interference devices) containing inter-grain nanobridges as Josephson elements by a focused ion beam (FIB) etching method and measured their transport properties. The entire structure including the SQUID loop was patterned only using a FIB. The beam energy was 30kV and the current was 0.9nA for larger structures and 34and 1.5pA for the nanobridge pattern. Each bridge with a nominal width of 100nm crossed a single grain boundary in the normal direction. The SQUID loop had a 3.1νm × 3.1νm hole with a 2νm average linewidth, corresponding to an inductance of 5.1pH. The nanobridges had a two-step transition with an increase in the resistivity of more than a decade and a substantial decrease in the critical current density. Current-voltage characteristics showed a resistively shunted junction behavior at all temperatures below T_c, which implies that the current in the inter-grain nanobridges was determined mainly by the Josephson coupling. These results are believed to be due to discernible damage on the grain boundary caused by FIB irradiation, resulting in the formation of a tunneling barrier in the boundary. The SQUID voltage showed well-behaved modulations in response to the external flux with maximum modulation depths of 80νV at 15.2K and 130νV at 5.9K.