All focused ion beam fabricatedMgB2 inter-grain nanobridge dc SQUIDs

Abstract

We have fabricated MgB2 dc SQUIDs (superconducting quantum interference devices) containing inter-grain nanobridgesas Josephson elements by a focused ion beam (FIB) etching method and measured theirtransport properties. The entire structure including the SQUID loop was patterned onlyusing a FIB. The beam energy was 30 kV and the current was 0.9 nA for larger structuresand 34 and 1.5 pA for the nanobridge pattern. Each bridge with a nominal width of 100 nmcrossed a single grain boundary in the normal direction. The SQUID loop had a3.1 µm × 3.1 µm holewith a 2 µm average linewidth, corresponding to an inductance of 5.1 pH. The nanobridges hada two-step transition with an increase in the resistivity of more than a decadeand a substantial decrease in the critical current density. Current–voltagecharacteristics showed a resistively shunted junction behavior at all temperatures belowTc, which implies that the current in the inter-grain nanobridges was determined mainly bythe Josephson coupling. These results are believed to be due to discernible damageon the grain boundary caused by FIB irradiation, resulting in the formation ofa tunneling barrier in the boundary. The SQUID voltage showed well-behavedmodulations in response to the external flux with maximum modulation depths of80 µV at 15.2 Kand 130 µV at 5.9 K.