All focused ion beam fabricated MgB₂inter-grain nanobridge dc SQUIDs

Abstract

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 30 kV and the current was 0.9 nA for larger structures and 34 and 1.5 pA for the nanobridge pattern. Each bridge with a nominal width of 100 nm crossed a single grain boundary in the normal direction. The SQUID loop had a 3.1 µm × 3.1 µm hole with a 2 µm average linewidth, corresponding to an inductance of 5.1 pH. 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 Tc, 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 80 µV at 15.2 K and 130 µV at 5.9 K.

Nano Superconducting Quantum Interference device: A powerful tool for nanoscale investigations

“…The SQUID consisted of a vanadium-superconducting loop including two InAs weak links obtained by a single nanowire contacted by vanadium electrodes (Fig.44c). [214], [218], [221], and [226] respectively).…”

“…Therefore, since its discovery, several SQUID sensors based on MgB 2 have been developed [224,225]. Micro-sized MgB 2 SQUIDs based on nanobridges (100 nm in width) with a loop area of about 10 µm 2 have been fabricated by using a FIB technique [226,227] (Fig.44d). The I-V characteristics were nonhysterestic in the full range of explored temperature [4.2 K- 33 K].…”

Superconducting transition width under magnetic field in MgB2 polycrystalline samples

“…This fact supports the above inference that the upturn is really linked with multiple superconducting transitions. In MgB 2 , two transitions have been occasionally reported, although the origin of this phenomenon is still unclear [16][17][18][19][20] but three transitions are rarely reported. A conceivable origin of the multiple transitions is that the MgB 2 grains are separated into three phases with different T C .…”

Micro-SQUIDs based on MgB2nano-bridges for NEMS readout

“…The long coherence length and the consequently high critical current density gives the advantage of easy manipulation of the junction barrier [11]. This was already demonstrated in nano-constriction on grained MgB 2 film with stable and reproducible technology [4,6,15], thus it is possible to use MgB 2 devices to readout NEMS. This new technology is promising over a wide spread of sectors from nanoscale metrology to many possible sensing applications.…”

“…The relatively high superconducting critical temperature of magnesium diboride (T c =39 K) and its intermetallic properties makes MgB 2 a suitable material to fabricate superconductor quantum interference device (SQUID) [1]. For more than a decade, MgB 2 SQUIDs have been successfully fabricated and studied by several groups [2][3][4][5][6][7]. Nowadays, MgB 2 -based SQUIDs are developed instruments for practical applications [8] and have been integrated into experimental systems, such as for magnetocardiography [9].…”