Niobium nanoSQUIDs based on submicron Josephson tunnel junctions: performance as a function of the temperature

Nanosized superconducting quantum interference devices (nano-SQUIDs) offer the possibility to investigate small spin populations and the magnetization of nanoparticles, opening new horizons in the interesting world of nanomagnetism. Due to the fabrication difficulties, most of current nanoSQUIDs consist of a square loop having a side length less than 1 ?m interrupted by two nanometric niobium constrictions (Dayem bridges). However, in the recent years there is a growing interest to the development of nanoSQUIDs based on a sub-micron Josephson tunnel junctions (JJs). Compared to typical nanoSQUIDs, the main advantages of the nanoSQUIDs based on JJs are a better control of the critical current, the high modulation depth, and the ultra-low noise, being based on a fully reliable niobium technology. In the present work, an experimental investigation as a function of the temperature (9-0.3 K) of the main characteristic of a niobium nanoSQUID will be presented. The nanosensor consists in a niobium superconducting loop (0.2 x 1.0 mm^2) interrupted by two sub-micron Nb/Al-AlOx/Nb Josephson junctions having an area of about (300x300 nm^2). These nanodevices have been fabricated by means of a Focused Ion Beam (FIB) sculpting method, used as lithographic technique to define the various elements of the SQUID. We have performed measurements of current-voltage, critical current-magnetic flux characteristics, switching current distributions from the zero voltage state and the related escape rates as function of the bias current, for different temperatures. The temperature behavior of the nanodevice critical current, the modulation depth, and the magnetic flux resolution will be also presented. The high critical current modulation depths and the low intrinsic dissipation exhibited by these device ensure a suitable sensitivity for nanoscale applications in the whole temperature range investigated.