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 μm 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.

As recently reported [1], we have developed different types of Nb/HfTi/Nb-nanoSQUIDs with a lateral junction size of about 200 nm x 200 nm and an inner loop size of about 840 nm x 840 nm. These nanoSQUIDs were designed as gradiometers, and implemented with gradiometric feedback loops, gradiometric transformers and rf filters to enhance their practical use. The noise in the white frequency range depends on the SQUID loop inductance, and is typically 115 nΦ₀/√Hz for parallel nanoSQUID gradiometers, or about 200 nΦ₀/√Hz for series nanoSQUID gradiometers. In combination with a SQUID series array which acts as a low-noise preamplifier, these nanoSQUIDs can be operated in the flux locked loop mode with high linearity in an ac magnetic field of up to a few mT. In this configuration, the excitation field is orientated either perpendicular or in-plane to the gradiometric SQUID loop. That is an experimental setup used e.g. for particle investigations. For the readout of nano-electromechanical resonator systems (NEMS), the field tolerance to a dc in-plane magnetic field is of considerable interest. We demonstrate the operation in dc in-plane fields of a few tens of mT with several measurements – most of them performed in a two-stage configuration. Furthermore, we present an experimental setup for NEMS readout, and discuss first measurements of this nanoSQUID-NEMS combination.
[1] S. Bechstein et al., ‘HfTi-nanoSQUID gradiometers with high linearity,‘ Appl. Phys. Lett. 106, 072601 (2015)

This work was partly supported by the DFG under Grant No. SCHU1950/5-1 and within the European Metrology Research Programme EMRP ‘MetNEMS’ NEW-08, which is jointly funded by the participating countries within EURAMET and the European Union.

Direct coupled high-Tc DC SQUIDs require the deposition of only a single superconducting layer, can be fabricated using a minimal number of technological steps and provide a magnetic field resolution of approximately 10 fT/rt(Hz) at 77 K, which is sufficient for applications in geomagnetic surveys, non-destructive evaluation and even biomagnetic measurements. If such SQUIDs are made using step edge Josephson junctions, then they potentially permit the simultaneous fabrication of many sensors on large area substrates, in close proximity to objects of interest that can be kept at room temperature.
The novel graphoepitaxial step edge Josephson junctions on buffered MgO substrates [1] have been used to overcome the poor reproducibility of conventional step edge high-Tc Josephson junctions, thanks to the self-arranged growth of two identical [100]-tilted 45 degree grain boundaries over a wide range of slope angles and step heights on MgO substrates. The implementation of such junctions has resulted in a doubling in voltage swings and IcRn-product, when compared with SQUIDs prepared using conventional step edge or bicrystal high-Tc Josephson junctions.
Additional improvements in SQUID parameters have been achieved by using serial arrays of SQUIDs. The use of a dual-SQUID configuration is a well-known technique for further improving voltage swings and flux noise in high-Tc SQUIDs. Unfortunately, the reproducibility of the parameters in conventional high-Tc junctions has not been sufficient to make use of the advantages of a dual-SQUID configuration, with the decoherence of Josephson oscillations in junctions that have different critical currents often preventing the doubling of voltage swings. Only graphoepitaxial SQUIDs possess the required properties, as demonstrated by measurements of electron transport and microstructure for different junction types. This contribution will describe our technology for the preparation of epitaxial metal-oxide heterostructures, graphoepitaxial high-Tc step edge Josephson junctions and dual type direct coupled DC SQUID magnetometers and gradiometers, as well as measurements of their electron transport properties and microstructures by high-resolution SEM, AFM and high-resolution TEM.
[1] M. I. Faley, Patent DE102012006825 (2012).

We describe aluminum-niobium-tungsten trilayer Nano-Superconducting Quantum Interference Devices (NanoSQUIDs) that can be read out continuously down, in temperature, to at least 230 mK. They show voltage oscillations up to at least 20 mT in field. A voltage modulation of 500 μV, voltage sensitivity of 2 mV/Φ₀, and white noise floor better than 5×10⁻⁵Φ₀/Hz^1/2 have been obtained. Flux noise places them between conventional low impedance SQUIDs and standard nanoSQUIDs. In modifying design parameters it is possible to suppress thermal hysteresis and to produce suspended Nanobrideges. High sensitivity and ease of implementation make this new kind of nanoSQUID attractive for magnetic detection schemes on the nanoscale and low temperature scanning SQUID microscopy.
D. Hazra, J. R. Kirtley, K. Hasselbach Applied Physics Letters 103, 093109 (2013)
D. Hazra, J. R. Kirtley, K. Hasselbach Applied Physics Letters 104, 152603 (2014)

D. Hazra and J. R. Kirtley thank the Nanosciences Fondation RTRA Grenoble for support in the framework of a SuperNanoCharac Project. K.H. acknowledges partial support in the framework of MICROKELVIN, the EU FRP7 low-temperature infrastructure under Grant No. 228464.

We report measurements of moderate size niobium-based SQUID Arrays (SQIFs) in both shielded and unshielded environments. These measurements demonstrate the ability of SQUID arrays to detect wide band transmissions of electromagnetic radiation. Specific examples will include simultaneous detection of AM (0.5 - 1.4 MHz) and FM (88 - 108 MHz) band transmitters. Performance metrics will include frequency rewsponse, gain, and linearity.

This work was supported by the Tactical SIGINT Technology Program.
Corresponding author: marcio.deandrade@navy.mil

The broadband RF detection capabilities of Josephson junctions can be enhanced by creating large 2D junction arrays using a combination of series and parallel connections to improve the voltage output and reduce the voltage noise compared to single junctions [1]. The majority of junction arrays demonstrated so far have used low-Tc junctions at helium temperatures, although ion-damaged junctions in YBCO have recently shown some good results [2]. The benefits of using high-Tc materials include the higher operating temperature and compact packaging which are readily available due to rapid developments in mini-cryocooler technology. Here we describe a set of test array circuits, based on YBCO step-edge junctions which have the advantage of relatively large normal resistances, the ability to be placed anywhere on a substrate and can be easily connected together via superconducting loops in a range of parallel and series connections [3, 4]. The size of the test array circuits range from 5 to 20,000 junctions distributed in superconducting loops of varying areas, some in a pseudo random pattern. The current-voltage and voltage-magnetic field performance of these arrays are characterized at 77 K in either liquid nitrogen or on a cryocooler. The RF response and sensitivity of some arrays will also be reported. These results are compared with simulations, adapted from low-Tc modelling packages, and implemented for high-Tc junction arrays. The effect of junction parameter spread and noise rounding at 77 K will be discussed.

[1] Berggren S A E, PhD thesis, 2012.
[2] Cybart S A et al. J. Appl. Phys., 112 063911 (2012).
[3] Lam S K H, Lazar J, Du J and Foley C P Supercond. Sci. Technol., 27 055011, (2014).
[4] Du J, Lam S K H, Mitchell E E and Foley C P, Supercond. Sci. Technol., 27, 095005, (2014).