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IEEE Transactions on Applied Superconductivity

Performance Optimization of a Three-Dimensional NanoSQUID Based on Niobium Tunnel Nanojunctions

Matteo Fretto, Matthias Schmelz, Antonio Vettoliere, V. Zakosarenko, Natascia De Leo, Ronny Stolz, C. Granata

Published May 29, 2018• 0 citations
Publisher

Abstract

We report results about an optimized three-dimensional nanoSQUID based on niobium tunnel nanojunctions having the loop suspended to reduce the parasitic capacitance. The SQUID loop has a size of 400 × 600 nm2while the dimension of the square tunnel nanojunctions is 150 × 150 nm2with a density of the critical current of about 35 × 103A/cm2. The nanodevice has been characterized at liquid helium temperature; it has shown nonhysteretic current-voltage characteristics and smooth voltage- magnetic flux characteristics resulting in a very stable operation in a wide range of bias points. The spectral density of the magnetic flux noise in the white region, measured with a two-stage noise measurement setup, was as low as 300 nΦ0/Hz1/2corresponding to a spin sensitivity of few Bohr magnetons for bandwidth unit.

DOI
10.1109/tasc.2018.2841906

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We report results about an optimized three-dimensional nanoSQUID based on niobium tunnel nanojunctions having the loop suspended to reduce the parasitic capacitance. The SQUID loop has a size of 400 × 600 nm2while the dimension of the square tunnel nanojunctions is 150 × 150 nm2with a density of the critical current of about 35 × 103A/cm2. The nanodevice has been characterized at liquid helium temperature; it has shown nonhysteretic current-voltage characteristics and smooth voltage- magnetic flux characteristics resulting in a very stable operation in a wide range of bias points. The spectral density of the magnetic flux noise in the white region, measured with a two-stage noise measurement setup, was as low as 300 nΦ0/Hz1/2corresponding to a spin sensitivity of few Bohr magnetons for bandwidth unit.
We report the performance of a reliable three-dimensional nanometer-sized Superconducting QUantum Interference Device (SQUID). The use of superconductor-isolator-superconductor Nb/Al-AlOx/Nb Josephson tunnel junctions together with small SQUID loop dimensions permits a high modulation depth of the SQUID's critical current and thus leads to very low intrinsic flux noise of the device. In particular, we present electrical characterization including detailed noise investigations. At 4.2 K, two-stage noise measurements with a SQUID as a low noise preamplifier result in a white flux noise of 51 nΦ0/Hz1/2, which is equivalent to an energy resolution of 1.3 h, with h being Planck's constant. Simulation of spin sensitivities results in about 1 μB/Hz1/2 for an electron spin positioned directly above the SQUID ring.
An integrated magnetic nanosensor based on a niobium dc SQUID (superconducting quantum interference device) for nanoscale applications is presented. The sensor, having a washer shape with a hole of 200 nm and two Josephson-Dayem nanobridges of 80 nm × 100 nm, consists of a Nb(30 nm)/Al(30 nm) bilayer patterned by electron beam lithography (EBL) and shaped by lift-off and reactive ion etch (RIE) processes. The presence of the niobium coils, integrated on-chip and tightly coupled to the SQUID, allows us to easily excite the sensor in order to get the voltage-flux characteristics and to flux bias the SQUID at its optimal point. The measurements were performed at liquid helium temperature. A voltage swing of 75 µV and a maximum voltage-flux transfer coefficient (responsivity) as high as 1 mV/Φ(0) were directly measured from the voltage-flux characteristic. The noise measurements were performed in open loop mode, biasing the SQUID with a dc magnetic flux at its maximum responsivity point and using direct-coupled low-noise readout electronics. A white magnetic flux noise spectral density as low as 2.5 μΦ(0) Hz(-1/2) was achieved, corresponding to a magnetization or spin sensitivity in units of the Bohr magneton of 100 spin Hz(-1/2). Possible applications of this nanosensor can be envisaged in magnetic detection of nanoparticles and small clusters of atoms and molecules, in the measurement of nanoobject magnetization, and in quantum computing.
The growing interest in the development of nano-superconducting quantum interference devices (nanoSQUIDs) and their applications in nanoscience demands a deep study of these ultra-high-sensitivity nanodevices. In this paper, the main features of niobium planar nanointerferometers as a function of the temperature have been investigated. In particular, two types of nanodevices have been realized and studied: one based on nanobridges and the other on sub-micron Josephson tunnel junctions (JTJs). The nanobridge-based nanointerferometer, fabricated by electron beam lithography (EBL), consists of a sub-micrometric loop (effective area of 0.5 μ m 2 ), interrupted by two Dayem nanobridges having a width and length of 50 and 80 nm respectively. The JTJs-based nanointerferometer has two Nb/Al-AlO x /Nb SNIS (superconductor/normal-insulator/superconductor) junctions with an area of 0.09 μ m 2 , connected by a rectangular loop (1 μ m × 0.2 μ m), and realized by the three-dimensional focused ion beam (3D FIB) sculpting technique. For both nanodevices, measurements of current–voltage characteristics and critical current versus external magnetic flux have been performed at operating temperatures ranging from 4.2 to 1.2 K. The critical current modulation depths, magnetic flux to current transfer factor (current responsivity) and magnetic flux resolution as a function of the temperature have been evaluated for both nanosensors. Furthermore, measurements of the voltage–magnetic flux characteristics and the spectral density of magnetic flux noise have been performed at T = 4.2 K.
We report on the design, the fabrication and the performance of an integrated magnetic nano-sensor based on niobium dc-SQUID (Superconducting QUantum Interference Device) for nanoscale applications is presented. The nano-sensors are based on nanometric niobium constrictions (Dayem bridges) inserted in a square loop having a side length of 200 nm. Measurements of voltage-flux characteristic, flux to voltage transfer factor and noise performances are reported. In small signal mode, the sensors have shown a magnetic flux noise spectral density of 1.5 μΦ 0 /Hz 1/2 corresponding to a spin sensitivity in unit of Bohr magneton of 60 spin/Hz 1/2 . Supercurrent decay measurements of these devices are also reported. Such measurements provide useful information for applications which employ the SQUID as a trigger where the sensor works on the zero voltage state. The experimental data, have shown an intrinsic current fluctuation less than 0.2% of the critical current at liquid helium temperature, corresponding to an intrinsic sensor magnetic flux resolution of a few mΦ 0 . In view of the nano-SQUID employments in the detection of small spin populations, the authors calculated the spin sensitivity and the magnetic response relative to the single spin, as a function of its position within the SQUID hole. The results show that the SQUID response depends strongly on the spin position.
We report on the development of highly sensitive SQUIDs featuring sub-micrometer loop dimensions. The integration of high quality and low capacitance SIS Nb/AlO x /Nb cross-type Josephson tunnel junctions results in white flux noise levels as low as 66 n Φ 0 Hz −1/2 , well below state-of-the-art values of their Nb-based counterparts based on constriction type junctions. Estimation of the spin sensitivity of the best SQUIDs yield S μ 1/2 < 7 μ B Hz −1/2 in the white noise region, suitable for the investigation of small spin systems. We discuss fabrication challenges, show results on the electrical characterization of devices with various pickup loops, and describe options for further improvement, which may push the sensitivity of such devices even to single spin resolution.
A superconducting quantum interference device (SQUID) miniaturized into the nanoscale is promising in the inductive detection of a single electron spin. A nano-SQUID with a strong spin coupling coefficient, a low flux noise, and a wide working magnetic field range is highly desired in a single spin resonance measurement. Nano-SQUIDs with Dayem bridge junctions excel in a high working field range and in the direct coupling from spins to the bridge. However, the common planar structure of nano-SQUIDs is known for problems such as a shallow flux modulation depth and a troublesome hysteresis in current–voltage curves. Here, we developed a fabrication process for creating three-dimensional (3-D) niobium (Nb) nano-SQUIDs with nanobridge junctions that can be tuned independently. Characterization of the device shows up to 45.9% modulation depth with a reversible current–voltage curve. Owning to the large modulation depth, the measured flux noise is as low as 0.34 μΦ 0 /Hz 1/2 . The working field range of the SQUID is greater than 0.5 T parallel to the SQUID plane. We believe that 3-D Nb nano-SQUIDs provide a promising step toward effective single-spin inductive detection.