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Applied Physics Letters

Development of a niobium nanosuperconducting quantum interference device for the detection of small spin populations

S. K. H. Lam, David L Tilbrook

Published Feb 14, 2003• 150 citations
Publisher

Abstract

Electron-beam lithography and reactive ion etching have been used to fabricate thin-film Au/Nb bridges with widths ∼50 nm. The Au layer was used as both a mask for etching the Nb superconducting bridge and as a resistive shunt in the completed devices. Using these junctions, a dc superconducting quantum interference device (SQUID) design with a hole size of 200 nm×200 nm (nano-SQUID) has also been fabricated and characterized. A flux noise of approximately 7×10−6 Φ0/Hz1/2 at 4.2 K has been achieved, from which a calculated spin sensitivity of 250 spin/Hz1/2 is predicted.

DOI
10.1063/1.1554770

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Electron-beam lithography and reactive ion etching have been used to fabricate thin-film Au/Nb bridges with widths ∼50 nm. The Au layer was used as both a mask for etching the Nb superconducting bridge and as a resistive shunt in the completed devices. Using these junctions, a dc superconducting quantum interference device (SQUID) design with a hole size of 200 nm×200 nm (nano-SQUID) has also been fabricated and characterized. A flux noise of approximately 7×10−6 Φ0/Hz1/2 at 4.2 K has been achieved, from which a calculated spin sensitivity of 250 spin/Hz1/2 is predicted.
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.
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.
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.
Focused ion-beam (FIB) technology has been used to fabricate miniature Nb DC SQUIDs (superconducting quantum interference devices) which incorporate resistively shunted microbridge junctions and a central loop with a hole diameter ranging from 1058 to 50 nm. The smallest device, with a 50 nm hole diameter, has a white flux noise level of 2.6 µΦ 0 Hz −1/2 at 10 4 Hz. The scaling of the flux noise properties and focusing effect of the SQUID with the hole size were examined. The observed low frequency flux noises of different devices were compared with the contribution due to the spin fluctuation of defects introduced during FIB processing and of thermally activated flux hopping in the SQUID washer.
Reduction of the size of the superconducting quantum interference device (SQUID) ring structure improves its sensitivity to magnetic moments. Nanobridge junctions are advantageous for SQUID miniaturization. However, many planar SQUIDs with nanobridge junctions have shallow flux modulation depths because of their nonideal Josephson current-phase relationships. Here, we fabricated a three-dimensional niobium nitride (NbN) miniaturized SQUID in which the nanobridge junction structures are much thinner than the superconducting ring. We grew a thick superconducting NbN film on a silicon wafer and embedded an insulating slit in the middle of the film. By setting two thin nanobridge junctions across this insulating slit, we were able to manufacture NbN miniaturized SQUIDs that, in principle, can become nano-SQUIDs. The flux modulation depths of these devices reached 36% at 4.2 K. In addition, the fabricated devices show reversible current-voltage characteristics in the absence of any shunting resistance.
We investigate electric transport and noise properties of microstrip-type submicron direct current superconducting quantum interference devices (dc SQUIDs) based on Nb thin films and overdamped Josephson junctions with a HfTi barrier. The SQUIDs were designed for optimal spin sensitivity Sμ1/2 upon operation in intermediate magnetic fields B (tens of mT), applied perpendicular to the substrate plane. Our, so far, best SQUID can be continuously operated in fields up to B≈±50 mT with rms flux noise SΦ,w1/2≤250 nΦ0/Hz1/2 in the white noise regime and spin sensitivity Sμ1/2≤29 μB/Hz1/2. Furthermore, we demonstrate operation in B = 0.5 T with high sensitivity in flux SΦ,w1/2≈680 nΦ0/Hz1/2 and in electron spin Sμ1/2≈79 μB/Hz1/2. We discuss strategies to further improve the nanoSQUID performance.