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 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.
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.
Dayem Bridge Based Nano-SQUID for High Sensitive Nanoscale Applications A recent progress in nanotechnology formulates the fabrication of SQUIDs loop less than 100 nm; its obtaining an adequate sensitivity to detect the magnetic nano-objects. These nano-SQUIDs can be employed to measure small clusters of atomic magnetic moment, to obtain an inductive readout sensor in single-photon detectors and they have been proposed as a magnetic qubit for quantum computing. The research activities at CNR-ICIB has been devoted to the design, fabrication and characterization of nano-SQUIDs based on niobium Dayem Bridge and its magnetization measurements of nanoparticles for advance study of nanomagnetism. Here, I have reported the designs, the fabrication processes and the experimental performances (voltage flux characteristics, spin noise performance) of these nano-devices and also the groundwork measurement of nanoparticles magnetization using switching current measurement method operating in CNR-ICIB laboratory. Design and Fabrication: Many devices are fabrication using advanced electron beam lithography techniques with conventional photolithography, lift off and reactive ion etching techniques. The fabricated devices can be divided into two categories, first bilayer device or non hysteretic device and second single layer device or hysteretic device. a)Non-Hysteretic Device: The bilayer device consists of a niobium loop with a hole of 200nm interrupted by two bridges having length and width dimensions of 100nm X 80nm, 80nm X 60nm etc. The superconducting loop has a washer shape in order to enhance the heat dissipation during the working operations when the sensor is current biased in resistive mode. The device includes a micrometric integrated niobium coil located very close to the nano-sensor in order to modulate, tune and operate the SQUID in FLL mode. b)Hysteretic Device: The single layer devices having two Dayem Bridges of 90nm x 250nm and loop sizes of 2, 1 and 0.75μm, consist of a single niobium layer 20nm thick pattern by electron beam lithography and shaped by lift-off. The SQUIDs were designed to have a hysteretic current-voltage characteristic in order to work as a magnetic flux-current transducer. Performances: a)Non-Hysteretic Device: An intrinsic voltage swing and a maximum responsivity of 75 μV and 1.5 mV/Φ0 can be estimated from the experimental data. Our interest is focused on the characterization in small signal mode, because the magnetic flux coupled into device is expected to be much smaller than Ф0. In fact, such devices are designed to measure very small magnetic fields arising from a local nano-object. The sensor exhibits a magnetic flux noise level of 1.5μΦ0/Hz1/2 in the white region corresponding to a spin noise, in unit of Bohr magneton, of Sn1/2=2aSΦ1/2/(μBμ0)~60 spin/Hz1/2 where a is the radius of the SQUID loop, μ0 is the magnetic vacuum permeability and μB = 9.27 × 10−24 J/T. This value is comparable with the best value reported in the literature. It is worth noting that these sensors show a wide linear region in the V- Φ characteristics; the non-linearity effects are not evident up to 0.02 Φ0. b)Hysteretic Device: The second single layer device shows hysteretic I-V characteristics which are used as magnetic flux-current transducer. The magnetic pattern determines the responsivity of the SQUID, defined as the variation of the critical current as function of the external magnetic flux variation dIc/dΦ; in the present work responsivity up to 30μA/Φ0 have been obtained. The maximum magnetic flux coupled to the 2 and 1 micron loop size is about 2 and 0.5 Φ0 respectively. A critical current modulation is about 20%. An intrinsic sensor magnetic flux resolution less than 1 mΦ0, can be estimated from the intrinsic current fluctuation. The main interest of such devices is magnetization measurement of small magnetic particles by switching current techniques. Simulation Results: We have computed performance of nano-superconducting-quantum-interference devices (SQUIDs) in view of their employment in the detection of small spin populations. The analysis has been focused on nano-SQUID sensors having a square loop with a side length of 200 nm. We have calculated the spin sensitivity and the magnetic response relative to the single Bohr magneton (single spin), as a function of its position within the SQUID hole. The results predict that the SQUID response depends strongly on the spin position. The projected information’s are very useful to optimize the sensor performance in view of the most nanomagnetism applications. Preliminary Measurements of Nanoparticle Magnetization: We have effectively employed non-hysteretic sensor for nanoparticle magnetization investigation in view of nano-magnetism applications. We have performed preliminary measurements with and without iron oxide nanoparticles on the SQUID loop showing that the presence of magnetic nanoparticles can be easily detected and the magnetic relaxation curve measured. To test the capability of our devices in order to detect the magnetization of nanoparticles, we deposited iron oxide nanoparticles, having an average diameter of about 16nm, on chips containing characterized sensors. Here we report the measurement performed on the device with 1μm loop. The presence of iron nanoparticles did not change appreciably the magnetic pattern obtained using the integrated coil (magnetic field normal to the SQUID plane), however the nanoparticle presence was evident by the critical current measurement performed with magnetic field generated using the external solenoid (magnetic field parallel to the SQUID plane). Using the measured responsivity for this particular device (27μA/Φ0) the critical current variation was converted to magnetic flux variation. It is worth to note that with the present electronic at 100Hz to acquire 104 measurements it takes about 100s. The above results clearly indicating that the sensor can be effectively used in nano-magnetism applications.
In the present paper, performance of nano-superconducting-quantum-interference devices (SQUIDs) has been investigated in view of their employment in the detection of small spin populations. The analysis has been focused on nano-SQUID sensors having a square loop with a side length of 200 nm. We calculate the spin sensitivity and the magnetic response relative to the single Bohr magneton (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; the ratio between the spin sensitivity evaluated in the center of the loop and the minimum one is as high as a factor of 3 for a spin at a reasonable distance z′ of 10 nm from the SQUID plane. Furthermore, the magnetic flux due to several hundred of spins has been evaluated by considering different random spin distributions within the SQUID hole. Due to the both nonuniform SQUID response and the random distribution process, the results show a statistical uncertainty which has been evaluated as a function of the spin number. The estimated informations are very useful to optimize the sensor performance in view of the most nanomagnetism applications.
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.
In the recent years, nanoscale superconducting quantum interference devices (nano-SQUIDs) played a fundamental role in the study of small spin systems. Nano-SQUIDs typically employ nano-Dayem bridges having dimensions (length L and/or width W) greater than coherence length $\ensuremath{\xi}$ of the superconducting film. They exhibit characteristics different from those of standard SQUIDs because the current-phase relationship (CPR) is nonsinusoidal, rendering most of the theoretical predictions based on the standard SQUID theory usually unreliable. Here, we present a noise theory of dc nano-SQUIDs based on Dayem nanobridges. We have computed the main characteristics of this quantum device including current-voltage and voltage-magnetic flux characteristics, magnetic flux-to-voltage transfer factor, and spectral densities of voltage and magnetic flux noise for L/$\ensuremath{\xi}$ ratios ranging from 1 (sinusoidal limit) to 3.5 (hysteretic limit). The CPRs have been computed by using the theory of Josephson weak links based on the Ginzburg--Landau equation. The results show a dependence of the magnetic flux noise spectral density on (L/$\ensuremath{\xi}$)4/3 involving a degradation of about a factor of five between the two extreme cases and are consistent with experimentally measured magnetic flux noises reported in the literature. These results provide useful information for both device physics and their applications.
Abstract Superconducting quantum interference devices (SQUIDs) with 3D nano-bridge junctions can be miniaturized into nano-SQUIDs that are able to sense a few spins in a large magnetic field. Among all device parameters, the inductance is key to the performance of SQUIDs with 3D nano-bridge junctions. Here, we measured the critical-current magnetic flux modulation curves of 12 devices with three design types using a current strip-line directly coupled to the SQUID loop. A best flux modulation depth of 71% was achieved for our 3D Nb SQUID. From the modulation curves, we extracted the inductance values of the current stripe-line in each design and compared them with the corresponding simulation results of InductEX. In this way, London penetration depths of 110 and 420 nm were determined for our Nb (niobium) and NbN (niobium nitride) films, respectively. Furthermore, we showed that inductances of 11 and 119 pH for Nb and NbN 3D nano-bridge junctions, respectively, dominated the total inductance of our SQUID loops which are 23 pH for Nb and 255 pH for NbN. A screening parameter being equal to one suggests optimal critical currents of 89.6 and 8.1 μ A for Nb and NbN SQUIDs, respectively. Additionally, intrinsic flux noise of 110 ± 40 n Φ 0 /(Hz) 1/2 is calculated for the Nb SQUIDs with 3D nano-bridge junctions by Langevin simulation.