Nanoscale Superconducting QUantum Interference Devices (SQUIDs) have spin sensitivities approaching that required to detect the flip of a single spin in close proximity. There is considerable interest in developing them for measuring the properties of small spin populations in magnetic systems. It is desirable that such measurements can be realised at sub-kelvin temperatures thereby allowing the study of magnetic systems that undergo phase changes at such temperatures. However, most nanoscale SQUIDs use nanobridges as the Josephson elements which limits the operating range to temperatures close to the transition temperature of the device. Well below this, the current-phase relation can become non-sinusoidal, and hot spots arising from the large critical current lead to hysteretic I-V characteristics. To extend the temperature range downwards, we have developed a range of nanoSQUIDs fabricated from alternative materials with lower transition temperatures including Ti/Au and Al/Ag bilayers patterned using lift-off and e-beam lithography (EBL). We report on their I-V characteristics, noise performance and behaviour in applied magnetic fields at temperatures down to 60 mK. We discuss theoretical analysis and computer modelling of the heat flow in the nanobridge structure, and consider the effects of bank geometry and kinetic inductance on the overall device performance. Finally, we present measurements of several magnetic systems: control lines, superconducting islands and ground planes and discuss the feasibility of magnetic measurements of novel materials of interest, including the heterointerface between lanthanium aluminate and strontium titanate.
Nanoscale Superconducting QUantum Interference Devices (SQUIDs) have spin sensitivities approaching that required to detect the flip of a single spin in close proximity. There is considerable interest in developing them for measuring the properties of small spin populations in magnetic systems. It is desirable that such measurements can be realised at sub-kelvin temperatures thereby allowing the study of magnetic systems that undergo phase changes at such temperatures. However, most nanoscale SQUIDs use nanobridges as the Josephson elements which limits the operating range to temperatures close to the transition temperature of the device. Well below this, the current-phase relation can become non-sinusoidal, and hot spots arising from the large critical current lead to hysteretic I-V characteristics. To extend the temperature range downwards, we have developed a range of nanoSQUIDs fabricated from alternative materials with lower transition temperatures including Ti/Au and Al/Ag bilayers patterned using lift-off and e-beam lithography (EBL). We report on their I-V characteristics, noise performance and behaviour in applied magnetic fields at temperatures down to 60 mK. We discuss theoretical analysis and computer modelling of the heat flow in the nanobridge structure, and consider the effects of bank geometry and kinetic inductance on the overall device performance. Finally, we present measurements of several magnetic systems: control lines, superconducting islands and ground planes and discuss the feasibility of magnetic measurements of novel materials of interest, including the heterointerface between lanthanium aluminate and strontium titanate.
Superconducting quantum interference devices (SQUIDs) incorporating thin film nanobridges as weak links have sensitivities approaching that required for single spin detection at 4.2 K. However, due to thermal hysteresis they are difficult to operate at much lower temperatures which hinder their application to many quantum measurements. To overcome this, we have developed nanoscale SQUIDs made from titanium-gold proximity bilayers. We show that their electrical properties are consistent with a theoretical model developed for heat flow in bilayers and demonstrate that they enable magnetic measurements to be made on a sample at system temperatures down to 60 mK.
Abstract SQUIDs (Superconducting Quantum Interference Devices) are macroscopic quantum devices capable of detecting and measuring a wide variety of physical parameters with unprecedented sensitivity. SQUIDs based on nanobridge weak links have shown increasing promise for quantum information and quantum sensing applications such as single spin detection. Focussed ion beam etched nanobridges have properties which can enhance nanoSQUID device performance but are often limited in terms of their non-hysteretic operating temperature range. Here we describe measurements of FIB-milled nanobridges, as single weak links or in nanoSQUIDs, made using either Ga, Xe or Ne ion beam sources. Their properties as a function of temperature, bias current, magnetic field and microwave power are measured and modelled according to a range of superconductivity models, as a means for improved understanding of the associated nanobridge parameters. We further propose techniques to extend the non-hysteretic operating temperature range of the devices.
We study operation of a superconducting quantum interference devices (SQUIDs) based on a new bilayer material. They can be used for the ultra-sensitive detection of magnetic momentum at temperatures down to milliKelvin range. Typically, thermal origin hysteresis of the symmetric SQUID current-voltage curves limits operating temperatures to T>0.6Tc. We used a new bilayer material for SQUID fabrication, namely proximity-coupled superconductor/normal-metal (S/N) bilayers (aluminum 25 nm/platinum 5 nm). Because of the 5 nm Pt-layer, Al/Pt devices show nonhysteretic behavior in a broad temperature range from 20 mK to 0.8 K. Furthermore, the Al/Pt bilayer devices demonstrate an order of magnitude lower critical current compared to the Al devices, which decreases the screening parameter (βL) and improves the modulation depth of the critical current by magnetic flux. Operation at lower temperatures reduces thermal noise and increases the SQUID magnetic field resolution. Moreover, we expect strong decrease of two-level fluctuators on the surface of aluminum due to Pt-layer oxidation protection and hence significant reduction of the 1/f noise. Optimized geometry of Al/Pt symmetric SQUIDs is promising for the detection of single-electron spin flip.
Nanoscale superconducting quantum interference devices (nano-SQUIDS) where the weak links are made from nanobridges, i.e., nanobridge SQUIDs (NBSs), are one of the most sensitive magnetometers for nanoscale magnetometry. Because of very strong nonlinearity in the nanobridge-electrode joints, the applied magnetic flux (${\mathrm{\ensuremath{\Phi}}}_{a}$)--critical current (Ic) characteristics of NBSs differ very significantly from conventional tunnel-junction SQUIDs, especially when the nanobridges are long and/or the screening parameter is large. However, in most of the theoretical descriptions, NBSs have been treated as the conventional tunnel-junction SQUIDs, which are based on the dc Josephson effect. Here, I present a model demonstrating that for long nanobridges and/or large screening parameters the ${I}_{c}({\mathrm{\ensuremath{\Phi}}}_{a})$ of a NBS can be explained by merely considering the fluxoid quantization in the NBS loop and the energy of the NBS; it is not necessary to take the Josephson effect into consideration. I also demonstrate that using the model, one can derive useful expressions such as the modulation depth and transfer function. I discuss the role of the kinetic inductance fraction ($\ensuremath{\kappa}$) in determining ${I}_{c}({\mathrm{\ensuremath{\Phi}}}_{a})$. I compare the predictions of the present model with the experimental data already published by several groups.
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.
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.