DAYEM BRIDGE BASED NANOSQUID FOR HIGH SENSITIVE NANOSCALE APPLICATIONS.
[Tesi di dottorato]
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.
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.
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