YBCO step-edge SQUIDs by magnetron sputtering technique

Summary YBCO step-edge junction d.c. SQUIDs have been realized by using the Inverted Cylindrical Magnetron Sputtering (ICMS) technique. This last represents a novel technology for high-T _c Josephson junctions (HTSC). Steps are obtained by standard ion milling procedure on LaAlO₃ (100) substrates using Nb-masks patterned by reactive ion etching. Measurements of currentvs. voltage, maximum d.c. Josephson currentvs. magnetic field and SQUID voltage response measurements have been performed, also as a function of the temperature. Operating temperature as high as 77K has been achieved. At 4.2K the SQUIDs show a maximum voltage of flux transfer function (∂V/∂ϕ)_max=870 μV/Ф₀ and a good periodicity of theV-ϕ modulation up to 20Ф₀ without any sign of hysteresis. The ratio between the step height (h) and the film thickness (d) seems to play a fundamental role in determining Josephson properties of the bridges, these conditions being more severe with respect to most of the data available in literature. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

A newly developed Fe-doped calcium sulfide nanoparticles with magnetic property for cancer hyperthermia

In this study, a magnetic iron-doped calcium sulfide (Fe–CaS) nanoparticle was newly developed and studied for the purpose of hyperthermia due to its promising magnetic property, adequate biodegradation rate, and relatively good biocompatibility. Fe–CaS nanoparticles were synthesized by a wet chemical co-precipitation process with heat treatment in a N₂ atmosphere, and were subsequently cooled in N₂ and exposed to air at a low temperature. The crystal structure of the Fe–CaS nanoparticles was similar to that of the CaS, which was identified by an X-ray diffractometer (XRD). The particle size was less than 40 nm based on a Debye–Scherrer equation and transmission electron microscope (TEM) examination. Magnetic properties obtained from the SQUID magnetometer demonstrated that the synthesized CaS was a diamagnetic property. Once the Fe ions were doped, the synthesized Fe–CaS converted into paramagnetism which showed no hysteresis loop. Having been heated above 600 °C in N₂, the Fe–CaS showed a promising magnetic property to produce enough energy to increase the temperature for hyperthermia. 10 mg/ml of the Fe–CaS was able to generate heat to elevate the media temperature over 42.5 °C within 6 min. The area of the hysteresis loop increased with the increasing of the treated temperature, especially at 800 °C for 1 h. This is because more Fe ions replaced Ca ions in the lattice at the higher heat treatment temperature. The heat production was also increasing with the increasing of heat treatment temperature, which resulted in an adequate specific absorption ratio (SAR) value, which was found to be 45.47 W/g at 37 °C under an alternative magnetic field of f = 750 KHz, H = 10 Oe. The in vitro biocompatibility test of the synthesized Fe–CaS nanoparticles examined by the LDH assay showed no cytotoxicity to 3T3 fibroblast. The result of in vitro cell hyperthermia shows that under magnetic field the Fe–CaS nanoparticles were able to generate heat and kill the CT-26 cancer cells significantly. We believe that the developed Fe–CaS nanoparticles have great potential as thermo-seeds for cancer hyperthermia in the near future.     

Voltage–current and voltage–flux characteristics of asymmetric high TC DC SQUIDs

We report measurements of transfer functions and flux shifts of 20 on-chip high T_C DC SQUIDs half of which were made purposely geometrically asymmetric. All of these SQUIDs were fabricated using standard high T_C thin-film technology and they were single layer ones, having 140 nm thickness of YBa₂Cu₃O_7?x film deposited by laser ablation onto MgO bicrystal substrates with 24° misorientation angle. For every SQUID the parameters of its intrinsic asymmetry, i.e., the density of critical current and resistivity of every junction, were measured directly and independently. We showed that the main reason for the on-chip spreading of SQUIDs’ voltage–current and voltage–flux characteristics was the intrinsic asymmetry. We found that for SQUIDs with a relative large inductance (L > 120 pH) both the voltage modulation and the transfer function were not very sensitive to the junctions asymmetry, whereas SQUIDs with smaller inductance (L ? 65–75 pH) were more sensitive. The results obtained in the paper are important for the implementation in the sensitive instruments based on high T_C SQUID arrays and gratings.     

Superconducting quantum interference devices made of Sb-doped Bi2Se3 topological insulator nanoribbons

We report the fabrication and characterization of superconducting quantum interference devices (SQUIDs) made of Sb-doped Bi₂Se₃ topological insulator (TI) nanoribbon (NR) contacted with PbIn superconducting electrodes. When an external magnetic field was applied along the NR axis, the TI NR exhibited periodic magneto-conductance oscillations, the so-called Aharonov-Bohm oscillations, owing to one-dimensional subbands. Below the superconducting transition temperature of PbIn electrodes, we observed supercurrent flow through TI NR-based SQUID. The critical current periodically modulates with a magnetic field perpendicular to the SQUID loop, revealing that the periodicity corresponds to the superconducting flux quantum. Our experimental observations can be useful to explore Majorana bound states (MBS) in TI NR, promising for developing topological quantum information devices.     

Ferromagnetic 0–<Emphasis Type="BoldItalic">π</Emphasis> Josephson junctions

We present a study on low-T_c superconductor-insulator-ferromagnet-superconductor (SIFS) Josephson junctions. SIFS junctions have gained considerable interest in recent years because they show a number of interesting properties for future classical and quantum computing devices. We optimized the fabrication process of these junctions to achieve a homogeneous current transport, ending up with high-quality samples. Depending on the thickness of the ferromagnetic layer and on temperature, the SIFS junctions are in the ground state with a phase drop of either 0 or π. By using a ferromagnetic layer with variable step-like thickness along the junction, we obtained a so-called 0–π Josephson junction, in which 0 and π ground states compete with each other. At a certain temperature the 0 and π parts of the junction are perfectly symmetric, i.e. the absolute critical current densities are equal. In this case the degenerate ground state corresponds to a vortex of supercurrent circulating clock- or counterclockwise and creating a magnetic flux which carries a fraction of the magnetic flux quantum Φ₀. PACS  74.50.+r; 74.78.Fk; 74.81.-g; 85.25.Cp     

Low-field magnetic response of multi-junction superconducting quantum interference devices

The magnetic states of multi-junction superconducting quantum interference device containing 2N identical conventional Josephson junctions are studied by means of a perturbation analysis of the non-linear first-order ordinary differential equations governing the dynamics of the Josephson junctions in these devices. In the zero-voltage state, persistent currents are calculated in terms of the externally applied magnetic flux Φ_ex . The resulting d.c. susceptibility curves show that paramagnetic and diamagnetic states are present, depending on the value of Φ_ex . The stability of these states is qualitatively studied by means of the effective potential notion for the system.     

Planar microwave biased RF-SQUIDs in niobium technology

A planar version of microwave biased SQUIDs is described. In this type of SQUID, a superconducting half-wavelength microstrip resonator serves as tank circuit, into which the SQUID is integrated. For evaluation of this type of SQUID, samples were prepared from thin Nb films on sapphire substrates, using tunnel junctions as Josephson elements. When operated in hysteretic mode, signal voltages of up to 80 V were achieved, corresponding to a flux noise of 4×10^–6 ₀/Hz and an energy resolution of 2×10^–31 J/Hz.     

Fabrication of magnetic gold nanorod particles for immunomagnetic separation and SERS application

The preparation and application of rod-shaped core–shell structured Fe₃O₄–Au nanoparticles for immunomagnetic separation and sensing were described for the first time with this study. To synthesize magnetic gold nanorod particles, the seed-mediated synthetic method was carried out and the resulting nanoparticles were characterized with transmission electron microscopy (TEM), ultraviolet visible spectroscopy (UV–Vis), energy-dispersive X-ray (EDX), and X-ray diffraction (XRD). Magnetic properties of the nanoparticles were also examined. Characterization of the magnetic gold nanorod particles has proven that the resulting nanoparticles were composed of Fe₃O₄ core and the gold shell. The rod-shaped gold-coated iron nanoparticles have an average diameter of 16 ± 2 nm and an average length of about 50 ± 5 nm (corresponding aspect ratio of 3). The saturation magnetization value for the magnetic gold nanorod particles was found to be 37 emu/g at 300 K. Rapid and room temperature reaction synthesis of magnetic gold nanorod particles and subsequent surface modification with E. coli antibodies provide immunomagnetic separation and SERS application. The analytical performance of the SERS-based homogenous sandwich immunoassay system with respect to linear range, detection limit, and response time is also presented.     

Biodegradable thermoresponsive polymeric magnetic nanoparticles: a new drug delivery platform for doxorubicin

The use of nanoparticles as drug delivery systems for anticancer therapeutics has great potential to revolutionize the future of cancer therapy. The aim of this study is to construct a novel drug delivery platform comprising a magnetic core and biodegradable thermoresponsive shell of tri-block-copolymer. Oleic acid-coated Fe₃O₄ nanoparticles and hydrophilic anticancer drug “doxorubicin” are encapsulated with PEO–PLGA–PEO (polyethylene oxide–poly d, l lactide-co-glycolide–polyethylene oxide) tri-block-copolymer. Structural, magnetic, and physical properties of Fe₃O₄ core are determined by X-ray diffraction, vibrating sample magnetometer, and transmission electron microscopy techniques, respectively. The hydrodynamic size of composite nanoparticles is determined by dynamic light scattering and is found to be ~36.4 nm at 25 °C. The functionalization of magnetic core with various polymeric chain molecules and their weight proportions are determined by Fourier transform infrared spectroscopy and thermogravimetric analysis, respectively. Encapsulation of doxorubicin into the polymeric magnetic nanoparticles, its loading efficiency, and kinetics of drug release are investigated by UV–vis spectroscopy. The loading efficiency of drug is 89% with a rapid release for the initial 7 h followed by the sustained release over a period of 36 h. The release of drug is envisaged to occur in response to the physiological temperature by deswelling of thermoresponsive PEO–PLGA–PEO block-copolymer. This study demonstrates that temperature can be exploited successfully as an external parameter to control the release of drug.     

Detection of magnetic nanoparticles with magnetoencephalography

Superconducting quantum interference devices (SQUIDs) have been widely utilized in biomedical applications due to their extremely high sensitivity to magnetic signals. The present study explores the feasibility of a new type of nanotechnology-based imaging method using standard clinical magnetoencephalographic (MEG) systems equipped with SQUID sensors. Previous studies have shown that biological targets labeled with non-toxic, magnetized nanoparticles can be imaged by measuring the magnetic field generated by these particles. In this work, we demonstrate that (1) the magnetic signals from certain nanoparticles can be detected without magnetization using standard clinical MEG, (2) for some types of nanoparticles, only bound particles produce detectable signals, and (3) the magnetic field of particles several hours after magnetization is significantly stronger than that of un-magnetized particles. These findings hold promise in facilitating the potential application of magnetic nanoparticles to in vivo tumor imaging. The minimum amount of nanoparticles that produce detectable signals is predicted by theoretical modeling and computer simulation.     

Tuning rf-SQUID flux qubit system's potential with magnetic flux bias

At an extremely low temperature of 20 mK, we measured loop current in a tunable rf superconducting quantum interference device (SQUID) with a dc-SQUID. By adjusting the magnetic flux applied to the rf-SQUID loop (Φ_f) and the small dc-SQUID (Φ_f^cjj), respectively, the potential shape of the system can be fully controlled in situ. Variations of transition step and overlap size in switching current with the barrier flux bias are analyzed, from which we can obtain some relevant device parameters and built up a model to explain the experimental phenomenon.     

YBaCuO BICRYSTAL JUNCTIONS AND DC SUPERCONDUCTING QUANTUM INTERFERENCE DEVICES

We have prepared yttria-stabilized-zirconia bicrystal substrates using a simple hot-pressing method. The grain-boundary junctions have been fabricated with YBa₂Cu₃O₇ thin films grown epitaxially on the bicrystals. The patterns are defined by conventional photolithography, The dc and microwave characteristics of the junctiorts and the dc superconducting quantum interference devices (SQUIDs) have been intensively studied. The current-voltage curves are bridge-typed with noise rounding near the critical current. Resistive tail has been observed in the resistance versus temperature curves. The results are compared with the theoretical prediction for classical Josephson junctions. It is found that the behavior of bicrystal junctions can be described in the frame of classical theory. The deviations are attributed to the nonuniformity of the junctions. The small loop dc SQUIDs demonstrate diffraction and interference effects with regard to the applied magnetic field. A large square-washer with a new configuration has been designed to enhance the effective area of dc SQUID as a magnetometer. It has achieved a magnetic field resolution down to 1 pT/(Hz)^1/2(at 10Hz) at 77K.     

Performance of 2-hole YBCO rf SQUID in flux-locked-loop mode at 77 K

A 2-hole rf SQUID has been fabricated using naturally present grain boundary Josephson weaklinks in a microbridge of bulk Y-Ba-Cu-O superconductor. Periodic oscillations in voltage-flux characteristics have been observed up to 80.5 K. The spectral density of flux noise of the SQUID is 8 × 10⁻⁴ Φ₀/√Hz at 100 Hz and 77 K for open-loop mode. The SQUID has been successfully operated in flux-locked-loop mode at 77 K, demonstrating the feasiblity of the device for practical applications. In the flux-locked-loop mode the stability as well as flux noise of the SQUID has been found to improve considerably as compared to that in the open-loop mode. Various SQUID parameters have also been estimated and reported here.     

Supercurrent and its Fano effect in a Josephson Aharonov-Bohm ring

The Josephson current through an Aharonov-Bohm (AB) interferometer, in which a quantum dot (QD) is situated on one arm and a magnetic flux Φ threads through the ring, has been investigated. In the presence of the magnetic flux, the relation between the Josephson current and the superconductor phase is complex, and the system can be adjusted to π junction by either modulating the magnetic flux or the QD’s energy level ε_d. Due to the electron-hole symmetry, the Josephson current I has the property I(ε_d,Φ)=I(-ε_d,Φ+π). The Josephson current exhibits a jump when a pair of Andreev bound states aligns with the Fermi energy. The condition for the current jump is given. Particularly, we find that the position of the current jump and the position of the maximum value of the critical current I_c are identical. Due to the interference between the two paths, the critical current I_c versus the QD’s level ε_d shows a typical Fano shape, which is similar to the Fano effect in the corresponding normal device. However they also show some differences. For example, the critical current never reaches zero for any parameters, while the current in the normal device can reach zero at the destruction point.     

Equivalent parameters of a Josephson junction in a microwave SQUID structure

The equivalent parameters of a Josephson junction in a microwave SQUID structure are calculated on the basis of relations obtained as a result of an analysis of the operation of an rf SQUID. This analysis is based on the sawtooth variation of the voltage on the resonator as a function of the constant flux bias. The quantitative characteristics permit regarding the Josephson junction as a linear impedance in the rf or microwave circuit, whose real and imaginary parts are controlled by the constant magnetic flux passing through the SQUID loop. Zh. Tekh. Fiz. 69, 106–112 (November 1999)     

The magnetic and hyperthermia studies of bare and silica-coated La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> nanoparticles

The magnetic nanoparticles of La_0.75Sr_0.25MnO₃ perovskite manganite with a controlled size were prepared via sol–gel procedure, followed by thermal treatment and subsequent mechanical processing of the resulting raw product. The prepared materials were structurally studied by the XRD and TEM methods and probed by DC magnetic measurements. The nanoparticles of the mean crystallite sizes 11–40 nm exhibit T _C in the range of ≈310–347 K and the sample possessing 20-nm crystallites was identified as the most suitable for hyperthermia experiments. In order to obtain a colloidally stable suspension and prevent toxic effects, the selected magnetic cores were further encapsulated into silica shell using tetraethoxysilane. The detailed magnetic studies were focused on the comparison of the raw product, the bare nanoparticles after mechanical processing and the silica-coated nanoparticles, dealing also with effects of size distribution and magnetic interactions. The heating experiments were carried out in an AC field of frequencies 100 kHz–1 MHz and amplitude 3.0–8.9 kA m⁻¹ on water dispersions of the samples, and the generated heat was deduced from their warming rate taking into account experimentally determined thermal losses into surroundings. The experiments demonstrate that the heating efficiency of the coated nanoparticles is generally higher than that of the bare magnetic cores. It is also shown that the aggregation of the bare nanoparticles increases heating efficiency at least in a certain concentration range.     

Structural,magnetic, and electrical properties of Gd-doped BiFeO<Subscript>3</Subscript> nanoparticles with reduced particle size

Pure and Gd-doped BiFeO₃ nanoparticles have been synthesized by sol–gel method. The significant effects of size and Gd-doping on structural, electrical, and magnetic properties have been investigated. X-ray diffraction study reveals that the pure BiFeO₃ nanoparticles possess rhombohedral structure, but with 10% Gd-doping complete structural transformation from rhombohedral to orthorhombic has been observed. The particle size of pure and Gd-doped BiFeO₃ nanoparticles, calculated using Transmission electron microscopy, has been found to be in the range 25–15 nm. Pure and Gd-doped BiFeO₃ nanoparticles show ferromagnetic character, and the magnetization increases with decrease in particle size and increase in doping concentration. Scanning electron microscopy study reveals that grain size decreases with increase in Gd concentration. Well-saturated polarization versus electric field loop is observed for the doped samples. Leakage current density decreases by four orders by doping Gd in BiFeO₃. The incorporation of Gd in BiFeO₃ enhances spin as well as electric polarization at room temperature. The possible origin of enhancement in these properties has been explained on the basis of dopant and its concentration, phase purity, small particle, and grain size.     

Metal-on-oxide nanoparticles produced using laser ablation of microparticle aerosols

A continuous aerosol process has been studied for producing nanoparticles of oxides that were decorated with smaller metallic nanoparticles and are free of organic stabilizers. To produce the oxide carrier nanoparticles, an aerosol of 3–6 μm oxide particles was ablated using a pulsed excimer laser. The resulting oxide nanoparticle aerosol was then mixed with 1.5–2.0 μm metallic particles and this mixed aerosol was exposed to the laser for a second time. The metallic micron-sized particles were ablated during this second exposure, and the resulting nanoparticles deposited on the surface of the oxide nanoparticles producing an aerosol of 10–60 nm oxide nanoparticles that were decorated with smaller 1–5 nm metallic nanoparticles. The metal and oxide nanoparticle sizes were varied by changing the laser fluence and gas type in the aerosol. The flexibility of this approach was demonstrated by producing metal-decorated oxide nanoparticles using two oxides, SiO₂ and TiO₂, and two metals, Au and Ag.     

Preparation and characterization of Fe3O4 magnetic composite microspheres covered by a P(MAH-co-MAA) copolymer

A magnetic core–shell-layered polymer microsphere (MPS) was successfully synthesized by a dispersion polymerization route, where the modified Fe₃O₄ nanoparticles (MFN) were used as a core, while poly(maleic anhydride-co-methacrylic acid) P(MAH-co-MAA) as a shell was covered on the surface of the Fe₃O₄ nanoparticles. Environmental scanning electron microscope (ESME) and transmission electron microscope (TEM) measurements indicate that the magnetic P(MAH-co-MAA)/Fe₃O₄ composite microspheres assume sphericity and have a novel core–shell-layered structure. The crystal particle sizes of the unimproved Fe₃O₄ and the MFN samples vary from 8 to 16 nm in diameter, and the average size is about 10.6 nm in diameter. The core–shell magnetic composite microspheres can be adjusted by changing the stirring speed. Since multiple Fe₃O₄ cores were coated with a proper percentage of P(MAH-co-MAA) copolymers, and therefore lower density was acquired for the MPS, which improved sedimentation and dispersion behavior. The saturated magnetization of pure Fe₃O₄ nanoparticles reaches 48.1 emu g⁻¹ and the value for composite nanoparticles was as high as 173.5 emu g⁻¹. The nanoparticles show strong superparamagnetic characteristics and can be expected to be used as a candidate for magnetism-controlled drug release.     

Performance of YBaCuO thick film rf SQUIDs at 77 K

RF SQUID behaviour has been observed at 77 K in YBaCuO thick films prepared by screen printing technique. A hole shunted with a microbridge type of geometry is patterned manually for observing rf SQUID behaviour. Flux noise spectrum is also studied and it is found to depend on the quality of the film. The spectral density of the flux noise in the white noise region is 1.7×10⁻³ Φ₀/√Hz at 77 K.