Keiji Enpuku

Flux-flow type Josephson oscillator for millimeter and submillimeter wave region

An oscillator which utilizes the effect of the vortex motion in long Josephson tunnel junctions, i.e., flux flow, has been presented in millimeter and submillimeter wave region. An electromagnetic wave generated by the oscillator is detected with a small tunnel junction as a detector with a refined coupling configuration. Quantitative evaluation of the detected power showed that the detected power attained the value of 10−6 W in the frequency range between 100 and 400 GHz, which is far superior to previous results. Frequency and magnetic field dependences of the present system were also measured, which showed that the output power was able to be controlled by the dc magnetic field. The present oscillator will be promising as the local oscillator in the integrated Josephson receiver systems.

Sonochemical synthesis of monodispersed magnetite nanoparticles by using an ethanol-water mixed solvent

The magnetite nanoparticles were synthesized in an ethanol-water solution under ultrasonic irradiation from a Fe(OH)(2) precipitate. XRD, TEM, TG, IR, VSM and UV/vis absorption spectrum were used to characterize the magnetite nanoparticles. It was found that the formation of magnetite was accelerated in ethanol-water solution in the presence of ultrasonic irradiation, whereas, it was limited in ethanol-water solution under mechanical stirring. The monodispersibility of magnetite particles was improved significantly through the sonochemical synthesis in ethanol-water solution. The magnetic properties were improved for the samples synthesized under ultrasonic irradiation. This would be attributed to high Fe(2+) concentration in the magnetite cubic structure.

DETECTION OF MAGNETIC NANOPARTICLES WITH SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE (SQUID) MAGNETOMETER AND APPLICATION TO IMMUNOASSAYS

A system is developed to magnetically measure biological antigen-antibody reactions with a superconducting quantum interference device (SQUID) magnetometer. In this system, antibodies are labeled with magnetic nanoparticles of γ-Fe2O3, and the antigen-antibody reactions are measured by detecting the magnetic field from the magnetic nanoparticles. A setup of the system is described, and the sensitivity of the system is studied in terms of detectable weight of nanoparticles. Magnetic particles as small as 600 pg can be detected at present. An experiment is also conducted to measure antigen-antibody reaction with the present system. It is shown that the sensitivity of the present system is better than that of the conventional optical method. A one order of magnitude improvement of sensitivity will be realized by the sophistication of the present system.

Effect of thermal noise on the characteristics of a high Tc superconducting quantum interference device

Effects of thermal noise on the characteristics of the dc superconducting quantum interference device (SQUID) have been studied. Numerical simulation on the SQUID characteristics operating at T=77 K has been performed by taking into account the thermal noise. It is shown that the voltage versus flux relation of the dc SQUID is degraded considerably with the thermal noise. The degradation becomes significant when the inductance of the SQUID increases. Due to this degradation, there exists significant limitation for the range of the inductance available at T=77 K, unlike the case at T=4.2 K. The maximum inductance should be around 200 pH in order to avoid significant degradation of the transfer function. This limited value of the inductance must be taken into account when we realize the SQUID coupled to an input coil. The analytical expression for the degradation of the transfer function due to the thermal noise is also obtained. The theoretical result explains experimental results reported recently.

Flux‐flow‐type Josephson oscillator for millimeter and submillimeter wave region. II. Modeling

A theoretical study is made of a travelling‐wave‐type oscillator, which utilizes a flux flow in a long Josephson junction for use as a local oscillator in the integrated superconducting receiver system. An internal electromagnetic field of the oscillator junction in the flux‐flow state is investigated both numerically and analytically. It is shown that the voltage amplitude of the internal oscillation increases gradually in the direction of the flux flow and reaches a maximum value at the junction end. An equivalent circuit of the oscillator is also obtained, which gives dependences of the emitted radiation on frequency, magnetic field, and load. It is shown that the output power attains the value of the order of 10−6 W in the frequency range between 100 and 500 GHz, and that the output power and the radiation frequency can be controlled by both the bias voltage and the applied magnetic field. These theoretical results explain quantitatively the experimental ones with a Pb‐alloy long junction of length 24 λJ.

Flux-flow-type Josephson oscillator for millimeter and submillimeter wave region. III. Oscillation stability

Numerical as well as experimental studies have been made of the oscillation stability for the travelling‐wave‐type oscillator, which utilizes a vortex motion in a long Josephson junction, i.e., a flux flow. For the Josephson oscillator, a steep current step in a dc I‐V characteristic improves the oscillation stability. Two kinds of effects which degrade the steepness of the current step have been investigated for the long junction of the overlap geometry. One is the self‐field effect which makes the current step inclined, and the other is the resonant motion of the vortices which induces a staircaselike structure on the current step. We have discussed the method for the reduction of these effects and proposed a novel junction geometry without these effects, i.e., an overlap junction with a projection on one junction side. With the new geometry, we have obtained current steps whose slope is about one hundred times steeper than those obtained with the conventional overlap geometry and whose height attains a...

Noise characteristics of a dc SQUID with a resistively shunted inductance. II. Optimum damping

Effects of a damping resistance on noise characteristics of a dc SQUID are studied theoretically, where the damping resistance is in shunt with a loop inductance of the SQUID. An analytical expression for the energy resolution of the SQUID is obtained, with which the relation between the damping resistance and the energy resolution is studied in detail. It is shown that an optimum value of the damping resistance exists, which is determined by the tradeoff between the improvement of the transfer function and the additional noise due to the damping resistance. Optimum values of the damping resistance are about twice the shunt resistance of the SQUID and vary slightly with SQUID modulation parameter β. For optimum damping, the energy resolution is improved, compared with the case without the damping resistance. This means that the SQUID with large β can be used without significant degradation of the performance, i.e., the previous restriction of β=1 can be loosened. The analytical results agree quantitatively with numerical ones.

Application of high T/sub c/ SQUID magnetometer to biological immunoassays

A high T/sub c/ SQUID system is developed for the application to biological immunoassay. In this application, magnetic nanoparticles are used as magnetic markers to perform immunoassay, i.e., to detect binding reaction between an antigen and its antibody. Design and set up of the system is described. Minimum detectable amplitude of the magnetic flux is 0.6 m/spl Phi//sub 0/ for the measurement bandwidth from 0.2 Hz to 5 Hz when we use a magnetometer. The system noise does not increase when the magnetic field of 0.8 mT is applied in parallel to the SQUID. An experiment to measure the antigen-antibody reaction shows that the sensitivity of the present system is 10 times better than that of the conventional method using an optical marker. When a gradiometer is used, the system noise decreased by a factor of 5, compared to the case of the magnetometer. This improvement indicates the usefulness of the gradiometer to suppress the residual environmental noise in the present system. Magnetic markers that have remanent magnetic moment are also studied in order to increase the signal.

Development of multisample biological immunoassay system using HTSSQUID and magnetic nanoparticles

We developed a prototype magnetic immunoassay system using a high temperature superconductor (HTS) superconducting quantum interference device (SQUID) to investigate the performance and usability of the magnetic immunoassay. The system is designed to measure multiple samples and liquid samples, and it can work in an unshielded environment at a medical facility. To reduce the disturbance from environmental noise, the SQUID and samples are covered with three-layers of permalloy magnetic shield. The SQUID and magnetic shield are set in an aluminum box which acts as an RF shield. A gradiometer with a 5 /spl times/ 10 mm pickup coil, which is cooled by liquid nitrogen through a sapphire/Cu rod, is used as a sensor. We also developed a nonmagnetic sample disk with 12 reaction cells and examined 12 samples in one measurement sequence. The measurement process is controlled by a computer, which perform data averaging. Fe/sub 3/O/sub 4/ nanoparticles with a 25-nm diameter were used as test samples. After applying a magnetic field of about 0.1 T, we measured the remanent magnetic field from the Fe/sub 3/O/sub 4/ nanoparticles. The present system could detect 30 pg of Fe/sub 3/O/sub 4/ nanoparticles. This result was obtained by averaging 100 trials under an unshielded laboratory environment. The measurement time for 100 trials was only 100 s.

Simulation and Quantitative Clarification of AC Susceptibility of Magnetic Fluid in Nonlinear Brownian Relaxation Region

The nonlinear Brownian rotational relaxation of magnetic fluids for the case of large excitation field was studied in relation to its biomedical applications. The Fokker–Planck equation, which describes the nonlinear behavior of magnetic fluids, was solved by numerical simulation when a large step or a sinusoidal field was applied. Deviations from the Debye theory were quantitatively clarified. First, it was shown that the response time of the magnetic fluids became shorter than the Brownian relaxation time for a larger excitation field, which can be expressed in terms of the field-dependent Brownian relaxation time. Next, the amplitude of the ac susceptibility became lower for larger excitation fields, and the frequency characteristic of the ac susceptibility moved to a higher frequency compared with that predicted by the Debye theory. Finally, higher harmonics occurred with increasing excitation fields. Approximate equations, that describe such nonlinear behaviors reasonably well, were also obtained. These equations are expected to be useful for developing biosensors based on Brownian relaxation.