In this work, the core size estimation technique of magnetic nanoparticles (MNPs) using the static magnetization curve obtained from a high-Tc SQUID magnetometer and a metaheuristic inversion technique based on the Particle Swarm Optimizer (PSO) algorithm is presented. The high-Tc SQUID magnetometer is constructed from a high-Tc SQUID sensor coupled by a flux transformer to sense the modulated magnetization signal from a sample. The magnetization signal is modulated by the lateral vibration of the sample on top of a planar differential detection coil of the flux transformer. A pair of primary and excitation coils are utilized to apply an excitation field parallel to the sensitive axis of the detection coil. Using the high-Tc SQUID magnetometer, the magnetization curve of a commercial MNP sample (Resovist) was measured in a logarithmic scale of the excitation field. The PSO inverse technique is then applied to the magnetization curve to construct the magnetic moment distribution. A multimodal normalized log-normal distribution was used in the minimization of the objective function of the PSO inversion technique, and a modification of the PSO search region is proposed to improve the exploration and exploitation of the PSO particles. As a result, a good agreement on the Resovist magnetic core size was obtained between the proposed technique and the non-negative least square (NNLS) inversion technique. The estimated core sizes of 8.0484 nm and 20.3018 nm agreed well with the values reported in the literature using the commercial low-Tc SQUID magnetometer with the SVD and NNLS inversion techniques. Compared to the NNLS inversion technique, the PSO inversion technique had merits in exploring an optimal core size distribution freely without being regularized by a parameter and facilitating an easy peak position determination owing to the smoothness of the constructed distribution. The combination of the high-Tc SQUID magnetometer and the PSO-based reconstruction technique offers a powerful approach for characterizing the MNP core size distribution, and further improvements can be expected from the recent state-of-the-art optimization algorithm to optimize further the computation time and the best objective function value.

We are developing an Ultra-Low Field (ULF) Magnetic Resonance Imaging (MRI) system with a tuned high-Tc (HTS)-rf-SQUID for food inspection. We previously reported that a small hole in a piece of cucumber can be detected. The acquired image was based on filtered back-projection reconstruction using a polarizing permanent magnet. However the resolution of the image was insufficient for food inspection and took a long time to process. The purpose of this study is to improve image quality and shorten processing time. We constructed a specially designed cryostat, which consists of a liquid nitrogen tank for cooling an electromagnetic polarizing coil (135mT) at 77K and a room temperature bore. A Cu pickup coil was installed at the room temperature bore and detected an NMR signal from a sample. The signal was then transferred to an HTS SQUID via an input coil. Following a proper MRI sequence, spatial frequency data at 64×32 points in k-space were obtained. Then, a 2D-FFT (Fast Fourier Transformation) method was applied to reconstruct the 2D-MR images. As a result, we successfully obtained a clear water image of the characters “TUT”, which contains a narrowest width of 0.5mm. The imaging time was also shortened by a factor of 10 when compared to the previous system.

Thermally activated magnetic-flux entry into a pickup coil through a flux dam in high Tc superconducting quantum interference device (SQUID) is studied. The behavior of this thermal activation is analyzed in terms of the circulating current flowing in the pickup coil. It is shown that the thermal activation can be prevented when the circulating current becomes much below a critical current of the flux dam. It is also shown that we need a long waiting time in order to realize this situation since the circulating current logarithmically decays with time in the case of the thermal activation. The relationship between the thermal activation and the circulating current is qualitatively confirmed with the experiment. We also show a method in order to forcibly reduce the circulating current instead of the thermal activation. In this case, we can prevent the thermal activation without the long waiting time.

High-Tc SQUID was applied to the detection of magnetized fine particle moving at high speed. Two types of SQUIDs were used. One was a large washer type and the other was a flux transformer type. Their Josephson junctions were step edge type. The iron particle was attached on a nylon wire and scanned under the SQUID. High-Tc SQUID detected an iron particle of 50 µm diameter running at 800 m/min. It was shown that the magnetic field measured by the SQUID was proportional to the volumer of the particle and is inversely proportional to the distance between the SQUID and the particle. This technique using high-Tc SQUID is hopeful not only to wire production line but also for the processing of food and medicine, etc.