Scanning SQUID Research Articles

STM-SQUID 顕微鏡

A SQUID microscope is one of the magnetic imaging methods to achieve both the good spatial resolution and quantitativity for magnetic imaging. Since the conventional SQUID microscopes adopt the low Tc superconducting SQUIDs, it is difficult to measure the samples under room temperature. We have developed a SQUID microscope with a scanning tunneling microscope (STM-SQUID) using a probe with a sharp tip made of permalloy and a high Tc superconducting SQUID. The submicron magnetic domains of ferromagnetic thin films were clearly observed under room temperature and the air condition. The STM-SQUID has also achieved the imaging of surface morphology with nanometer scale and magnetic field with submicron scale simultaneously.

Nano-superconducting quantum interference devices with continuous read out at milliKelvin temperatures

We describe aluminum-niobium-tungsten trilayer Nano-Superconducting Quantum Interference Devices (NanoSQUIDs) that can be read out continuously down, in temperature, to at least 230 mK. They show voltage oscillations up to at least 20 mT in field. A voltage modulation of 500 μV, voltage sensitivity of 2 mV/Φ0, and white noise floor better than 5×10−5 Φ0/Hz have been obtained. Flux noise places them between conventional low impedance SQUIDs and standard nanoSQUIDs. High sensitivity and ease of implementation make this new kind of nanoSQUID attractive for magnetic detection schemes on the nanoscale and low temperature scanning SQUID microscopy.

Ferromagnetic exchange, spin–orbit coupling and spiral magnetism at the LaAlO3/SrTiO3 interface

The electronic properties of the polar interface between insulating oxides is a subject of great current interest. An exciting new development is the observation of robust magnetism at the interface of two non-magnetic materials LaAlO_3 (LAO) and SrTiO_3 (STO). Here we present a microscopic theory for the formation and interaction of local moments, which depends on essential features of the LAO/STO interface. We show that correlation-induced moments arise due to interfacial splitting of orbital degeneracy. We find that gate-tunable Rashba spin-orbit coupling at the interface influences the exchange interaction mediated by conduction electrons. We predict that the zero-field ground state is a long-wavelength spiral and show that its evolution in an external field accounts semi-quantitatively for torque magnetometry data. Our theory describes qualitative aspects of the scanning SQUID measurements and makes several testable predictions for future experiments.

Analysis on Magnetic Images of Solar Cell by Laser-SQUID Microscope

We have developed a laser-superconducting quantum interference device (laser-SQUID) microscope based on a scanning SQUID probe microscope with a laser excitation system. This microscope can image the local magnetic field distributions detected by the SQUID above a semiconducting sample, which is caused by the photocurrent from the laser excitation spot. In this study, we analyzed the laser-SQUID microscope images of a polycrystalline silicon solar cell equipped with comb-shaped top electrodes. Moreover, these images were obtained by using not only a sample-scanning mode but also a probe-scanning mode. In the sample-scanning mode, the images correspond to the changes of the magnetic field intensity against the positional changes of the laser spot over the sample. On the other hand, in the probe-scanning mode, the images correspond to the magnetic field distribution around a laser spot excited at a fixed position on the sample. Those results indicated that the photocurrent mainly reached the electrode neighbor to the laser spot, and diffused from the electrode to all the other electrodes. Due to the current flow along the electrodes, the image taken by the sample-scanning mode showed the gradually inclined background magnetic signal.

Imaging the Distribution of Magnetic Nanoparticles on Animal Bodies Using Scanning SQUID Biosusceptometry Attached With a Video Camera

To image magnetic nanoparticles (MNPs) on animal bodies, physicians often use magnetic resonance imaging to determine the superparamagnetic characteristics of MNPs during preoperative analysis. However, magnetic resonance imaging is unsuitable for other biomedical applications, such as the curative surgical resection of tumors or pharmacokinetic studies of MNPs, because of the requirement of nonmetal environments and high financial cost of frequent examination, respectively. Thus, researchers have proposed other nonmagnetic imaging technologies, such as fluorescence, using multimodal MNPs with nonmagnetic indicators. The development of a magnetic instrument based on the other magnetic characteristics of MNPs avoids the disadvantages of multimodal MNPs, including the biosafety risk. On the basis of the alternating current susceptibility of MNPs, previous research has demonstrated the magnetic examination of scanning superconducting-quantum-interference-device biosusceptometry (SSB). This study, using a low-noise charge-coupled-device type of a video camera, reports the integration of SSB and charge-coupled-device to immediately image the magnetic signals on animal bodies or organic tissue. This real-time imaging by SSB increases the usefulness of MNPs for more clinical applications, including the imaging-guided curative surgical resection of tumors.

Vortex states in de facto mesoscopic Mo80Ge20 pentagon plates

We present direct observations of vortex configurations in Mo80Ge20 pentagonal plates using a scanning SQUID microscope. The vortex distribution tends to adapt to one of five symmetric axes in a pentagon, as expected from the sample geometry. We found that, for a vorticity of six, the vortices form a two-shell structure in a small pentagon, and a single-shell structure in a larger pentagon. We revealed how the vortex configuration is strongly modified by the presence of a pinning site at the center of the pentagon. In particular, a two-shell structure is formed in the pinning-site sample with a vorticity of L = 4, while the second shell appeared in the pinning site-free sample with a vorticity up to L = 6. Furthermore, upon geometric confinement, the vortices attempt to form a symmetrical configuration reflecting the sample geometry even in the presence of a pinning site (or sites). The experimental results for pentagons are in good agreement with our theoretical predictions based on the nonlinear Ginzburg–Landau equation both for a pin-free pentagon and a pentagon with a pin at the center.

Vortex imaging in amorphous Mo80Ge20 pentagons

We present the direct observation of vortex configuration in Mo80Ge20 pentagons using the scanning SQUID microscope. Systematic measurements allow us to reveal how vortices are confined in a restricted sample as well as how vortex arrangement evolves with increasing the applied magnetic field. Vortex distribution in pentagon tends to take one of five symmetric axes of pentagon expected from sample geometry. We found that the rule of shell filling is different among various different sizes of the pentagons. For vorticity of six, vortices form a two-shell structure in small pentagon, while single shell structure in larger pentagon. We also demonstrate that the vortex dragging effect can be caused by the superconducting pick-up coil in a weak pinning specimen.

Characterization of Superconducting Thin Films by Scanning SQUID Microscopy

Characterization of Superconducting Thin Films by Scanning SQUID Microscopy

Nano-sized SQUID-on-tip for scanning probe microscopy

We present a SQUID of novel design, which is fabricated on the tip of a pulled quartz tube in a simple 3-step evaporation process without need for any additional processing, patterning, or lithography. The resulting devices have SQUID loops with typical diameters in the range 75–300 nm. They operate in magnetic fields up to 0.6 T and have flux sensitivity of 1.8 μΦ0/Hz1/2 and magnetic field sensitivity of 10−7 T/Hz1/2, which corresponds to a spin sensitivity of 65 μB/Hz1/2 for aluminum SQUIDs. The shape of the tip and the small area of the SQUID loop, together with its high sensitivity, make our device an excellent tool for scanning SQUID microscopy: With the SQUID-on-tip glued to a tine of a quartz tuning fork, we have succeeded in obtaining magnetic images of a patterned niobium film and of vortices in a superconducting film in a magnetic field.

A Noninvasive Method to Determine the Fate of Fe3O4 Nanoparticles following Intravenous Injection Using Scanning SQUID Biosusceptometry

Magnetic nanoparticles (MNPs) of Fe3O4 have been widely applied in many medical fields, but few studies have clearly shown the outcome of particles following intravenous injection. We performed a magnetic examination using scanning SQUID biosusceptometry (SSB). Based on the results of SSB analysis and those of established in vitro nonmagnetic bioassays, this study proposes a model of MNP metabolism consisting of an acute metabolic phase with an 8 h duration that is followed by a chronic metabolic phase that continues for 28 d following MNP injection. The major features included the delivery of the MNPs to the heart and other organs, the biodegradation of the MNPs in organs rich with macrophages, the excretion of iron metabolites in the urine, and the recovery of the iron load from the liver and the spleen. Increases in serum iron levels following MNP injection were accompanied by increases in the level of transferrin in the serum and the number of circulating red blood cells. Correlations between the in vivo and in vitro test results indicate the feasibility of using SSB examination for the measurement of MNP concentrations, implying future clinical applications of SSB for monitoring the hematological effects of MNP injection.

Agreement between local and global measurements of the London penetration depth

Recent measurements of the superconducting penetration depth in Ba(Fe1−xCox)2As2 appeared to disagree on the magnitude and curvature of Δλab(T), even near optimal doping. These measurements were carried out on different samples grown by different groups. To understand the discrepancy, we use scanning SQUID susceptometry and a tunnel diode resonator to measure the penetration depth in a single sample. The penetration depth observed by the two techniques is identical with no adjustments. We conclude that any discrepancies arise from differences between samples, either in growth or crystal preparation.

Reconstructing 3D profiles of flux distribution in array of unshunted Josephson junctions from 2D scanning SQUID microscope images

By using a specially designed algorithm (based on utilizing the so-called Hierarchical Data Format), we report on successful reconstruction of 3D profiles of local flux distribution within artificially prepared arrays of unshunted NbAlOxNb Josephson junctions from 2D surface images obtained via the scanning SQUID microscope. The analysis of the obtained results suggest that for large sweep areas, the local flux distribution significantly deviates from the conventional picture and exhibits a more complicated avalanche-type behavior with a prominent dendritic structure.

Theory for measurements of penetration depth in magnetic superconductors by magnetic force microscopy and scanning SQUID microscopy

We investigate the magnetic field distribution near the surface of a magnetic superconductor when a magnetic source is placed close to the superconductor. The magnetic field distribution can be measured by magnetic force microscopy and scanning SQUID microscopy, from which one can extract both the penetration depth $\lambda_L$ and magnetic susceptibility $\chi$. When the magnetic moments are parallel to the surface, one extracts $\lambda_L/\sqrt{1-4\pi \chi}$. When the moments are perpendicular to the surface, one obtains $\lambda_L$. By changing the orientation of the crystal, one thus is able to extract both $\chi$ and $\lambda_L$.

Scanning SQUID susceptometry of a paramagnetic superconductor

Scanning SQUID susceptometry images the local magnetization and susceptibility of a sample. By accurately modeling the SQUID signal we can determine physical properties such as the penetration depth and permeability of superconducting samples. We calculate the scanning SQUID susceptometry signal for a superconducting slab of arbitrary thickness with isotropic London penetration depth $\ensuremath{\lambda}$ on a nonsuperconducting substrate, where both slab and substrate can have a paramagnetic response that is linear in the applied field. We derive analytical approximations to our general expression in a number of limits. Using our results, we fit experimental susceptibility data as a function of the sample-sensor spacing for three samples: (1) $\ensuremath{\delta}$-doped SrTiO${}_{3}$, which has a predominantly diamagnetic response, (2) a thin film of LaNiO${}_{3}$, which has a predominantly paramagnetic response, and (3) the two-dimensional electron layer at a SrTiO${}_{3}$/LaAlO${}_{3}$ interface, which exhibits both types of response. These formulas will allow the determination of the concentrations of paramagnetic spins and superconducting carriers from fits to scanning SQUID susceptibility measurements.

Complete tailor-made inverse filter for image processing of scanning SQUID microscope

By introducing a numerical image processing technique, the resolution of scanning SQUID microscope (SSM) has been improved beyond the “naive” limit determined by the size of the pickup (sensor) coil. Our image processing is developed by taking account of the specific characteristics of SSM apparatus, including detailed shape of the coil and its perfect diamagnetism, in a tailor-made manner. The actual experiment has been done for nano-scale superconducting Pb network, and the magnetic field structures apparently smaller than the size of the pickup coil were made visible by our method.

The Magnetic Imaging of Oil Paintings

We propose a new technique for authentication of oil paintings, using a scanning SQUID or fluxgate technique to measure its magnetic field. The paintings are pre-magnetized in an homogeneous field of 100 G. It was observed that the response depends on the ferromagnetic properties of each paint independent of its colour. This shows that a magnetic image could be used as a magnetic signature for authentication purposes.

Direct observation of vortices by scanning SQUID microscope on small superconducting Mo80Ge20 circular disks

The amorphous superconducting films, which tend to have weakened pinning centers, are remarkably useful as model substance in studying nanostructured superconductors. The amorphous Mo80Ge20 films have been deposited by a sputtering apparatus using a Mo80Ge20 target and the small Mo80Ge20 plates were fabricated with the aid of photolithography. The vortex distribution in a Mo80Ge20 disk has been investigated by means of a scanning SQUID microscope. We found that vortices form different configurations as well as shell structures, which evolves with the increase in applied magnetic fields. The observed results are in good agreement with theoretical studies predicted for vortices in mesoscopic circular disks. We also applied a novel numerical method to improve the spatial resolution of the scanning SQUID microscope.

High–Resolution Magnetic Field Measurement Using an STM–SQUID

Abstract We have developed an STM–SQUID microscope that combined a scanning tunneling microscope (STM) and a SQUID probe microscope for the purpose of high–resolution magnetic field measurement. We were able to obtain a spatial resolution of 1 μm or less by polishing the tip of the needle on the SQUID probe microscope to 100 nm or less, and by using a magnetic probe, which was the flux guide, as the STM probe. In this study, we took magnetic images of typical ferromagnetic materials, such as bulk iron and bulk nickel using the STM–SQUID microscope. We successfully observed the magnetic domain structures of these magnetic materials. However, topographic artifacts were also observed in the STM–SQUID images due to the background magnetic field from the sample. The future subject for resolution and quality improvement became clear.

Laser SQUID Microscope for the Evaluation of Solar Cell

A laser SQUID microscope is one of the semiconductor inspection tools. The main feature of this microscope is noncontact detection of the defects. In the previous work, we showed that it could clearly reveal the electrically inactive grain boundaries [1]. In this work, for the further understanding of the imaging mechanism, we investigated a piece of a polycrystalline silicon solar cell as a model sample, which was cut without including the top electrodes. In the magnetic images taken by the laser SQUID microscope, the contrast change was recognized at the grain boundaries similarly to the previous results. The detected distributions of the magnetic field over the whole sample were drastically changed depending on the relative position between the needle probe and the laser spot. We also scanned the needle over the sample with the laser spot fixed on a certain sample position by optical fiber in order to estimate the photocurrent path from the laser spot. These magnetic images imply that the photocurrent induced by laser irradiation did not flow simply. We found that the laser SQUID microscope images were affected by the complicated factors.

SQUID microscopy of magnetic field induced in solar cell by laser spot irradiation

A solar cell with surface stripe electrodes was investigated by laser–superconducting quantum interference device microscopy (laser-SQUID microscopy) using two scan methods: the standard method and our new approach. In the standard method, the sample was raster scanned while the positions of the laser irradiation spot and the SQUID were fixed. The resulting magnetic images reflected some defects related to the grain boundaries on the solar cell. Background contrast fluctuations also exist in the images. For a better understanding of these fluctuations, we developed a method to investigate the photocurrent distributions on the solar cell around the laser spot. In this method, the sample was raster scanned with the laser spot fixed to a certain position by means of an optical fiber. We converted the magnetic images of the sample to photocurrent images. The results showed that the anisotropic photocurrent mainly flowed along the electrode near the laser spot rather than in the area around the spot. Therefore, the arrangement of the surface stripe electrodes affected the magnetic images obtained by the standard method in laser-SQUID microscopy.