Scanning SQUID Research Articles

Investigation of flux trapping into moats with various moat depths by a scanning SQUID microscope

We have observed flux trapping in high- T c thin films with moats fabricated by focused Ga ion beam etching using a scanning SQUID microscope (SSM) system. Flux exclusion effect was investigated for moats or moat patterns with various moat depths on a 190-nm-thick NdBa 2Cu 3O y (NBCO) sputtered thin film and a 120-nm-thick La 0.2Y 0.9Ba 1.9Cu 3O y (La-YBCO) sputtered thin film. It was found that magnetic flux can be trapped into a moat with a moat depth smaller than the film thickness. In addition, we confirmed that the array of 136-μm-long and 4-μm-wide moats certainly suppress flux trapping inside the moat-surrounded 160 × 160 μm 2 area even when its moat depth is only 19% of the film thickness of 190 nm.

Faceting and critical current densities of [001] high-angle tilt boundaries in YBCO films

The [001] high-angle tilt boundaries in YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films deposited on SrTiO/sub 3/ (STO) bicrystal substrates have been studied by Scanning Squid Microscopy (SSM) and transmission electron microscopy (TEM). Generally critical current density across a boundary decreases with increasing misorientation. However, misorientation angle is found to be insufficient to predict the J/sub c/ behavior of a YBCO grain boundary. The two inclination angles of the adjacent grains are found to be important parameters. In two separate cases of same misorientation but different inclinations, the boundaries exhibit drastically different superconducting behaviors and this has been observed by SSM imaging of Josephson vortices. The YBCO film boundary is not an exact copy of the design boundary of the substrate bicrystal template. YBCO boundaries meander due to island formation during film growth. These microscopically wavy boundaries contain straight segments of atomistic facets. Three energetic criteria that affect faceting have been identified. The particular criterion that a facet plane cannot be too deviated from the design boundary plane (/spl phi/<28/spl deg/) explains the strong effect of the design inclination angles on J/sub c/ despite extensive faceting. The original scattered values of J/sub c/ each calculated from individual fluxes of the same design boundary are likely to be a bona fide effect corresponding to individual facet having its own J/sub c/ and /spl lambda//sub j/ values.

Multi channel high-T/sub c/ scanning SQUID microscope

We have constructed and tested a multichannel scanning SQUID microscope. An array of up to 8 high-T/sub c/ YBCO SQUIDs are mounted on a single chip at the end of a 77 K cold finger. Each SQUID loop measures 30 /spl mu/m by 60 /spl mu/m, and the SQUID's are spaced by about 200 /spl mu/m. The normal to the surface of the chip (and the SQUID loop) is aligned parallel to the main scanning direction. A vacuum space and a thin (<25 /spl mu/m) sapphire window separate the SQUID chip from the sample, which is in air at room-temperature. The microscope has been tested by imaging defects in wires and short circuits in computer chips. We discuss the advantages of the multichannel system over the single channel system, as well as some of the obstacles encountered.

Ultimate limits to magnetic imaging

An inverse transformation based on the Fast Fourier Transform (FFT) can be used to convert two-dimensional (2-D) images of magnetic field into corresponding images of the 2-D source currents that generated the field. We discuss the ultimate limits to the spatial resolution that can be obtained in such current density images when information about the sample is incorporated into the inversion process. We discuss the key parameters and compare our theory to experimental data obtained from a high-T/sub c/ scanning SQUID microscope.

YBa/sub 2/Cu/sub 3/O/sub X/ submicron Josephson junctions on bicrystal substrates

A technological process based on e-beam lithography and ion beam etching of YBa/sub 2/Cu/sub 3/O/sub X/ (YBCO) film through carbon mask is applied to fabricate a series of YBCO dc SQUIDs on SrTiO/sub 3/ 30/spl deg/-bicrystal substrates. The width of Josephson junctions used in SQUIDs ranges from 0.6 /spl mu/m to 1 /spl mu/m. The character of the temperature dependence of the junctions critical current enables one to confirm the realization of d-wave pairing mechanism as well as the presence of localized states in the grain boundary region. The diffraction pattern of submicron junctions is discussed in relation to their further application in SQUIDs for task of scanning SQUID microscopy in high magnetic fields.

HTS SQUID microscope head with permalloy flux guide

In order to improve the spatial resolution of an HTS SQUID microscope, permalloy rods with sharpened tip angles of 50/spl deg/ to 170/spl deg/ were placed in front of a SQUID as magnetic flux guides. A current carrying meander line on a printed circuit board with lines and spaces of two widths, 0.3 mm and 0.5 mm, was used as a magnetic field source. The magnetic field distribution was better resolved with the smaller tip angle. Also, a rod with a smaller tip diameter of curvature of 30 /spl mu/m could resolve the magnetic field distribution better than one with a 100 /spl mu/m diameter. It was further found that using a magnetic shield around the magnetic flux guide to suppress the inefficient magnetic field allowed the magnetic field image to be better resolved. The limits of the system's spatial resolution were examined with a 100 /spl mu/m wide meander line. Magnetized laser-printed characters of font size from 2 points to 5 points were also imaged. It was found that this method has a higher magnetic sensitivity and spatial resolution than a large SQUID system, and allows the position and distance from the sample to be adjusted.

LTS SQUID microscope with micron spatial resolution

We describe a multichannel LTS SQUID microscope with micron spatial resolution. The system achieves micron resolution by the use of small (14 /spl mu/m) detection coils and narrow gap between the coils and the object(s) being scanned. Samples are mounted inside an exchange-gas can at the lower end of a cryogenic probe. This houses all of the cryogenic portions of the microscope and is filled with helium exchange gas. A 5 mm/spl times/5 mm scanning stage is used to scan the sample, which is at cryogenic temperatures. Stepping motors on the scanning stage allow step sizes as small as 0.16 /spl mu/m. The SQUIDs are mounted on a cantilever structure with nine separate detection coils on the end of the structure. Flux noise of the SQUIDs is better than 5 /spl mu//spl Phi//sub o///spl radic/Hz. Sensitivity is better than 100 pT//spl radic/Hz with a bandwidth of dc-10 kHz. Open architecture software provides control of all critical system components, along with data acquisition and analysis. We have demonstrated spatial resolutions better than 2 /spl mu/m. We discuss the impact of the external field coils for susceptibility measurements.

Blumberget al.Reply:

We confirm that all the results of scanning SQUID, tunneling, ARPES, penetration depth and Raman experiments are consistent with a nonmonotonic d_{x^2-y^2} superconducting order parameter proposed in Phys. Rev. Lett., 88, 107002 (2002).

Development of scanning SQUID microscope for studying room temperature samples

Scanning SQUID microscope based on niobium junction DC SQUID has been developed. The SQUID is mounted inside the insulation vacuum of a cryostat, which is separated from room temperature samples by a 65 μm thick sapphire window. The spatial resolution of the microscope is about 100– 150 μm and the magnetic field sensitivity is on the order of 3– 60 pT/ Hz in magnetically unshielded environment. Various samples are being studied with the microscope.

A scanning SQUID microscope in a dilution refrigerator

We have designed and built a scanning SQUID microscope in a dilution refrigerator, capable of magnetic imaging at temperatures down to 20 mK . As sensors we use susceptometer SQUIDs with two pickup loops and on-chip field coils to allow measurement of both the magnetic susceptibility of the sample and the magnetic field at the sample surface on a mesoscopic length scale. The instrument is useful for studying superconductivity and magnetic effects in novel materials and electronic coherence effects (such as persistent currents) in mesoscopic systems.

Development of high- Tc SQUID microscope

We have developed a SQUID microscope using a high- T c SQUID sensor. The SQUID sensor has a sensitivity of 1 pT and the effective sensor area is 0.08 mm 2 . An X– Y stage moves over 30×30 mm 2 area and its resolution is 0.05 μm at most. The SQUID sensor is cooled by liquid nitrogen, and can be held at 77 K for 1 h . The distance between the sensor and a sapphire window is less than 100 μm . The sample temperature is limited to room temperature. The device is controlled by a LabVIEW-6i software and a motion controller. The performance of the SQUID microscope is examined by a tiny Ni rod.

Scanning SQUID microscopy of superconducting La 2− xSr xCuO 4 single crystals

We have measured local magnetic images of La 2− x Sr x CuO 4 (LSCO) single crystals by the scanning SQUID microscope. For the LSCO single crystals with 0.11< x<0.16, any magnetic structure above T c, as previously reported on LSCO thin film specimens, could not be detected presently. At around the optimally doped region, we found that the vortex patterns were clearly observed at 3 K though they totally diminished at temperatures higher than T c. Preliminary measurements on the “unique” underdoped LSCO single crystals, which have spatial Sr concentration gradient, revealed that the magnetic images changed at around x=0.12.

Flux quantization in a superconducting microdisk

Magnetic flux distribution of a YBa2Cu3O7−δ microdisk with 50 μm in diameter and 0.48 μm in thickness has been studied by scanning SQUID microscopy. All vortices observed in the microdisk with high-κ each carries flux quantum φ0 and are loosely distributed in magnetic fields up to 30 μT. Number of the vortices increases one by one with increasing magnetic field. Average magnetic field for each increase in the number is greater than that expected from the size of the microdisk, which is probably due to expulsion of magnetic field from the rim of the microdisk. These results show that multi-vortex states are stable in the microdisk.

Scanning SQUID microscopy on composition-spread NdSrMnO films under irradiation

Local magnetic properties of composition-spread Nd 1− x Sr x MnO 3 (NSMO; x=0.4–0.6) films were surveyed by a scanning SQUID microscope under irradiation. We have found that the spontaneous magnetization of the FM phase is significantly enhanced by the laser irradiation ( λ=532 nm ), while the phase boundary is essentially unchanged.

Magnetic domain structure of growth temperature-gradient Sm 2Mo 2O 7 thin film investigated by scanning SQUID microscopy

Recently spin frustrated pyrochlore A 2B 2O 7 has attracted much attention. In this study, we have observed the domain structure of pyrochlore molybdate Sm 2Mo 2O 7 epitaxial thin film fabricated with growth temperature-gradient method by a scanning SQUID microscope. The temperature dependence of domain structure has been investigated in detail.

High-Tc rf SQUID for magnetic microscopy

The spectral density of magnetic flux noise SΦ1/2(f ) is investigated for high-Tc rf SQUIDs with a pumping frequency of 390–457 MHz, placed inside three-layer Permalloy and superconducting shields. A superconducting interferometer with inner dimensions of the pickup loop of 100×100 μm is prepared by thin-film technology with a YBaCuO–PrBaCuO–YBaCuO Josephson junction of the ramp-edge type. It is shown that with the use of a cooled preamplifier, the energy sensitivity of SQUIDs in the “white” noise region (at frequencies above 1 kHz) is 4×10−30 J/Hz and is mainly determined by the intrinsic noise of the high-Tc superconducting interferometer and the shields. At low frequencies the predominant noise is from external fields penetrating directly into the shields. The addition of a ferromagnetic antenna to the SQUID microscope increases the intrinsic noise of the magnetometer to 8×10−30 J/Hz at frequencies above 1 kHz.

磁気顕微鏡による磁束観測

Recently, vortices confined into micro-scale superconductors with shapes like a disk, triangle, square, etc., have attracted much attention because of the quantum phase transition of the self-organized vortex arrangement occurring within such geometrical constraints. Such a transition can be observed using a scanning SQUID microscope with high spatial resolution. We have successfully improved spatial resolution by incorporating a microfabrication technique that reduces both the size of the pick-up coil of the micro DC-SQUID and the standoff distance between the pick-up coil and the sample surface. Using this microscope, we have studied vortex arrangements in micro-scale superconductors made of Nb and YBa2Cu3O7−δ films with various sizes and geometrical shapes. A peculiar oscillating behavior of diamagnetic magnetization corresponding to the particular vortex state was observed.

Quantized Vortices and Diamagnetic Precursor to the Meissner State in High-Tc Superconductors

We report the magnetic imaging for underdoped and optimally-dopedLa2−xSrxCuO4 (LSCO) thin films on single substrates and nearly optimallydoped YBa2Cu3O7−x (YBCO) thin films on tricrystal substrates in the temperature range both below and above Tc using scanning SQUID microscopy. Below Tc, clear integer- and half-integer quantized vortices were observable. Above Tc, however, the inhomogeneous diamagnetic domains appeared. The local diamagnetic domains that led to the Meissner state were found in the broad temperature range for underdoped samples and in the narrow limited temperature range for optimally-doped samples. The results provide evidence that local diamagnetic domains are closely related to the pseudogap state. The continuous connection of the domain state above Tc with the state of a half-integer vortex at the tricritical point in the YBCO film below Tc also indicates that the diamagnetic domains are also closely related to the occurrence of dx2-y2-wave superconductivity.

ＳＱＵＩＤを用いたバイオ免疫診断

A SQUID system for application to the biological immunoassay process is shown. In this system, the biological binding-reaction between an antigen and its antibody is detected using a magnetic marker and a SQUID magnetometer; that is, the binding reaction is detected by measuring the magnetic field from the marker. A so-called SQUID microscope was used in order to achieve a close distance between the cooled SQUID and the room-temperature sample. Three methods have so far been developed for measurement: susceptibility, relaxation and remanence. The measurement method is chosen by the properties of the magnetic marker. It is pointed out that a marker that is optimized for the immunoassay should be developed. For this purpose, we have developed a new marker made of an Fe3O4 particle having a diameter of 25 nm. Since the new marker can keep a remanence after a field of 0.1 T is applied, we use the remanent field of the marker to detect the binding reaction. We conducted an experiment to detect an antigen called Interleukin 8 (IL8). It was shown that the present system can detect IL8 at a weight of 0.1 pg.

Research and some applications of high-Tc SQUIDS

In the work, we studied the noise characteristics of electronic gradiometers in unshielded environments. In the off-axis electronic gradiometers for biomagnetic measurements, we optimized low frequency noise and performed two-dimensional magnetic mapping of magnetic signal from human heart. The measured magnetocardiography (MCG) signal was averaged according to the simultaneously recorded electrocardiography signal to further enhance the signal-to-noise ratio. The off-axis configuration in our gradiometer system offers the flexibility for multichannel SQUID-based MCG applications. In the study of the SQUID microscope for circuit detection in unshielded environment, we fabricated SQUIDS, considered the design of cryostat, and used the lock-in technique to examine the circuit board. We also examined the magnetic field pattern from the magnetized magnetic thin film.