Scanning SQUID Research Articles

Vortex Trapping and Expulsion in Thin-Film Type-II Superconducting Strips

Vortex trapping is investigated in thin-film strips of superconducting material. We present a model for the critical field above which vortex trapping occurs in these strips. This model includes the pairing energy of vortex-antivortex pairs in addition to the energy of single vortices. Experimental verification of the model with a scanning SQUID microscope shows very good agreement between the model and experiments on YBa2Cu3O7-delta and Nb strips. Statistical analysis of the vortex distribution in the strips above the critical field has been performed and a comparison has been made between Nb and YBa2Cu3O7-delta for the distributions in the lateral and longitudinal directions.

High Tc SQUID System for Detection of Small Metallic Contaminant in Industrial Products

A three channel High-Tc SQUID system for detection of magnetic contaminants in industrial products was developed. Finding ultra-small metallic contaminants is a big issue for manufacturers producing commercial products such as lithium ion batteries. When the contamination does occur, the manufacturer of the product suffers a great loss to recall the tainted products. The outer dimension of metallic particles less than 100 micron can not be detected by X-ray imaging, which is commonly used as the inspection method. Therefore, a highly sensitive detection system for small contaminants is required. We developed a detection system based on a three channel high-Tc SQUID microscope with a high performance magnetic shield. Three SQUIDs were installed in one microscope-type cryostat with a 3" sapphire vacuum window which separates the SQUID and atmosphere. This design enables the SQUID to approach an object to be measured as close as 1 mm. The minimal detectable size of the particle is also highly dependent on the magnetic field noise of the SQUID. Therefore, we employed double transformer coupling, which lowers the noise. One transformer was placed at 77 K and the other was at room temperature (RT). As a result, the magnetic field noise of the SQUID was reduced by 36%. Finally we could successfully measure small iron particles as small as 30 micron. This detection level was difficult to achieve when using a conventional X-ray detection method.

Persistent Currents in Normal Metal Rings

The authors have measured the magnetic response of 33 individual cold mesoscopic gold rings, one ring at a time. The response of some sufficiently small rings has a component that is periodic in the flux through the ring and is attributed to a persistent current. Its period is close to h/e, and its sign and amplitude vary between rings. The amplitude distribution agrees well with predictions for the typical h/e current in diffusive rings. The temperature dependence of the amplitude, measured for four rings, is also consistent with theory. These results disagree with previous measurements of three individual metal rings that showed a much larger periodic response than expected. The use of a scanning SQUID microscope enabled in situ measurements of the sensor background. A paramagnetic linear susceptibility and a poorly understood anomaly around a zero field are attributed to defect spins.

Fluctuation superconductivity in uniform and nonuniform rings

Due to fluxoid quantization, the average current around a superconducting ring close to its critical temperature is an oscillatory function of the magnetic field. For the case of uniform one-dimensional rings, there is an exact theory that enables us to evaluate this current. Using a scanning SQUID device, it has been possible to isolate magnetic signals that are seven orders of magnitude smaller than the applied field. This technique was applied to several samples, intended to be uniform. A numerical simulation method which we have recently developed enables us to evaluate the average fluctuating current also in the case that the ring is not uniform. We find that the behavior of most rings is indeed in agreement with the assumption that they are uniform, but for some of the rings nonuniformity must be invoked. The difference between the behavior of a uniform and a nonuniform ring is most pronounced near half-integer number of fuxoids, and close to the temperature at which a critical point has been found.

History dependent vortex configurations in superconducting disks of amorphous MoGe

We have investigated spatial configurations of vortices in superconducting disks of amorphous MoGe as a function of field history. Employing a scanning SQUID microscope technique, we observe quasi-symmetric distributions of the vortices imposed by the disk geometry. For a field cool measurement, a single vortex state appears after the Meissner state and it is followed by multi-vortex states at higher fields. In a zero field cool (ZFC) measurement, however, different behaviors are observed. On increasing magnetic field, the Meissner state switches directly to the multi-vortex state without showing the single vortex state. The corresponding magnetization curves of the disks are strikingly history dependent. These results indicate that the ZFC procedure results in an unusual penetration of the vortices in the small superconducting disks.

Limits on the Superconducting Order Parameter in NdFeAsO1-xFy from Scanning SQUID Microscopy

Identifying the symmetry of the superconducting order parameter in the recently-discovered ferrooxypnictide family of superconductors, RFeAsO{sub 1-x}F{sub y}, where R is a rare earth, is a high priority. Many of the proposed order parameters have internal {pi} phase shifts, like the d-wave order found in the cuprates, which would result in direction-dependent phase shifts in tunneling. In dense polycrystalline samples, these phase shifts in turn would result in spontaneous orbital currents and magnetization in the superconducting state. We perform scanning SQUID microscopy on a dense polycrystalline sample of NdFeAsO{sub 0.94}F{sub 0.06} with T{sub c} = 48K and find no such spontaneous currents, ruling out many of the proposed order parameters.

823 高温超電導SQUID顕微鏡を用いた非破壊評価法の開発(GS-3 計測・検査(2))

823 高温超電導SQUID顕微鏡を用いた非破壊評価法の開発(GS-3 計測・検査(2))

Spontaneous Spin Accumulation in Singlet-Triplet Josephson Junctions

We study the Andreev bound states in a Josephson junction between a singlet and a triplet superconductors. Because of the mismatch in the spin symmetries of pairing, the energies of the spin-up and -down quasiparticles are generally different. This results in imbalance of spin populations and net spin accumulation at the junction in equilibrium. This effect can be detected using probes of local magnetic field, such as the scanning SQUID, Hall, and Kerr probes. It may help to identify potential triplet pairing in (TMTSF)2X, Sr2RuO4, and oxypnictides.

Magnetic properties of nanostructured MnZn ferrite

Mn 0.5Zn 0.5Fe 2O 4 nanoparticles (10–30 nm) have been prepared via mechanochemical processing, using a mixture of two single-phase ferrites, MnFe 2O 4 and ZnFe 2O 4. SQUID measurements (field-cooled magnetization curves and hysteresis loops) were performed to follow the mechanically induced evolution of the MnFe 2O 4/ZnFe 2O 4 mixture submitted to the high-energy milling process. The resulting single MnZn nanoferrite phase was characterized by SQUID ( M– H curve), Faraday balance ( M– T curve) and transmission electron microscopy. The magnetic characteristics of the mechanosynthesized material were compared with those of bulk Mn 0.5Zn 0.5Fe 2O 4. It was found that the saturation magnetization of nanostructured Mn 0.5Zn 0.5Fe 2O 4 (87.2 emu/g) is lower than that of the bulk Mn 0.5Zn 0.5Fe 2O 4, but, the Néel temperature of the sample (583 K) is higher than that of the bulk Mn 0.5Zn 0.5Fe 2O 4.

Improvement of spatial homogeneity in IBAD based YBCO coated conductors

Prior to the development of fabrication technique for the chemical vapor deposited (CVD) YBa 2Cu 3O 7-δ coated conductor on a IBAD-Gd 2Zr 2O 7, investigations on the improvement of spatial homogeneity have been done. By using spatially resolved measurements and combined multiple microanalysis techniques with length scale of several μm, physical and transport properties of the CVD samples have been investigated before and after fabrication modification. Structural inhomogeneity was visualized using thermoelectric voltage imaging (TVI) technique using a laser scanning microscope. Laser scanning microscopy at superconducting temperature is used to visualize flux flow dissipation; furthermore, mappings of 2D local current flow density distribution have been done using a scanning SQUID microscopy. It has been shown that the superconducting layer consisted of YBCO matrix with localized defects originating from the buffer layer. This led to current non-uniformity and caused high flux flow dissipation within the vicinity of the defects. Process conditions have been modified effectively based on those insights. After fabrication modification, our measurement analyses shows that the texturing of the YBCO layer improved significantly and the appearance of spatially distributed obstacles that are responsible for non-uniform current distribution and localized dissipation are reduced. Our complementing, quick yet non-invasive technique not only can quantify the improvement of YBCO homogeneity but also shed light on the basic understanding of the current limiting mechanism in the IBAD based coated conductors.

Shell structures of vortices in superconducting disks of an amorphous MoGe film

We have investigated spatial configurations of vortices in superconducting disks of an amorphous MoGe film. A scanning SQUID microscope allows us to image vortices magnetically in the disks. Results on the disks with 56 μm in diameter demonstrate that quasi-symmetric distributions imposed by the circle geometry appear for vorticity up to five. Meanwhile, for vorticity more than six, vortices form shell structures, in which one vortex (two vortices) is located in the center and the others form a ring around.

Dynamical reentrance and geometry imposed quantization effects inNb–AlOx–Nb Josephson junction arrays

In this paper, we report on different phenomena related to the magneticproperties of artificially prepared highly ordered (periodic) two-dimensionalJosephson junction arrays (2D-JJAs) of both shunted and unshuntedNb–AlOx–Nb tunnel junctions. By employing mutual-inductance measurements and using ahigh-sensitivity home-made bridge, we have thoroughly investigated (both experimentallyand theoretically) the temperature and magnetic field dependence of complex ACsusceptibility of 2D-JJAs.After a brief description of the measurement technique and the numerical simulationsmethod, we proceed to demonstrate that the observed dynamic reentrance (DR)phenomenon is directly linked to the value of the Stewart–McCumber parameterβC. By simultaneously varying the inductance related parameterβL, we obtain aphase diagram βC–βL (which demarcates the border between the reentrant and non-reentrantbehavior) and show that only arrays with sufficiently large value ofβC will exhibit DR behavior.The second topic of this paper is related to the step-like structure (with the number of stepsn = 4 corresponding to the number of flux quanta that can be screened by themaximum critical current of the junctions) which has been observed in thetemperature dependence of AC susceptibility in our unshunted 2D-JJA withβL (4.2 K) = 30 andattributed to the geometric properties of the array. The steps are predicted to manifest themselves in arrays withβL(T) matching a‘quantization’ condition βL(0) = 2π(n+1).In conclusion, we demonstrate the use of the scanning SQUID microscope for imaging thelocal flux distribution within our unshunted arrays.

High resolution magnetic imaging: MicroSQUID Force Microscopy

Magnetic imaging at the micrometer scale with high sensitivity is a challenge difficult to be met. Magnetic force microscopy has a very high spatial resolution but is limited in magnetic resolution. Hall probe microscopy is very powerful but sensor fabrication at the one micron scale is difficult and effects due to discreteness of charge appear in the form of significant 1/f noise. SQUID microscopy is very powerful, having high magnetic resolution, but spatial resolution is usually of the order of 10 μm.The difficulties lay mostly in an efficient way to couple flux to the sensor. The only way to improve spatial resolution is to place the probe close to the very edge of the support, thus maximising coupling and spatial resolution. If there has been found a way to bring close the tip, there must be also found a reliable a way to maintain distance during scanning.We want to present recent improvements on scanning microsquid microscopy: Namely the improved fabrication of microSQUID tips using silicon micro machining and the precise positioning of the micrometer diameter microSQUID loop by electron beam lithography. The microSQUID is a microbridge DC SQUID, with two opposite microbridges. The constrictions are patterned by high-resolution e-beam lithography and have a width of 20 nm and a length of about 100 nm.The distance control during scanning is obtained by integrating the microSQUID sensor with a piezoelectric tuning fork acting as a force sensor allowing to control height and even topographic imaging. The detector is placed in a custom built near field microscope and the sample temperature can be varied between 0.1 Kelvin and 10 K.The microscope is used to study magnetic flux structures in unconventional superconductors and will be used to observe thermal domains in superconducting detectors in the voltage state.

Scanning SQUID Microscopy on Polycrystalline SmFeAsO0.85and NdFeAsO0.94F0.06

The order parameter of the recently-discovered ferric arsenide family of superconductors remains uncertain. Some early experiments on polycrystalline samples suggested line nodes in the order parameter, however later experiments on single crystals have strongly supported fully-gapped superconductivity. An absence of nodes does not rule out unconventional order: {pi} phase shifts between the separate Fermi sheets and time-reversal symmetry-breaking components in the order parameter remain possibilities. One test for unconventional order is scanning magnetic microscopy on well-coupled polycrystalline samples: d- or p-wave order would result in orbital frustration, leading to spontaneous currents and magnetization in the superconducting state. We have performed scanning SQUID microscopy on SmFeAsO{sub 0.85} and NdFeAsO{sub 0.94}F{sub 0.06}, and in neither material do we find spontaneous orbital currents, ruling out p- or d-wave order.

Imaging spontaneous currents in superconducting arrays of π-junctions

Superconductors separated by a thin tunneling barrier exhibit the Josephson effect that allows charge transport at zero voltage, typically with no phase shift between the superconductors in the lowest energy state. Recently, Josephson junctions with ground state phase shifts of pi proposed by theory three decades ago have been demonstrated. In superconducting loops, pi-junctions cause spontaneous circulation of persistent currents in zero magnetic field, analogous to spin-1/2 systems. Here we image the spontaneous zero-field currents in superconducting networks of temperature-controlled pi-junctions with weakly ferromagnetic barriers using a scanning SQUID microscope. We find an onset of spontaneous supercurrents at the 0-pi transition temperature of the junctions Tpi = 3 K. We image the currents in non-uniformly frustrated arrays consisting of cells with even and odd numbers of pi-junctions. Such arrays are attractive model systems for studying the exotic phases of the 2D XY-model and achieving scalable adiabatic quantum computers.

Synthesis of layered hydroxides using controlled ammonia vapor diffusion

Layered hydroxides Co(OH) 1.65Br 0.35·0.5H 2O ( 1), Co(OH) 1.60(DDS) 0.36Cl 0.04·2.1H 2O ( 2), Mg 2Al(OH) 6NO 3·3H 2O ( 3) and Mg(OH) 2 ( 4) with unique morphologies have been synthesized by low temperature, controlled ammonia vapor diffusion. The materials were characterized by X-ray powder diffraction (XRD), field emission scanning microscope (FESEM) and SQUID magnetometry. Crystallites of ( 1) and ( 4) grow as thin films perpendicular to the air/water interface, while crystals of ( 2) and ( 3) share an unusual coral-like morphology. The magnetic data is consistent with a 3D ferrimagnet ground state at low temperature for both ( 1) and ( 2). The saturation magnetization ( M SAT) at 5 K is observed to be 0.88 and 0.59 μ B for ( 1) and ( 2), respectively. These values are consistent with a model containing two antiferromagnetically coupled magnetic sublattices, one comprising of octahedral cobalt atoms and one comprising tetrahedral cobalt atoms.

STM-SQUID probe microscope

We have developed a STM-SQUID probe microscope. A highTC SQUID probe microscope was combined with a scanning tunneling microscope forinvestigation of samples at room temperature in air. A high permeability probe needlewas used as a magnetic flux guide to improve the spatial resolution. The probewith tip radius of less than 100 nm was prepared by microelectropolishing. Theprobe was also used as a scanning tunneling microscope tip. Topography of thesample surface could be measured by the scanning tunneling microscope withhigh spatial resolution prior to observation by SQUID microscopy. The SQUIDprobe microscope image could be observed while keeping the distance from thesample surface to the probe tip constant. We observed a topographic image and amagnetic image of Ni fine pattern and also a magnetically recorded hard disk.Furthermore we have investigated a sample vibration method of the static magneticfield emanating from a sample with the aim of achieving a higher signal-to-noise(S/N) ratio.

VORTEX DOPING INTO FINITE-SIZED SUPERCONDUCTING NETWORKS

Superconducting finite-sized Pb square networks with 10×10 square holes fabricated by electron beam lithography have been investigated in view of vortex doping into superconducting networks. Vortex images observed by a SQUID microscope were compared with predictions from the Ginzburg-Landau theory. Exactly reversed pattern was confirmed to appear both in the vortex-doping x into an empty network and in the antivortex doping 1- x into a fully occupied network ( x =1/4). We also confirmed a checker-board crystal of vortices at half filling both theoretically and experimentally.

High-throughput screening of magnetic properties of quenched metallic-alloy thin-film composition spreads

A novel method for capturing and characterizing high-temperature phases of metallic-alloys in thin-film composition spreads has been developed. A high-vacuum (10 −8 Torr) high-temperature annealing and quenching furnace system has been developed that allows formation of high-temperature phases in thin-film composition-spread samples deposited on 3 in. silicon wafers. Scanning SQUID microscopy and magneto-optical Kerr effect measurements were used to map the remnant magnetization and the hysteresis loops of the spreads of magnetic materials quenched at high temperatures. The combination of these techniques can be used to obtain comprehensive information on the magnetic properties of various metallic-alloy systems.

A trial usage of microprobes for improvement of a scanning SQUID microscope

A scanning SQUID microscope is a powerful tool for direct observations of vortices. Magnetic sensitivity is high enough, but spatial resolution is limited by the size of pickup coils. To improve the spatial resolution, a trial usage of microprobe on the pickup coil was achieved. Obtained images are expanded about four times, and the attached microprobe acts as a magnetic lens.