Scanning Hall Probe Measurements Research Articles

Low AC Loss Inkjet-Printed Multifilamentary YBCO Coated Conductors

Impressive performance has been achieved in $(\text{RE})\text{Ba}_{2}\text{Cu}_{3}\text{O}_{7-\delta}$ (REBCO) coated conductors, but for many applications, the high cost and ac losses remain prohibitive. Inkjet printing methods combine scalability and low equipment cost with high-resolution patterning, potentially addressing both issues by enabling the production of multifilamentary coated conductors without subtractive processing. The successful production of multifilamentary superconducting $\text{YBa}_{2}\text{Cu}_{3}\text{O}_{7-\delta}$ (YBCO) structures by inkjet printing of a low-fluorine YBCO precursor solution on SS/ABAD-YSZ/CZO substrates is reported. Two approaches have been developed. In the first method, YBCO filaments were directly deposited on the buffered substrate by piezoelectric printing; and in the second approach, CeO2 tracks were first printed to pattern a subsequently overprinted YBCO film, creating a multifilamentary structure by an inverse technique. Scanning Hall probe measurements have been used to compare the filamentary structures and critical currents achieved by both methods, and a $J_{c}$ of up to 3 MA cm−2 has been obtained at 77 K. For the inverse printing approach, the ac hysteresis losses have been also measured and compared with theoretical models.

Irreversible magnetization switching at the onset of superconductivity in a superconductor ferromagnet hybrid

We demonstrate that the magnetic state of a superconducting spin valve, that is normally controlled with an external magnetic field, can also be manipulated by varying the temperature which increases the functionality and flexibility of such structures as switching elements. In this case, switching is driven by changes in the magnetostatic energy due to spontaneous Meissner screening currents forming in the superconductor below the critical temperature. Our scanning Hall probe measurements also reveal vortex-mediated pinning of the ferromagnetic domain structure due to the pinning of quantized stray fields in the adjacent superconductor. The ability to use temperature as well as magnetic field to control the local magnetisation structure raises the prospect of potential applications in magnetic memory devices.

Inkjet printing of multifilamentary YBCO for low AC loss coated conductors

Considerable progress has been made with the development of REBCO coated conductors in recent years, and high performance conductors are available commercially. For many applications, however, the cost remains prohibitive, and AC losses discourage their selection for higher frequency applications. Chemical solution deposition (CSD) methods are attractive for low-cost, scalable preparation of buffer and superconductor layers, and in many respects inkjet printing is the method of choice, permitting non-contact deposition with minimal materials wastage and excellent control of coating thickness. Highly textured coatings of YBCO and Gd-doped CeO2 have previously been reported on buffered metal substrates. Inkjet printing also introduces the possibility of patterning - directly depositing two and three dimensional structures without subtractive processing - offering a low-cost route to coated conductors with reduced AC losses. In this contribution, the inkjet deposition of superconducting YBCO tracks is reported on industrially relevant buffered metal substrates both by direct printing and an inverse patterning approach. In the latter approach, ceria tracks were printed reported, which are a candidate both for resistive filament spacers and buffer layers. TFA-based precursor solutions have been printed on SS/ABAD-YSZ/CeO2 and Ni-W/LZO/CeO2 RABiTS substrates, and the resulting multifilamentary samples characterised by microscopy and scanning Hall probe measurements. The prospects for future inkjet-printed low AC loss coated conductors are discussed, including control of interfilamentary resistivity and bridging, transposed filamentary structures and stabilisation material.

Batch Patterning of SubMillimeter Features in Hard Magnetic Films Using Pulsed Magnetic Fields and Soft Magnetizing Heads

In this paper, we report a method to impart spatially defined magnetic patterns into hard magnetic films using pulsed magnetic fields. Differing from prior investigations on magnetic patterning, a single magnetizing head is used rather than a sandwich structure with two magnetizing heads. Arrays of 250μm-wide and 125μm-wide magnetic stripes with alternating out-of-plane polarity (perpendicular magnetization) are patterned in a 10μm-thick Co-rich Co-Pt alloy (Co80Pt20) film using three different microstructured, 2-mm-thick, Fe-Co magnetizing heads and pulsed magnetic fields ranging from 0.1 to 1 T. A magneto-optical imaging film is used to view the magnetization patterns at the surface of the Co-Pt film. Scanning Hall probe measurements of the resulting magnetic structures indicate a ~5mT peak magnetic field 100 m above the surface which compares favorably with numerical simulations of the field patterns at the same height.

What are the internal field and the vortex density along the edges of a coated conductor or a superconducting bridge carrying current?

In this contribution, I show that, due to demagnetizing effects, the internal field and the vortex density along the edges of coated conductors and superconducting thin film bridges carrying current can be very large. To support this claim, using relations derived between the current and the self and internal fields, the self fields along the edges of YBa2Cu3O7−δ bridges and coated conductors are calculated as functions of the transport current. The resulting values are compared to those obtained from magneto-optical and scanning Hall probe measurements. The possible consequences on the behaviour of coated conductors in the critical state are discussed.

A simple model for the estimation of hysteresis energy loss in a coated conductor under thesimultaneous application of a magnetic field and current

A simple model for estimating the hysteresis energy loss of coated conductorsunder a general load line was studied. We took advantage of the characteristic lineIb(Ha) to determine the major parameters used in this model. The value ofIb(Ha) was based on the scanning Hall probe measurements (SHP) on aSm1Ba2Cu3O7 − δ coated conductor. During SHP measurement, a magnetic field (Ha) andcurrent (Ia) were applied simultaneously and were varied along 11 different loadlines. From the values of SHP measurements, the current density profiles,J(x, Ha, Ia), were calculated using a numerical inversion method. We define the quantityIb = ∫ − ww|J(x, Ha, Ia)| dx and we calculatedIb at many points(Ha, Ia) in every load line. Wefound that when Ia is less than Ib and the flux trap regions are absent, the values ofIb for all points(Ha, Ia) constitute asingle line Ib(Ha), which can be easily extrapolated to a high field. This line provided a major parameter forour model.

Real-space observation of magnetic domain structure at metamagnetic tansition in a triple-layer ruthenate Sr4Ru3O10

We report the real-space observation of magnetic domains in the ac-plane at the metamagnetic transition in a triple-layer ruthenate Sr4Ru3O10 by magneto-optical imaging and scanning Hall probe measurements. We find anisotropic magnetic domain structures with weak c-axis correlation at low fields. The magnetic inhomogeneities are removed above the metamagnetic transition field. These changes of magnetic structure support the phase-separation scenario for the ultra-sharp steps in magnetoresistance.

Estimation of hysteresis loss in Sm1Ba2Cu3O7−δ coated conductor via scanning Hall probe measurements with simultaneously applied fieldand current

The hysteresis loss in a Sm1Ba2Cu3O7−δ coated conductor was estimated from magnetic field profilesmeasured by the scanning Hall probe method. Current,Ia, and magneticfield, Ba, were appliedsimultaneously; Ba was applied in the normal direction with respect to the tape surface.Ba andIa werevaried from Bpeak to −Bpeak andfrom Ipeak to−Ipeak, respectively,with the ratio α = Ia/Ba fixed during the variation. Three values ofα were taken for the three load lines. The values ofBpeak/Ipeak were varied from 0 mT/0 A to 10.7 mT/116 A, 99.1 mT/50 A, and 298.2 mT/46.1 A,respectively, for the three load lines. From the measured values of magnetic fieldprofiles, the current profiles were calculated by the iterative inversion method.From the current profiles, the flux density profiles and the hysteresis loss,Q, were then calculatedfor various values of Ipeak(= αBpeak) in each load line. The results were compared with theoretical calculations based on Brandt’s model. WhenBpeak was about 300 mT,the estimated values of Q were several times smaller than the theoretical values ofQ with the self-fieldIc0. The low value ofQ in this case is due tothe field dependent Ic and the saturation effect of the current profiles, which results in significant reduction of the induced magneticmoment, M.

Calculations of AC current losses and AC magnetic losses from the scanning Hall probe measurements for a coated conductor

We measured the field profiles near the surface of a coated conductor (CC) by using the scanning Hall probe method. The external currents, represented by I a, or the external fields, represented by H a, were applied during the measurements. The field was applied in the normal direction with respect to the tape surface. The measured field profiles could be expressed by H( x, X a), where the variable, X a, was either I a or H a. The x-axis was parallel to the in-plane transverse direction of the CC tape. The CC tape was fabricated by the IBAD-PLD method. The applied field (current), H a ( I a), was decreased stepwise from H peak to − H peak (from I peak to − I peak). From H( x, X a), we calculated the current profile, J( x, X a), by the iterative inversion method. From J( x, X a) and the corresponding flux densities, B( x, X a) we calculated the AC current loss, Q C( I peak), and the AC magnetic loss, Q M( H peak), for various values of I peak and H peak, respectively. The characteristic functions were defined by f C = π Q C / μ 0 I c 2 and f M = π Q M / μ 0 I c 2 , where I c is the critical current. We found that the calculated values of f C ( f M) were similar to the experimental values of f C ( f M) which were directly measured for the IBAD-PLD CCs. On the other hand, the values of f C ( f M) were smaller than (larger than) the values estimated from the experimentally observed Q C ( Q M) in RABiTS ™-MOD CCs.

Scanning Hall probe measurements of field distributions of a coated conductor underapplied fields

We measured the field profiles near the surface of a coated conductor (CC) under variousapplied fields by using the scanning Hall probe method. The field, applied inthe normal direction, was increased from zero to 171.5 Oe and then decreased to−58.8 Oe. We could not analyse our data completely by the direct use of Brandt’s calculation butby a modification with unusual field dependences of the introduced parameters.Since Brandt’s original calculation was based on homogeneous films, it was notsuitable for CCs with coarse granular structures. The modified calculations withappropriate parameters are related to the coarse granular structures. Those parameters,D,Jc,and R, represent the three characteristics of the flux penetration network: the averagedistance of flux penetrations, the density of critical sheet currents, and the range ofmeandering of the flux penetration front, respectively. The external field dependences ofthese parameters were different from those of the classical critical state model.

Grain Boundaries in Multi-Seeded Melt-Grown Superconductors

Future applications of high temperature superconductors require bulk materials of a complex shape. The multi-seeded-melt-growth process (MSMG) represents a promising technique for obtaining qualitatively well oriented bulk materials with different kinds of shape. In the MSMG process, several seeds are placed on a precursor pellet, from which the growth of the bulk starts. A certain problem of the MSMG process is that grain boundaries become inevitable when the growth fronts of two neighboring seeds collide. These grain boundaries are responsible for a reduction of the critical currents and pose a problem for high current applications. By polishing the sample step by step, the influence of the grain boundaries was investigated by scanning Hall probe measurements and by the magnetoscan technique. Additionally, optical microscopy and electron microscopy were employed to investigate the details of the microstructure.

Magnetic flux penetration into melt-textured superconductors

Scanning Hall probe measurements were made on melt-textured Y-123 monoliths undersequentially increasing activation fields. The remnant field profiles are compared tomeasurements of the local response of the superconductor to a small magnetic field inducedby a permanent magnet (‘magnetoscan’). Optical microscopy is employed to relate the fluxpatterns with the microstructure of the sample.

Current distribution in Y/sub 1/Ba/sub 2/Cu/sub 3/O//sub 7-γ/ disk-shaped thin film by scanning hall probe measurements

We have measured the shielding and trapped (remanent) magnetic field profiles both perpendicular (H/sub z/(r)) and parallel (H/sub r/(r)) to the planes of a zero-field-cooled disk-shaped YBCO thin film. H/sub z/(r) and H/sub r/(r) were obtained using transverse and radial scanning Hall probes at various temperatures ranging from 30 K to 70 K and at small magnetic fields up to 750 G. The current distributions in the samples were inferred from a direct comparison of our experimental profiles to the theoretical ones obtained using the critical state models by Mikheenko et al. (1993) and Zhu et al. (1993). The models are in good agreement with the experimental trapped field profiles; however field dependence of the current should be taken into account for the shielding profiles.

Phase transitions in isolated vortex chains

In very anisotropic layered superconductors (e.g. Bi$_2$Sr$_2$CaCu$_2$O$_x$) a tilted magnetic field can penetrate as two co-existing lattices of vortices parallel and perpendicular to the layers. At low out-of-plane fields the perpendicular vortices form a set of isolated vortex chains, which have recently been observed in detail with scanning Hall-probe measurements. We present calculations that show a very delicate stability of this isolated-chain state. As the vortex density increases along the chain there is a first-order transition to a buckled chain, and then the chain will expel vortices in a continuous transition to a composite-chain state. At low densities there is an instability towards clustering, due to a long-range attraction between the vortices on the chain, and at very low densities it becomes energetically favorable to form a tilted chain, which may explain the sudden disappearance of vortices along the chains seen in recent experiments.

Transport currents measured in ring samples: test of superconducting weld

The critical current densities in bulk melt-textured YBa 2Cu 3O x and across superconducting “weld” joints are measured using scanning Hall probe measurements of the trapped magnetic field in ring samples. With this method, critical current densities are obtained without the use of electrical contacts. Large persistent currents are induced in ring samples at 77 K, after cooling in a 3 kG field. These currents can be determined from the magnetic field they produce. At 77 K a supercurrent exceeding 2000 A (about 10 4 A/cm 2) was induced in a 2 cm diameter ring; this current produces a magnetic field exceeding 1.5 kG in the bore of the ring. We demonstrate that when a ring is cut, and the cut is repaired by a superconducting weld, the weld joint can transmit the same high supercurrent as the bulk.

Scanning Hall probe measurements on single- and double-sided sputtered YBCO films for microwave applications

We have investigated the quality and the homogeneity of YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) films up to /spl phi/=2" diameter and t=360 nm in thickness with a scanning Hall probe. The YBCO films were grown by high oxygen pressure sputtering with heater temperature compensation up to T=1020/spl deg/C, resulting in a constant growth temperature for both film sides. Typical /spl phi/=1" double-sided films on LaAlO/sub 3/ substrates revealed inductively T/sub c/=87.8(88.2) K and J/sub c/=4(4.5) MA/cm/sup 2/ for the first (second) deposited side. Surface resistance measurements at 87 GHz resulted in R/sub s/(4.2 K)=2.6 (1.6) m/spl Omega/. At 19 GHz, R/sub s/(4.2 K)=0.2 m/spl Omega/ with moderate field dependence up to B/sub s/=15 mT was obtained for both sides. The scanning Hall probe measurements have been carried out after cooling the film in an external magnetic field and then switching it off. The local J/sub c/ values deduced from the measured remanent induction B were in good agreement with inductive data taken at corresponding positions. Different kinds of defects and inhomogeneities were investigated with a spatial resolution of 1 mm.

Magnetic flux mapping, magnetization, and current distributions of YBa2Cu3O7 thin films by scanning Hall probe measurements

Mapping of the magnetic flux distribution in a square-shaped superconducting YBa2Cu3O7 thin film was carried out using a scanning micro-Hall probe with a spatial resolution of about 25 μm. Calculation procedures for converting the measured flux map to the film current distribution are discussed. The flux penetration with the applied field perpendicular to the film surface was studied in initially zero-field-cooled samples. With full flux penetration, the film sheet currents follow the sample edges, whereas with partial flux penetration, the sheet currents are more complex and include multiply connected patterns. In both cases the sheet currents change their flow direction abruptly along the sample diagonals where the shielding is more effective, and this results in distinctive minima of the flux penetration. The saturation magnetization for a square-shaped sample of side 2a was found to be identical to that for disc and cylindrical samples of radius a. It is shown that the multiply connected current patterns corresponding to various parts of the hysteresis cycle can be reconstructed from the superposition of two appropriate virgin current distributions. The critical current density was dependent on magnetic flux density, and it was found to follow the Kim model.