Critical current in PLD-YBCO coated conductors investigated byhigh-resolution Hall scan measurements

Mayraluna Lao 1 * , Johannes Hecher 1 , Patrick Pahlke 2 , Max Sieger 2 ,enhuHnebuR 2 , Michael Eisterer1

1Atominstitut, TU Wien, Stadionallee 2, 1020, Vienna, Austria2 Institute for Metallic Materials, IFW Dresden, PO Box 270116, D-01171 Dresden, Germany

*E-mail: mlao@ati.ac.at

The limitation of the global critical current density resulting from low angle grain boundaries (LAGBs)in magnetic fields and low temperatures is still one of the open issues for high-temperature supercon-ductor wires and tapes. The growing demand on generating large magnetic fields and the suitabilityof (RE)Ba 2Cu 3O7 (RE = rare earth element) coated conductors (CC) call for the optimization of thecurrent-carrying capacity of these materials at low temperatures. A lot of research has been devoted onthe vortex pinning landscape that enhances the critical current density, Jc , and minimizes its anisotropy[1, 2]. However, grain boundaries with low misorientation angles (i. e. < 10◦ ) are always present andeither contribute to pinning [3] or limit current flow and decrease J c [4]. The main goal of this work isto determine at which magnetic fields and temperatures GBs limit Jc of CCs.

We use scanning Hall probe microscopy (SHPM) as a straightforward technique that allows the directimaging of the magnetic field profiles of superconducting films and tapes. The measured field profilesprovide information on the homogeneity of the superconducting layer. The spatial variation of J c canbe calculated from the field profile by implementing an algorithm that inverts the Biot-Savart law [5].

In this work, YBa 2Cu 3O7− δ (YBCO) coated conductors grown by pulsed laser deposition (PLD)technique on a rolling assisted biaxially textured (RABiT) NiW substrates were investigated. The Ni-5at%W has chemically deposited La 2Zr2O7 and CeO2 bu�er layers while PLD-Y2O3/YSZ/CeO2 wasused for the Ni-9%W tape. The SHPM is equipped with an 8 T superconducting magnet in a He gasflow cryostat where the temperature can be stabilized between 3 K and 300 K. The spatial resolutionof the SHPM in xyz -directions is about 1 m stepwidth with a similar distance of the Hall probe to thesample sur face. Another scanning Hall probe device was used to measure remanent field profiles in aliquid nitrogen (LN 2 ) bath with a lower resolution of ≥ 50 m where the distance between the Hall probeand the sample is ≈ 15 m.

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Figure 1: Remanent field profiles of PLD-YBCO on (a) ABAD-SS substrate and (b) Ni-5at%W substratein LN2 (stepwidth of 200 µm). The arrows illustrate the current flow obtained by inversion of the fieldprofile. The insets show SEM images of the sample surfaces.

Figure 1 shows the trapped field profile of two PLD-YBCO samples in LN2. YBCO-NiW tapes areknown to have large superconducting grains in the range of 20-80µm as shown by the scanning electronmicroscope (SEM) image in the inset of Figure 1b. Their field profiles show more granularity than thePLD-YBCO tape in Figure 1a, which is based on a stainless steel substrate with a YSZ buffer layerproduced by alternating beam assisted deposition (ABAD-YSZ). This fabrication technique results inan average grain size of 0.7-1.1µm (Figure 1a). Further confirmed by a higher resolution Hall scanimage of a PLD-YBCO spot on Ni9%W (Figure 2), instead of a global current flow around the wholesuperconducting area, the current percolates locally around a grain or cluster of grains.

To determine what property of the superconducting layer dictates the local current flow, the remanentfield profile of a small PLD-YBCO spot with a diameter of ≈ 120µm was measured and compared toan electron backscatter diffraction (EBSD) map of the same spot as shown in Figure 3. The EBSD

1

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YBCO on Ni9%W - 4.2 K, remanent field

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Figure 2: (a) Remanent field profile of a YBCO spot on Ni9%W at 4.2 K. The scan has a stepwidth of10µm. Enlarged areas in the (b) central part and (c) lower right part are shown with the Jc vector mapsto illustrate the local current flow within grains and clusters of grains.

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Figure 3: (a) Remanent field profile of a ≈ 120 µm YBCO spot on Ni5%W substrate at 4.2 K and scanstepwidth of 3µm. (b) The corresponding EBSD map of the same spot showing the grain boundarymisorientation angles and single grain orientation.

map shows the grain boundary misorientation angles and the orientation of YBCO in a single grain. Bycomparing these two images, it is evident that the cluster of magnetic grains is separated by a boundarywith a misorientation angle of 5◦ and only a weak magnetic field was detected in the area with largegrain misorientation. A large angle grain boundary is denoted by a black arrow in Figure 3.

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Figure 4: Linescans at y=0.8 mm of the 2-mm YBCO spot of figure 2 at (a) remanent state at differenttemperatures, (b) with an applied magnetic field of 1 T and different temperatures and (c) at T = 4.2K and different applied magnetic fields.

Line scans were done above the YBCO spot of Figure 2. Each scan is made along the x-direction aty=0.8 mm. The granular structure of the field profiles shown in Figure 4a-c persists at temperatures upto 77 K and applied fields of up to 5 T. In the remanent state (Figure 4a), the variations at the magneticboundaries are almost the same from 64 K down to 4.2 K. With an applied field of 1 T (Figure 4b),

2

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one can see that the peaks are amplified due to the larger superconducting signal at lower temperatures.However, the boundaries do not disappear even at higher temperatures (i. e. 77 K). At 4.2 K (Figure4c), the granular texture of the profile is also found to persist up to an applied field of 5 T. These resultsclearly show that the limitation of the boundaries formed between clusters of grains or single grainsindeed limits the J c not just at lower fields but also in a wider range of field and temperature.

In summary, with the high resolution SHPM measurements on PLD-YBCO tapes with RABiT NiWsubstrate (both on magnetic Ni5%W and non-magnetic Ni9%W), the granular texture of the depositedYBCO layer was resolved in the magnetic field profile. The granularity in the field profiles was found toappear at all fields and temperatures.

References[1] K Matsumoto, P Mele, Supercond. Sci. Technol. , 23 014001, 2010.

[2] S R Foltyn, L Civale, J L MacManus-Driscoll, Q X Jia, B Maiorov, H Wang and M Maley, Nat. Mat. ,6 pp. 631-642, 2007.

[3] T Horide and K Matsumoto, Appl. Phys. Lett. , 101 112604, 2012.

[4] M Weigand, N A Rutterand J H Durrell, Supercond. Sci. Technol., 26 105012, 2013.

[5] F Hengstberger, M Eisterer, M Zehetmayer, H W Weber, Supercond. Sci. Technol. , 22 025011, 2009.

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IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), October 2015. EUCAS 2015 poster 3A-M-P-03.08. EUCAS Young Investigator Prize 2015 in Materials.

Not submitted to IEEE Trans. Appl. Supercond.