La 0.7 Sr 0.3 MnO 3 : A single, conductive-oxide buffer layer for the development ofYBa 2 Cu 3 O 7−δ coated conductorsT. Aytug, M. Paranthaman, B. W. Kang, S. Sathyamurthy, A. Goyal, and D. K. Christen Citation: Applied Physics Letters 79, 2205 (2001); doi: 10.1063/1.1405424 View online: http://dx.doi.org/10.1063/1.1405424 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/79/14?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Superconductivity suppression in YBa2Cu3O7-δ/La0.7Sr0.3MnO3 bilayer films J. Appl. Phys. 116, 233905 (2014); 10.1063/1.4904442 Stray field enhanced vortex dissipation in YBa2Cu3O7/La0.7Sr0.3MnO3/YBa2Cu3O7 hetero-epitaxial tri-layer Appl. Phys. Lett. 103, 012601 (2013); 10.1063/1.4813110 Robust high-temperature magnetic pinning induced by proximity in YBa2Cu3O7−δ /La0.67Sr0.33MnO3 hybrids J. Appl. Phys. 109, 073921 (2011); 10.1063/1.3573491 Effect of Y Ba 2 Cu 3 O 7 − δ film thickness on the dielectric properties of Ba 0.1 Sr 0.9 Ti O 3 in Ag ∕ Ba 0.1 Sr0.9 Ti O 3 ∕ Y Ba 2 Cu 3 O 7 − δ ∕ La Al O 3 multilayer structures J. Appl. Phys. 97, 014108 (2005); 10.1063/1.1821644 Spin-polarized quasiparticle injection devices using Au/YBa 2 Cu 3 O 7 /LaAlO 3 /Nd 0.7 Sr 0.3 MnO 3heterostructures Appl. Phys. Lett. 71, 1718 (1997); 10.1063/1.120014

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La0.7Sr0.3MnO3: A single, conductive-oxide buffer layer for the developmentof YBa 2Cu3O7Àd coated conductors

T. Aytug,a) M. Paranthaman, B. W. Kang, S. Sathyamurthy, A. Goyal, and D. K. ChristenOak Ridge National Laboratory, Oak Ridge, Tennessee 37831

~Received 24 May 2001; accepted for publication 20 July 2001!

Coated conductor applications in power technologies require stabilization of the high-temperaturesuperconducting~HTS! layers against thermal runaway.ConductiveLa0.7Sr0.3MnO3 ~LSMO! hasbeen epitaxially grown on biaxially textured Ni substrates as asinglebuffer layer. The subsequentepitaxial growth of YBa2Cu3O72d ~YBCO! coatings by pulsed laser deposition yielded self-fieldcritical current densities (Jc) of 0.53106 A/cm2 at 77 K, and provided good electrical connectivityover the entire structure~HTS1conductive-buffer1metal substrate!. Property characterizations ofYBCO/LSMO/Ni architecture revealed excellent crystallographic and morphological properties.These results have demonstrated that LSMO, used as asingle, conductivebuffer layer, may offerpotential for use in fully stabilized YBCO coated conductors. ©2001 American Institute ofPhysics. @DOI: 10.1063/1.1405424#

The recent development of second-generation wire tech-nology utilizing rolling-assisted biaxially textured substrates~RABiTS! has emerged as a promising approach by which tofabricate high temperature superconducting~HTS! wires/tapes for high-current, high-field applications operating atliquid nitrogen temperatures.1,2 The RABiTS utilizes the bi-axial texture of the metal substrate as a template to depositepitaxial buffer layers that provide a barrier to chemical in-teractions with Ni, and yield chemically and structurallycompatible surfaces for the epitaxial growth of HTS films.The resulting biaxially textured HTS coating provides supe-rior critical current density (Jc) performance, since the per-vasive ‘‘weak-link’’ problem associated with high-anglegrain boundaries is ameliorated.3,4

Potential applications of HTS coated conductors involvethe efficient production, distribution, and storage of electricalpower. For these, it would be desirable to use conductive-oxide buffer layers to electrically couple the thin HTS layerto the thick underlying metal substrate, thereby providingstability against thermal runaway in the event of an overcur-rent situation. To date, however, development has focused onbuffer layers of insulating oxides and multilayers,2,5–7 andhigh-quality YBa2Cu3O72d ~YBCO! films have been grownreproducibly on RABiTS standard architecture ofCeO2 /YSZ/CeO2 /Ni.2,5,6,8,9In contrast, the development ofconductive-oxide buffer layers for coated conductors has justbegun.10–12 Recently we demonstrated the epitaxial growthof electrically well-coupled YBCO films on a conductiveSrRuO3 /LaNiO3 bilayer structure that was deposited onsingle crystal oxides@either SrTiO3 ~STO! or LaAlO3

~LAO!# and on biaxially textured Ni substrates.11,12

A major challenge for the fabrication of these multilay-ered buffer architectures is to maintain the structural andchemical integrity of all individual sublayers. Generally,deposition of a complete buffer-layer sequence requires acombination of vacuum deposition techniques.2,5–9 This re-

quirement increases the overall process complexity and cost,requiring multiple intermediate thermal cyclings and expo-sure of samples to ambient environment during transfer fromone deposition system to another. Ideally, development of asingle, conductivebuffer layer, which can be fabricated by aneasily scaleable deposition technique, is highly desirable.

In recent years, the study of colossal magnetoresistance~CMR! in perovskite-structured, doped lanthanum manga-nese oxides has generated great interest in fabricating thesematerials as thin film heterostructures for various technologi-cal applications.13–16 The variant La0.7Sr0.3MnO3 ~LSMO!,apart from its CMR properties, is also an electrically conduc-tive oxide with good thermal stability.17 Moreover, thepseudocubic lattice parameter of 3.9 Å is a close match toYBCO and other HTS materials. Therefore, it is of interest toinvestigate the viability of LSMO as a single, conductivebuffer interface on RABiTS for YBCO-coated conductors.Here, we report the fabrication of high-quality YBCO coat-ings on a single, conductive buffer layer deposited on biaxi-

a!Electronic mail: aytugt@ornl.gov

FIG. 1. Typical XRDu–2u patterns~a! for a 600 nm thick LSMO film on abiaxially textured Ni substrates and~b! for a 200 nm thick YBCO film on aLSMO/Ni multilayer.

APPLIED PHYSICS LETTERS VOLUME 79, NUMBER 14 1 OCTOBER 2001

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ally textured, high-purity Ni tapes. The structural, electrical,and superconducting properties of these short prototype con-ductors are reported.

Biaxially textured Ni substrates were obtained from ran-domly oriented high purity~99.99%! Ni bars, which are firstmechanically deformed by cold rolling and then annealed invacuum at 1000 °C for 1 h to obtain the desired~100! cubetexture. The deposition of LSMO buffer layers was per-formed with a rf-magnetron sputtering system using the ox-ide target. Typical sputtering conditions for LSMO consistedof a sputter-gas mixture of Ar and O2 at a ratio of Ar/O2

515/2, with a total pressure of 3 mTorr, and a substratetemperature in the range of 550–600 °C. A deposition rate of'0.7 Å/s resulted, yielding film thicknesses of 400–700 nm.

Pulsed laser deposition~PLD! was used for the deposi-tion of YBCO films utilizing a XeCl excimer laser system,operated with an energy density of'4 J/cm2, at a repetitionrate of 10 Hz. During YBCO deposition, substrates weremaintained at 790 °C in 120 mTorr of O2. After deposition,the samples were first cooled to 600 °C at a rate of 7 °C/min;then the O2 pressure was increased to 550 Torr, and thesamples were cooled to room temperature at the same rate.Typical YBCO film thicknesses were 200 nm. Crystal struc-tures were characterized with a Huber high-resolution x-raydiffractometer ~XRD!, and microstructural analyses wereconducted on a JEOL model JSM-840 scanning electron mi-croscope~SEM!. Secondary ion mass spectrometry~SIMS!depth-profile analyses were made to document possible cat-ion contamination of YBCO. Electrical property character-izations were made using a standard four-probe technique,with values ofJc assigned at a 1mV/cm criterion.

The XRD u–2u spectrum for a LSMO buffer layer de-posited on biaxially textured Ni is plotted in Fig. 1~a!, whileFig. 1~b! plots the pattern for a YBCO film on a LSMO/Nisubstrate. In both cases, predominantly (00l ) reflections areindexed, indicating excellentc-axis texture. After YBCOdeposition, small impurity peaks at 37° and 44°, belonging toNiO, are detected, the consequence of which to the electricalproperties of the samples will be discussed later. The relativein-plane ~f scan! and out-of-plane~v-rocking curve! tex-tures of all three layers are documented in Figs. 2~a! and

2~b!, respectively. Thef-scan full width at half maximum~FHWM! values~Df! of 9°–10° for Ni~111!, LSMO~111!,and YBCO~113! imply that both oxide layers closely repli-cate the in-plane texture of Ni. The layers assume in-planeepitaxial orientations, YBCO@110#//LSMO@110#//Ni@110#.The c-axis alignment was determined byv-rocking curvemeasurements as shown in Fig. 2~b!. The data reveal goodout-of-plane crystallographic texture among all layers. TheFWHM values for YBCO~005!, LSMO~002!, and Ni~002!are 6.7°, 7.2°, and 7°, respectively.

The surface morphology of LSMO buffers is smooth,uniform, crack free, and has dense coverage, including Nigrain boundary regions, as illustrated in Fig. 3~a!. The sur-face microstructure of the same sample after YBCO deposi-tion is shown in Fig. 3~b!, where a dense and uniform mi-crostructure is also observed. These results demonstrate thatLSMO buffer layers can be grown epitaxially on biaxiallytextured Ni substrates, and can provide a morphologicallyexcellent template for the growth of YBCO films.

Electrical transport and superconducting property char-acterizations were conducted on the same sample and theresults are shown in Fig. 4, which displays the magnetic fielddependence ofJc at 77 K, with the field applied parallel tothec axis. At zero applied field, the sample exhibits a highJc

of 0.53106 A/cm2. This value is approximately one half thatobtained on RABiTS having both the ‘‘standard trilayer in-sulating buffer architecture, ‘‘CeO2 /YSZ/CeO2 /Ni, and ‘‘bi-

FIG. 2. XRD ~a! f scans and~b! v-rocking curves for the YBCO/LSMO/Nistructure showing that the oxide layers tend to replicate the in-plane andout-of-plane textures of the Ni substrate.

FIG. 3. SEM micrographs of~a! the grain boundary region of LSMO/Nistructure and~b! the surface morphology of the YBCO film grown on thesame conductive buffer structure shown in~a!.

2206 Appl. Phys. Lett., Vol. 79, No. 14, 1 October 2001 Aytug et al.

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layer conductive architecture,’’ SrRuO3 /LaNiO3. The SIMSdepth profile analyses of YBCO/LSMO/Ni samples have in-dicated a low level Sr contamination of YBCO, from theLSMO layer. We speculate that the observed lowerJc behav-ior results from the combination of a somewhat broader dis-tribution of grain-to-grain misorientations~resulting from thetexture of the metal tape! and the observed contamination ofYBCO with Sr, although the latter has not produced amarked depression ofTc . Efforts are underway to confirmthis issue. Nevertheless, these results establish a benchmarkfor performance that it was attained on a RABiTS structurehaving a single, conductive buffer layer. The inset of Fig. 4displays the temperature dependent, overall resistivity andelectrical coupling among the layers. Note that the net resis-tivity ( rnet) is calculated from the thickness of the conduc-tive structure ~YBCO1LSMO1Ni substrate!. For directcomparison, data for LSMO/Ni, as well as data for bare,biaxially textured Ni substrate, are included. The supercon-ducting transition temperature (Tc) of YBCO/LSMO/Ni is89 K. Excellent electrical contact at the LSMO–Ni interfaceis evident by the similarrnet–T behavior before and afterdeposition of the LSMO layer. After YBCO deposition, how-ever, rnet of the entire structure has increased beyond thatexpected from ideally coupled, parallel layers, given byrnet

5dtot@dM /rM1dB /rB1dS/rS#21. Here, thedi and r i val-ues are thicknesses and resistivities of the various layers~i5M, B, and S for the metal tape, buffer layer, and supercon-ducting layer, respectively!. This increase can be ascribed tothe presence of some~111! and ~002! textured NiO at thesubstrate–buffer interface, which was detected in the XRDspectra of Fig. 1~b!. Nevertheless, the net resistivity value ofonly 2–3mV cm near liquid nitrogen operating temperaturesshould provide significant stabilization. For example, a tran-sient event that forced complete current sharing from aYBCO coating carrying 100A /cm width would dissipate aheat flux of only;4 W/cm2, which is well below the critical

heat flux of ;10 W/cm2 for nucleate boiling of liquidnitrogen.18

In summary, we have demonstrated fabrication of high-quality epitaxial YBCO coatings on a single, conductivebuffer layer of LSMO deposited on textured Ni substrates.The crystalline structure and the surface quality of this con-ductive architecture are excellent. For 200 nm thick YBCO,self-fieldJc values of 0.53106 A/cm2 were achieved at 77 K.Most significantly, electrical property characterizations re-vealed that there is good metallic connectivity between theYBCO coatings and the Ni substrate. The results in this workunderscore the great potential and additional feasibility ofLSMO buffer layers for the development of fully conductiveRABiTS-based YBCO coated conductors.

This work was supported by the U.S. DOE Office ofEnergy Efficiency and Renewable Energy at the Oak RidgeNational Laboratory, managed by U.T.-Battelle, LLC underContract No. DE-AC05-00OR22725.

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FIG. 4. Magnetic field dependence ofJc for YBCO/LSMO/Ni at 77 K. Theinset shows the temperature dependence of the net resistivity of the YBCO/LSMO/Ni structure. Also, shown for comparison are thernet–T data foras-grown LSMO/Ni and for a pure biaxially textured Ni substrate.

2207Appl. Phys. Lett., Vol. 79, No. 14, 1 October 2001 Aytug et al.

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