Full text of DOI:10.1557/JMR.2002.0322

Single buffer layers of LaMnO3 or La0.7Sr0.3MnO3 for the
development of YBa2Cu3O7−␦-coated conductors:
A comparative study
T. Aytug,a) M. Paranthaman, H.Y. Zhai, H.M. Christen, S. Sathyamurthy, and D.K. Christen
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
R.E. Ericson
3M Company, St. Paul, Minnesota 55144
(Received 27 March 2002; accepted 19 June 2002)
Single, epitaxial buffer layers of insulating LaMnO3 (LMO) or conductive
La0.7Sr0.3MnO3 (LSMO) have been grown by sputter deposition on biaxially textured
Ni and Ni–alloy substrates. We report baseline investigations of their compatibility
with the YBa2Cu3O7−␦ (YBCO) coatings and demonstrate biaxially textured YBCO
films grown by pulsed-laser deposition on these single-buffered tapes. Superconducting
property characterizations revealed better properties for YBCO films on LMO-buffered
tapes relative to those grown on LSMO layers. Self-field critical current densities (Jc)
exceeding 1 × 106 A/cm2 at 77 K have been obtained for the YBCO (200 nm) films on
LMO-buffer layers. These results offer prospects for the use of single, LMO-buffered
metal tapes in the development of practical YBCO-coated conductors.
Recently, good superconducting performance has
been demonstrated in continuously processed, high-
temperature superconducting (HTS) coated conductors
on the basis of the approach of rolling-assisted biaxially
textured metal substrates (RABiTS).1,2 The RABiTS
technique utilizes the biaxial texture of the metal sub-
strate produced by thermomechanical processing2 as a
template for the epitaxial deposition of one or more
buffer layers. This buffer then provides a structural tem-
plate and a chemical barrier for the subsequent epitaxial
growth of a HTS coating. Presently, the RABiTS-based
coated conductors use Ni or dilute Ni–alloys as the tape
substrates. These templates impart sufficient crystalline
texture to the HTS material to ameliorate the pervasive
problem of weak-linked, high-angle grain boundaries.3,4
Potential applications of HTS-coated conductors in-
volve the efficient production, distribution, and storage
of electrical power. Ideally, for these applications
conductive-oxide buffer layers are preferred, since they
can provide electrical coupling between the thin HTS
layer and the thick underlying metal substrate, thereby
improving the electrical and thermal stability of the
structure against a transient to the dissipative regime. To
date, however, developed buffer layers have comprised
insulating oxides and multilayers.1,2,5–7 Previously, high-
quality YBa2Cu3O7−␦ (YBCO) films have been grown
reproducibly on the RABiTS standard architecture
of CeO2/YSZ/CeO2–(or Y2O3)/Ni.2,5–6 Development of
conductive-oxide buffer layers on RABiTS has begun
recently with the demonstration of an epitaxial SrRuO3/
LaNiO3 bilayer structure on Ni tapes.8 The YBCO coat-
ings deposited on the SrRuO3/LaNiO3/Ni architec-
ture exhibited good electrical connectivity over the entire
structure and yielded critical current density (Jc) values
(at 77 K, self-field) exceeding 1.2 × 106 A/cm2.
The fabrication of these multilayered buffer architec-
tures however, presents major challenges. Generally,
deposition of a complete buffer-layer sequence involves
a combination of vacuum deposition techniques1–8 and
exposure of samples to multiple intermediate thermal cy-
clings and ambient environment. These requirements ad-
versely affect the structural and chemical integrity of all
individual sublayers and increase the overall process
complexity. To overcome these difficulties, it is impor-
tant to develop a single buffer layer, ideally conductive,
that can be fabricated by an easily scalable deposition
technique. Recently, we investigated viability of the
doped rare earth manganate compound La0.7Sr0.3MnO3
(LSMO) as a buffer layer for YBCO-coated conductors,
due to its electrical conductivity, good thermal stability,
and structural compatibility with YBCO.9 In that study,
YBCO films deposited on highly textured LSMO buffer
layers on Ni tapes showed superconducting transition
temperatures (Tc ס 87–89 K) and Jc (77 K, self-field)
values ≈ 5 × 105 A/cm2. These values are somewhat
a)e-mail: aytugt@ornl.gov
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depressed compared to those obtained on either the stan-
at a rate of 5 °C/min; then the O2 pressure was increased
to 550 torr, and the samples were cooled to room tem-
perature at the same rate. Typical YBCO film thicknesses
were 200 nm. Crystal structures were characterized with
a Philips model XRG3100 x-ray diffractometer (XRD),
and microstructural analyses were conducted on a JOEL
model, JSM-840 scanning electron microscope (SEM).
SIMS depth–profile analyses were made to document
possible cation contamination of YBCO. Electrical prop-
erty characterizations were made using a standard four-
probe technique on samples with dimensions of 15 mm
in length and 5 mm in width, with values of Jc assigned
at a 1-␮V/cm criterion.
A typical XRD ␪–2␪ scan for a 300-nm-thick LMO
layer deposited on biaxially textured Ni is given in Fig. 1,
with the inset showing a logarithmic-scale (111) pole
figure of the same LMO film. The presence of only (00l)
reflections indicates that LMO is oriented with its c axis
normal to the plane of the substrate, and the pole figure
reveals a single-domain epitaxy, with the in-plane
LMO[110]//substrate[110]. The ␻-scan rocking curve on
the LMO(002) and ␾ scan on the LMO(111) have
yielded out-of-plane and in-plane peak-width full width
at half-maximum of ⌬␻ ס 9° and ⌬␾ ס 10°, respec-
tively. Both distribution widths are comparable to those
of the underlying Ni (⌬␻ ס 9.4° and ⌬␾ ס 9.8°) sub-
strate. Similar crystallographic relations were observed
for the growth behavior of LMO films on Ni–alloy tapes.
In addition to excellent crystalline quality, LMO layers
show a crack-free, homogeneous, and dense microstruc-
ture. This point is displayed in Figs. 2(a) and 2(b), which
show grain boundary regions for 300-nm-thick LMO-
buffered Ni and Ni–alloy substrates. Similar surface mor-
phologies were observed for films deposited at various
thicknesses ranging from 60 to 300 nm. Figures 2(c) and
2(d) show typical surface morphologies of two YBCO
films deposited on LMO(300 nm)/Ni and LMO(300 nm)/
Ni–alloy substrates, respectively, where dense and
dard CeO2/YSZ/CeO2–(or Y2O3)/Ni or the conduc-
tive SrRuO3/LaNiO3/Ni structure [Tc ס 90–92 K, Jc
(77 K) Ն1 × 106 A/cm2]. The reason for this slight deg-
radation was attributed to low-level Sr contamination of
YBCO from the LSMO layers, as documented by sec-
ondary ion mass spectroscopy (SIMS). In light of the
above observations, it is reasonable that the parent com-
pound LaMnO3 (LMO), if fabricated epitaxially on Ni
and/or Ni–alloy, would resolve the Sr-contamination is-
sue. While LMO is a semiconducting perovskite oxide,
not a conductive buffer layer as LSMO, its pseudocubic
lattice parameter (3.91 Å) is closely matched to that of
the orthorhombic YBCO (3.86 Å). These characteristics
point to possible viability as a single buffer layer for
YBCO-coated conductors.
Here we report for the first time the growth of YBCO
coatings on a single buffer layer with Jc (77 K) values
above 1 × 106 A/cm2, deposited both on biaxially tex-
tured Ni and on mechanically strengthened Ni–
(3 at.% W–1.7 at.% Fe) alloy (referred to as Ni–alloy). A
comparative study of the chemical compatibility and the
superconducting properties (Tc, Jc) of YBCO films
grown on single layers of both LSMO- and LMO-
buffered Ni and Ni–alloy tapes is presented.
Biaxially textured Ni (99.99%) and Ni–alloy sub-
strates were obtained by progressive cold rolling of poly-
crystalline, randomly oriented Ni and Ni–alloy bars,
followed by a texturing anneal at 1100 °C (for pure Ni)
and 1250 °C (for Ni–alloy). The 1-h heat treatment in a
reducing, forming gas (96% Ar + 4% H2) atmosphere
yielded the desired (100) cube texture. Depositions of the
LSMO and LMO buffer layers were conducted with an
rf-magnetron sputtering system. The sputter targets were
made from single-phase LSMO and LMO powders, pre-
pared by solid-state synthesis, and lightly packed into a
copper target tray. Typical sputtering conditions con-
sisted of a sputter-gas mixture of forming gas and
(2–5) × 10−5 torr of H2O at a total pressure of 3 m torr.
Substrate temperatures were in the range of 575–625 °C.
Water vapor provided oxygen for the stability of the
oxides and forming gas helped suppress the oxidation
of the metal tapes. The LMO film thickness was varied in
the range of 60 to 300 nm to investigate the effect on
microstructure and on the superconducting properties of
the subsequent HTS layer. The thickness of LSMO layers
was maintained at around 300 nm. The deposition rate
for both buffer materials using our laboratory equipment
was approximately 0.56 Å/s.
The YBCO films were grown by pulsed laser deposi-
tion (PLD), using a KrF excimer laser system operated at
an energy density of approximately 2 J/cm2 and a repeti-
tion rate of 15 Hz. During YBCO deposition, the sub-
strates were maintained at 780 °C in 120 mtorr of O2.
After deposition, the samples were first cooled to 500 °C
FIG. 1. Typical XRD ␪–2␪ pattern for a 300-nm-thick LMO buffer
layer on biaxially textured Ni substrates. The inset shows a (111)
logarithmic-scale pole figure of the same film.
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FIG. 2. SEM micrographs of 300-nm-thick LMO films deposited directly on biaxially textured (a) Ni and (b) Ni–alloy substrates and the surface
morphology of YBCO film grown on LMO-buffered (c) Ni and (d) Ni–alloy tapes.
uniform microstructures are also observed. Overall, these
results on LMO layers are very similar to those previ-
ously observed on the LSMO layers.9
The qualifying test for the suitability of any buffer
layer structure on any metal tape is chemical compatibil-
ity with the YBCO coatings. Figures 3(a) and 3(b) com-
pare the results of SIMS depth profile measurements on
YBCO/LSMO/Ni and YBCO/LMO/Ni, respectively.
Both samples show well-resolved interfaces and well-
defined layers and effectively block Ni diffusion to
within a thickness of few hundred angstroms, as evi-
denced by the sharp increase of the Ni signal near the
buffer–substrate interface. For the YBCO/LSMO/Ni
structure [Fig. 3(a)], the La and Mn signals remain at the
noise level, while a significant broadening of the Sr sig-
nal within the YBCO layer indicates contamination. On
the other hand, Fig. 3(b) clearly shows a cation contami-
nation-free YBCO coating on the LMO/Ni structure. In
fact, these SIMS results are in accordance with the dif-
ferences observed in the electrical and superconducting
properties of YBCO films deposited on LSMO9 or LMO
buffer layers. In Fig. 4, we compare the magnetic field
dependence (H||c axis) of the transport Jc at 77 K for
YBCO films deposited on LMO/Ni–(Ni–alloy) with
those on LSMO/Ni–(Ni–alloy) architectures. To observe
the effect of LMO thickness on the superconducting
FIG. 3. SIMS depth profiles for (a) YBCO/LSMO/Ni and (b) YBCO/
LMO/Ni multilayer structures.
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biaxially textured Ni and Ni–alloy substrates. Although
YBCO films on both buffers have good crystalline struc-
ture, the Tc and Jc of the YBCO on LMO layers have
shown significantly higher values than those of the
YBCO films grown on LSMO layers. These results are
observed to be in accordance with the low-level Sr con-
tamination of the YBCO originating from the LSMO
layers. Our results also indicated that the LMO layer
thicknesses in the range of 60–300 nm do not signifi-
cantly effect the superconducting properties of the
YBCO coatings. For epitaxial YBCO films on LMO/Ni–
(Ni–alloy) tapes, typical self-field Jc(77 K) values of
1 × 106 A/cm2 have been achieved. The present results
provide initial support for implementation of LMO
buffer layers in the development of second-generation
HTS wires on the basis of the RABiTS approach to
YBCO-coated conductors.
FIG. 4. Magnetic field dependence of Jc, measured at 77 K, for the
YBCO films on LMO-buffered Ni and Ni–alloy substrates. Also
shown for comparison are typical Jc (H, 77 K) data for YBCO/LSMO/
Ni and YBCO/LSMO/Ni–alloy tapes. The inset shows the resistive
superconducting transition region of the same YBCO/LMO/Ni and
YBCO/LSMO/Ni structures.
ACKNOWLEDGMENTS
This work was supported by the United States Depart-
ment of Energy, Division of Materials Sciences,
Office of Science, Office of Power Technologies-
Superconductivity Program, and Office of Energy Effi-
ciency and Renewable Energy. The research was
performed at the Oak Ridge National Laboratory, man-
aged by U.T.-Battelle, LLC, for the United States
Department of Energy under Contract No. DE-AC05-
00OR22725.
properties of YBCO layers, we also present the Jc–H
behavior of two samples having 60- and 300-nm-thick
LMO layers. At zero applied field, YBCO films of thick-
ness approximately 200 nm grown on LMO support
high-Jc Ն1.0 × 106 A/cm2, whereas the YBCO films on
1
LSMO show approximately ⁄
2
that value. The transport
properties are largely independent of LMO thickness
in the range of 60–300 nm. Specifically, the self-field
Jc of the YBCO/LMO(60 nm)/Ni–alloy is above 1.2 ×
106 A/cm2, which is comparable to the best Jc values
obtained on RABiTS having both the “standard trilayer
insulating” buffer architecture, CeO2/YSZ/CeO2–
(or Y2O3)/Ni, and “bilayer conductive” architecture,
SrRuO3/LaNiO3. The present results establish a new
benchmark for performance since they were attained on
a RABiTS structure having a single buffer layer. The
inset of Fig. 4 shows the superconducting transition re-
gion of the temperature-dependent resistivity curves for
the same YBCO/LMO/Ni and YBCO/LSMO/Ni sam-
ples. Differences in the net resistivity values between two
samples are due to the electrical coupling between
the HTS layer and the metal substrate through the con-
ductive LSMO layer, providing an overall reduced resis-
tivity. While the YBCO/LSMO/Ni exhibits a Tc of only
82 K, that of the YBCO/LMO/Ni structure is 90 K, fur-
ther supporting a low-level of Sr contamination from the
LSMO layer. It is well documented that Sr impurities in
the superconducting structure can significantly reduce
Tc.10,11 This conclusion is also consistent with the lower Jc
values observed for the YBCO films on LSMO layers.9
In summary, we have made a comparative study of the
chemical compatibility and superconducting properties
(Tc, Jc) of YBCO films grown on single layers of
La0.7Sr0.3MnO3(LSMO) and LaMnO3(LMO) buffered
REFERENCES
1. D.P. Norton, A. Goyal, J.D. Budai, D.K. Christen, D.M. Kroeger,
E.D. Specht, Q. He, B. Saffian, M. Paranthaman, C.E. Kalbunde,
D.F. Lee, B.C. Sales, and F.A. List, Science 274, 755 (1996).
2. A. Goyal, R. Feenstra, F.A. List, M. Paranthaman, D.F. Lee,
D.M. Kroeger, D.B. Beach, J.S. Morell, T.G. Chirayil,
D.T. Verebelyi, X. Cui, E.D. Specht, D.K. Christen, and
P.M. Martin, J. Met. 19 (July 1999).
3. D. Dimos, P. Chaudari, J. Mannhart, and F.K. Legoues, Phys.
Rev. Lett. 61, 219 (1988).
4. D.T. Verebelyi, D.K. Christen, R. Feenstra, C. Cantoni, D.F. Lee,
M. Paranthaman, P.N. Arendt, R.F. DePaula, J.R. Groves, and
C. Prouteau, App. Phys. Lett. 76, 1755 (2000).
5. M. Paranthaman, A. Goyal, F.A. List, E.D. Specht, D.F. Lee,
P.M. Martin, Q. He, D.K. Christen, D.P. Norton, J.D. Budai, and
D.M. Kroeger, Physica C 275, 266 (1997).
6. F.A. List, A. Goyal, M. Paranthaman, D.P. Norton, E.D. Specht,
D.F. Lee, and D.M. Kroeger, Physica C 302, 87 (1998).
7. Q. He, D.K. Christen, R. Feenstra, D.P. Norton, M. Paranthaman,
E.D. Specht, D.F. Lee, A. Goyal, and D.M. Kroeger, Physica C
314, 105 (1999).
8. T. Aytug, B.W. Kang, C. Cantoni, E.D. Specht, M. Paranthaman,
A. Goyal, D.T. Verebelyi, D.K. Christen, J.Z. Wu, R.E. Ericson,
C.L. Thomas, C-Y. Yang, and S.E. Babcock, J. Mater. Res. 16,
2661 (2001).
9. T. Aytug, M. Paranthaman, B.W. Kang, S. Sathyamurthy,
A. Goyal, and D.K. Christen. Appl. Phys. Lett. 79, 2205 (2001).
10. M. Oda, T. Murakami, Y. Enomoto, and M. Suziki, Jpn. J. Appl.
Phys. 26, L804 (1987).
11. T. Wada, S. Adachi, T. Mihara, and R. Inaba, Jpn. J. Appl. Phys.
26, L706 (1987).
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