Critical Current Density of YBa{2}Cu{3}O{7-d} Low-Angle Grain Boundaries in Self-Field

Abstract

A study has been perfomed on the superconducting critical current density J_{c} flowing across low angle grain boundaries in epitaxial thin films of YBa{2}Cu{3}O{7-d} (YBCO). The materials studied were dual grain boundary rings deposited on SrTiO{3} and containing 2, 3, 5 and 7 degree tilt boundaries. The current density in self-field was determined by magnetometric methods at temperatures from 5 K to T_c. We conclude that at the higher temperatures of coated conductor applications, there is limited potential for improving J_{c} by reducing the grain boundary angle below about 3 degrees. Comment: 10 pp. total, including REVTEX text, 1 table, and 3 figures

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arXiv:cond-mat/0102081v1 [cond-mat.supr-con] 5 Feb 2001
Critical Current Density of YBa2Cu3O7−δLow-Angle Grain
Boundaries in Self-Field
D. T. Verebelyi, C. Cantoni, J. D. Budai and D. K. Christen
Oak Ridge National Laboratory, Oak Ridge, TN 37831-6061
H. J. Kim and J. R. Thompson
Department of Physics, University of Tennessee, Knoxville, TN 37996-1200 and
Oak Ridge National Laboratory, Oak Ridge, TN 37831-6061
(18 September 2000)
Typeset using REVTEX
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Abstract
A study has been perfomed on the superconducting critical current den-
sity Jc flowing across low angle grain boundaries in epitaxial thin films of
YBa2Cu3O7−δ (YBCO).The materials studied were dual grain boundary
rings deposited on SrTiO3and containing 2o, 3o, 5oand 7otilt boundaries.
The current density in self-field was determined by magnetometric methods
at temperatures from 5 K to Tc. We conclude that at the higher temperatures
of coated conductor applications, there is limited potential for improving Jc
by reducing the grain boundary angle below ∼ 3o.
PACS numbers (1999 scheme): 74.60.Jg, 74.50.+s, 74.76.Bz, 74.60.Ge
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Well-aligned grains are necessary for optimum critical current density Jc in high-
temperature superconductors.A large misalignment of adjacent grains suppresses the
order parameter at the grain boundary (GB) and results in Josephson tunneling behavior
[1] and a near-exponential decrease in Jcwith misorientation angle [2–4].
YBa2Cu3O7−δ(YBCO) low-angle grain boundaries (θ ? 4o), the boundary is comprised of a
periodic array of dislocation cores separated by only slightly perturbed material [5] provid-
ing a strong conduction channel wider than the in-plane coherence length. This low-angle
regime is particularly important to understanding transport in the highly-textured coated
conductors. Both of the most prominent coated conductor methodologies, rolling-assisted
biaxially textured substrate (RABiTS) [6,7] and ion-beam assisted deposition (IBAD) [8],
can provide this level of texture. Our previous transport results concluded that a 2oGB
was inherently different from a 5oGB due to the weak-linked characteristics of the 5oGB
[9].In that study, grain-like voltage-current relations were observed, and no reduction in
Jccould be measured across the 2oGB compared with its adjacent grains. However, those
results were regarded as inconclusive since the estimated grain boundary voltages were less
than those of the grain for reasonable sample geometry. Due to the important implications
for coated conductors and the fundamentals of epitaxial YBa2Cu3O7−δ(YBCO) films, we
have investigated the properties of low-angle grain boundaries utilizing a high sensitivity
magnetometer technique. From their magnetic moments, we deduce the persistent currents
flowing in thin YBCO rings with and without 2o, 3o, 5o, and 7o[001]-tilt boundaries, and
compare these results.
Films were prepared by pulsed laser deposition [7] of YBCO on SrTiO3(STO) bicrystal
substrates with a single [001]-tilt boundary. Using standard optical photolithography tech-
niques, rings were produced with an outside diameter of 3 mm and a width of 100 µm. A
ring was patterned across the GB of the bicrystal, resulting in two GB’s included in each
”GB ring.” For comparison, a companion ring was patterned beyond the grain-boundary
region of each substrate to provide a control sample with the intragrain properties, which
will be referred to as the ”grain ring.” The nominal thickness of the films was 200 nm.
Magnetic measurements were conducted with a Quantum Design MPMS-7 magnetome-
ter. The separated, individual ring samples on rectangular STO substrates were attached
to Si mounting disks using cellulose nitrate cement and placed into a Mylar tube for sup-
port. The magnetic field H, applied perpendicular to the plane of the ring, was ramped to
3000 G at 5 K to induce the maximum circulating persistent currents within the ring. The
applied field was then reduced to zero, inducing maximum currents in the opposite sense.
The resulting magnetic moment was measured while increasing the temperature from 5 K
to 95 K in 1 K steps. For the grain rings, Jcwas calculated using the modified Bean critical
state model,
Jc=
where mring is the magnetic moment in units of emu, V is the volume of a cylinder
with the outside radius R of the ring, and V′is the volume of the ”missing” cylinder with
inside radius R′. For the GB ring, we determined the current across the boundary from
the measured magnetic moment by removing the background signal arising from currents
circulating within the ”strip” of YBCO but not crossing the GB.
verified by measurements on an open circuit ring in which a line was etched across its entire
100 µm width; from these data, the Bean model, with a different geometrical factor, also
In the case of
30 mring
RV −R′V′,Eq.1
(This procedure was
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yields the grain Jcof a homogeneous YBCO strip.)
the corrections are illustated by the ratio of the apparent GB Jc(as deduced from the Bean
relation Eq. 1) to the actual Jcof the boundary, all at 5 K.
We first consider and discuss some surprising findings on the lowest angle grain bound-
ary ( 2o), then return to the measurements on the remaining low angle GB materials. The
present results, obtained by magnetometry of the 2oGB ring, confirmed our previous trans-
port measurements by showing no discernible reduction in Jc when compared with the
companion grain ring.Figure 1 shows the temperature dependence of Jcfor two sets of
samples with 2oGB and their companion grain rings.
quantified the exact YBCO misorientations: 1.83o[001]-tilt and 0.13o[100]-tilt. From the
Franck formula, the calculated dislocation spacing for a 2oGB is 11 nm, with a maximum
strong channel span of 10 nm approximated from visual inspection of transmission electron
micrograph images. This corresponds to a minimum reduction of only 10% in effective GB
length, which is within the expected accuracy range of the data considering geometrical and
experimental errors.A possible conclusion is that the dislocation cores along the GB are
no more than 1 nm in size with respect to suppression of the order parameter.
This simplistic view, however, may be overshadowed by the intrinsic structure of YBCO
films on a cubic substrate. The twin structure of YBCO could provide an explanation
for undiminished Jc values across a 2oGB. Twin boundaries are formed in YBCO films
at the tetragonal-to-orthorhombic structural transition upon cooling in oxygen from high
temperature. Variations in twin structure are dependent on strain, ambient oxygen pressure
and cooling rate [10]. Twin boundaries in films are found to be preferentially aligned along the
?110? and?110?of lattice-matched cubic substrates. In this case, the orthorhombic nature
of the lattice is accommodated by a ∼ 1.8omisalignment of the (110) planes on either side
of a (110) twin boundary [11]. Thus a YBCO film on a SrTiO3(001)single crystal actually
consists of domains with 4 distinct, symmetry-related in-plane orientations. Figure 2 shows
a x-ray phi scan through the YBCO (225) peak including both sides of a 5obicrystal. Each
group of three peaks represent one grain of the bicrystal. The strong central peaks at 0oand
5.1oarise from the two twin domains for which the YBCO {110} planes are aligned with the
substrate [HH0] direction. The difference of 5.1obetween these peaks corresponds to the
[001]-axis bicrystal tilt misorientation. The smaller peaks located at ±0.9o(as given by [2
tan−1(b/a)−90o] ) from the central peaks are reflections from the two possible twin domains
associated with the twin planes aligned with the [HH0] substrate direction.
true for the adjacent grain across the GB. We note that although the x-ray analysis does
not describe the morphology of domains, it does reveal that twinned YBCO films on single
crystal substrates are populated with possible GB misorientation angles of 0.9o, 89.1o, and
90o. Thus the YBCO boundaries located above the intersection of a bicrystal substrate with
a tilt boundary of θoare not composed of simply a single well-defined GB of angle θ. Instead,
different YBCO domains can impinge with a variety of misorientation angles, including
θ ±0.9oand θ ±89.1o. It is important to note that twin boundaries supply no dislocations,
while the interface between the suggested twin domains would produce dislocations with
an expected 12 nm period.Such a dilute array of dislocations easily accommodates a
superconducting coherence length between dislocations, resulting in the typically observed
strongly non-linear, well coupled Abrikosov superconductor.
a single 2oGB to the matrix would not be expected to provide additional distinguishable
In Table I, the relative magnitudes of
High resolution x-ray diffraction
This is also
In this view, the addition of
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dissipation.
Let us now consider more generally the critical current density for grain boundaries with
slightly larger angles. In transport studies, prominant tunneling behavior appears to domi-
nate transport chacteristics for a GB angle above 3-4o, where the grain-like channel between
the dislocation cores becomes smaller than a coherence length. The present magnetometry
study on GB rings of 2o, 3o, 5o, and 7oprovides precise results for the temperature-dependent
Jcin the regime of low angle boundaries. Figure 3 shows the temperature dependence of
Jcfor 3o, 5o, and 7oGB rings, compared to their companion grain rings.
lists the Jcvalues at 5 K for each misorientation angle.
their tunneling conduction in transport studies via a linear differential V − J characteris-
tic; in these magnetometry studies, a signature of tunneling is a complex in-field Jcwith
a pronounced dependence on the magnetic field history.
the 2oGB found grain-like, power law characteristics indicative of strong, tunneling-free
conduction. The magnetometry studies of both the 2oand 3orings revealed in-field Jc’s
with a qualitatively simple, monotonic falloff with H and with minimal-to-no dependence
on field history. These results indicate then, a robust current conduction for GB’s up to 3o,
but pronounced tunneling behavior for GB angles of 5oand above. For the 3oGB, the Jc
lies a factor of 2-3 below the grain value, which can be accounted for in part by a reduced
cross-section of ”good” material by dislocations.
Qualitatively, Fig. 3 shows a systematic reduction of GB current density with angle.
The inset in Fig. 3 shows quantitatively the angle-dependence of Jcat temperatures T =
5, 64, and 77 K. At low temperatures, the Jcrises rapidly as the GB angle decreases in
this regime of low angles. For comparison, the (calculated) depairing current density at
T=0 is included in the inset. At the higher temperatures of coated conductor applications,
there is limited potential for improving Jcby reducing the grain boundary angle below ∼ 3o.
The present materials, in the temperature range of LN2, already possess the technologically
desirable current densities >106A/cm2.In this low angle range, the intragrain Jcbecomes
the limiting factor, and should become the focus of further improvements, especially in the
presence of magnetic fields.
We thank D.M. Feldmann and D.C. Larbalestier for scientific discussions and for provid-
ing the 3osubstrate. Research co-sponsored by the DOE Division of Materials Sciences, the
DOE Office of Energy Efficiency and Renewable Energy, Power Technologies, under contract
DE-AC05-00OR22725 with the Oak Ridge National Laboratory, managed by UT-Battelle,
LLC.
Also, Table I
The 5oand 7otilt GB’s manifest
In contrast, transport studies of
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FIGURE CAPTIONS
Fig 1. Magnetometer measurements of persistent current density vs. temperature for
two 1.8ograin boundary rings and their companion grain rings. Transport data from a 1.8o
single grain boundary are shown for comparison.
Fig. 2. X-ray φ scan though the YBCO {225} peak on a 5.1obicrystal showing the
in-plane angular relation between various YBCO domains.
Fig 3. Magnetometer measurements of critical current density vs. temperature for 2.8o,
5.1oand 7ograin boundary rings and their companion grain rings from the same deposition
and substrate. Inset: Jc versus grain boundary angle at T = 5, 64, and 77 K, in self field.
=========================================
Table I. Comparison of critical current density at 5 K for YBCO grain boundaries and
their companion grains.Data are calculated values of Jc from the maximum magnetic
moment of persistent currents in a narrow ring of YBCO on SrTiO3substrates, in self field.
Misorientation
(degrees)
1.8
1.8
2.8
2.8
5.1
5.1
7
GB Ring
Jc(MA/cm2) Jc(MA/cm2)
35
36
9.1
11.5
0.95
1.4
0.85
Grain Ring (JGB
c
/ JGrain
c
)(JGB,corr
c
/ JGB,Bean
c
)
39
37
19a
23
18
34
31
0.90
0.97
0.48
0.50
0.053
0.041
0.027
1
1
1
0.99
0.48
0.56
0.47
aValue measured with an ”open circuit” ring in strip geometry.
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020406080100
0
10
20
30
40
Fig. 1 (D. Verebelyi et al)
1.8 o GB
1.8 o Grain
1.8 o GB
1.8 o Grain
1.8 o GB
(transport)


Jc (MA/cm2)
T (K)

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0
2000
4000
6000
-202468
Intensity (arb)
φ (deg)
Left
Grain
(110)
Right
Grain
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036
0.1
1
10
100
020 406080100
0.01
0.1
1
10
100
Fig. 3 (D. Verebelyi et al)


J (MA/cm2)
θ (deg)
Jdepair
5 K
64 K
77 K
θ (o) GB grain
2.8
2.8
5.1
5.1
7







Jc (MA/cm2)
T (K)