Supercond. Sci. Technol. 13 (2000) 173–177. Printed in the UK
PII: S0953-2048(00)06357-0
Synthesis and characterization of
thallium-based 1212 films with high
critical current density on LaAlO3
substrates
J Y Lao†, J H Wang†, D Z Wang‡, S X Yang‡, Y Tu‡, J G Wen‡,
H L Wu‡, Z F Ren‡, D T Verebelyi§, M Paranthaman§, T Aytug§,
D K Christen§, R N Bhattacharyak and R D Blaugherk
† SUNY at Buffalo, Department of Chemistry, Buffalo, NY 14260-3000, USA
‡ Boston College, Department of Physics, Chestnut Hill, MA 02467, USA
§ Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
k National Renewable Energy Laboratory, Golden, CO 80401, USA
Received 22 July 1999
Abstract. Growth of epitaxial Cr-doped (Tl,Bi)-1212 films on LaAlO3 substrates by pulsed
laser deposition (PLD) and post-deposition annealing in static air has been achieved. The
films exhibit a high transport Jc of over 1.5 × 106 A cm−2 at 77 K and self-field. Tc values
measured by the four-probe method are in the range 94–100 K. ICP emission spectroscopy
measurement shows a thallium deficiency, with about 0.15 atom bismuth per formula in the
film. XRD θ–2θ, φ and ω scans show that the dominant phase is 1212, with excellent epitaxy.
The film surface has typical PLD morphology as analysed by SEM. TEM analysis found
some inclusions in the film, and HREM found that three 1201 layers exist between the
interface of 1212 phase and substrate.
1. Introduction
Since the discovery of high-temperature superconductivity
[1], a great deal of effort has been devoted to fabricating longlength, flexible superconductors for potential applications
such as transmission cables, motors, generators, etc. In this
regard, the powder-in-tube method has been successful for
synthesis of Bi-2212, Bi-2223, Tl-1212 and Tl-1223 tapes
[2–4]. However, these efforts have been hindered by the
intrinsic properties such as the strong temperature decay
of the irreversibility field in BSCCO-type superconductors
and weak-link effects in Tl-based superconductor tapes.
Recently, two techniques have been developed for fabricating
biaxially textured substrates for the growth of strongly linked,
high-Jc YBCO and Tl-1223 films. One is known as ionbeam-assisted deposition, which was pioneered by Iijima
et al [5] and Reade et al [6]; the other is known as rollingassisted biaxially textured substrates, which was invented at
Oak Ridge National Laboratory [7]. Both methods have
produced YBCO films with Jc values over 106 A cm−2 at
77 K and self-field, while the latter method has also produced
(Tl,Bi)-1223 films with Jc over 4 × 105 A cm−2 at 77 K and
self-field [8].
Among the Tl-based superconductors, the Tl-1223
compound has received the most attention for film synthesis
because Tl-1223 is a single Tl layer system with high Tc
0953-2048/00/020173+05$30.00
© 2000 IOP Publishing Ltd
and Jc and good performance in magnetic fields. However,
the single Tl layer superconducting material Tl-1212 has
the shortest insulating distance between the superconducting
CuO2 layers among all the Tl-based superconductors.
Actually, Tl-1212 is structurally most similar to YBCO
[9]. This short insulating distance could possibly lead
to reduced anisotropy through stronger interlayer coupling,
less severe thermally activated flux motion, therefore higher
critical current density, and better performance in magnetic
fields. In addition, the Tl-1212 compound apparently
is thermodynamically preferred to the Tl-1223 film, as
evidenced by the initial formation of Tl-1212 prior to
transformation into Tl-1223 films. In previous work on the
synthesis of (Tl,Bi)-1223 films, we spent a great deal of
effort to eliminate the intergrowth of the lower-Tc Tl-1212
phase. On the other hand, the synthesis of high-Jc Tl-1212
films has not been successful compared with Tl-1223 films,
owing to the lower values of Jc and Tc . The reason is
that the undoped Tl-1212 material has a formal copper
oxidation state of +2.5, which is higher than the generally
accepted +2.2 optimum valence. This high copper valence
results in a lower Tc of about 80 K for TlBa2 CaCu2 O7 [10].
Fortunately, there are two ways to lower the Cu valence: one
is to anneal the samples in an oxygen-reduced atmosphere
and the other is to dope the compound by elemental
substitution. Until now, TlBa2 CaCu2 O7 superconducting
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J Y Lao et al
films have been successfully grown on LaAlO3 substrates
in a two-zone thallination furnace followed by annealing in
nitrogen [11]. The resulting film had Tc of about 100 K and
Jc just over 105 A cm−2 at 77 K. Another type of Tl-1212
system, TlSr 2 CaCu2 O7 , was reported to be superconducting
at 70–80 K [12]. With elemental substitutions, such as Pb
or Bi for Tl [13–15], rare earths and Y for Ca [16, 17],
the pure 1212 phase can be formed with Tc above 90 K.
In fact, epitaxial (Tl, Pb)Sr 2 Ca0.8 Y0.2 Cu2 O7 thin films have
been grown by off-axis magnetron sputtering in the presence
of Tl vapour on LaAlO3 and NdGaO3 (001) substrates with
Tc of 93 K [18]. Interestingly, the 3d element, Cr, was
reported to be doped into the TlSr2 CaCu2 O7 compound
to produce Tl-1212 superconducting films by a two-step
process: laser ablation of a Tl-free target, followed by
post-ablation annealing in air at 860–870 ◦ C for 15–20 h in
the presence of Tl1.3 Sr 2 CaCr 0.2 Cu2 Oz pellets [19, 20]. The
annealed film had Tc values in the range 98–102 K and Jc
approaching 1 × 106 A cm−2 at 77 K, measured by a selfinductance method.
Recently, we studied the growth of Tl-1212 with high Tc
and Jc as an alternative to Tl-1223. Among all the possible
doping choices, we found that a combination of Cr and Bi is
the best. Here we report the synthesis of Cr-doped (Tl,Bi)1212 film with transport Jc of up to 1.5 × 106 A cm−2
at 77 K and self-field. The total annealing time in static
air was less than 1 h, which is 10 times shorter than
the previously reported times [20]. More importantly, the
annealing temperature window of 875–925 ◦ C is much larger
than that of (Tl,Bi)-1223, 865–875 ◦ C. As far as we know,
the currently observed Jc values are the highest ever reported
for Tl-1212 films. The potential for applications, such as
transmission cables, is very promising.
ArF 193 nm excimer laser, with an energy range from 90 to
120 mJ/pulse at a laser repeat rate of 4 Hz for 60 min with
an oxygen pressure around 25 mTorr. The substrate used
was an LaAlO3 (001) single crystal. The resulting precursor
film was then processed ex situ in a muffle furnace in static
air by placing the sample on a gold plate situated between
two (Tl0.85 Bi0.3 )Sr 2.0 Ca0.85 Cr 0.15 Cu2 O7 semicircular pellets
to maintain the partial pressure of Tl2 O. The assembly was
then wrapped in silver foil with adequate space for vapour
diffusion and heated at 885–905 ◦ C for 35–45 min, which
resulted in superconductive (Tl, Bi)Sr 2.0 Ca0.85 Cr 0.15 Cu2 O7
film. The as-annealed film thickness is between 0.4 and
0.7 µm.
The phase structure and out-of-plane mosaic distribution
were measured by XRD θ –2θ and ω scans, while the inplane epitaxial alignment between the film and substrate was
determined by φ scans. All the XRD measurements were
made using Cu Kα x-rays and a four-circle diffractometer
with a graphite-diffracted beam monochromator. The
film composition was measured by the ICP emission
spectrometer method. For the four-probe electrical transport
measurements, low-resistance contacts were formed by
depositing Ag pads onto the sample, followed by annealing
for 1 h at 500 ◦ C in 1 atm of pure oxygen. Jc at 77 K was
measured as a function of magnetic field using a voltage
criterion of 1 µV cm−1 . The measurements of Jc (H )
were made with magnetic fields aligned perpendicular to the
film plane. The film thickness was determined by a stylus
profiler which uses a stylus as the detector. The surface
microstructural morphology was examined by SEM. The film
cross section was examined by TEM to check the film for
defects and epitaxiality from the substrate.
3. Results and discussion
2. Experimental details
Superconducting (Tl, Bi)Sr 2.0 Ca0.85 Cr 0.15 Cu2 O7 films were
prepared by a pulsed laser ablation method using a
reacted superconducting source target, followed by postdeposition annealing in a muffle furnace in static air.
In fabricating the reacted source target, a prepowder of
Sr 2.0 Ca0.85 Cr 0.15 Cu2 O7 was first prepared by grinding a
stoichiometric mixture of SrCO3 , Cr 2 O3 , CaO and CuO.
The mixture was then heated in an alumina crucible for
40 h at 920 ◦ C with regrinding after each 10 h of heating.
Then a uniform mixture, with a stoichiometric composition
corresponding to Tl1.0 Sr 2.0 Ca0.85 Cr 0.15 Cu2 O7 , was prepared
by grinding the mixture of the prepowder with Tl2 O3
according to the nominal composition. This mixture was
subsequently compressed at a pressure of 1.75 × 108 Pa
into a 1.9 cm diameter pellet, sandwiched between two
gold plates, wrapped in silver foil and reacted at 885 ◦ C
for 3.0 h in a muffle furnace with stationary air to become
superconducting. The superconducting pellet was then
pulverized, compensated with an additional amount of Tl2 O3
for the Tl loss, compressed at a pressure of 7.0 × 108 Pa into
a 1.9 cm diameter pellet, again wrapped as described above
and heated at 865 ◦ C for 30 min in the same way as in the
previous step to become the laser ablation source target for
film fabrication. The target was then laser ablated by an
174
The ICP emission spectroscopy measurements showed that
the as-deposited film has only 0.45–0.55 atom Tl per
formula because of the high volatility of thallium during
pulsed laser deposition. The as-annealed films have good
superconductive properties although they are Tl deficient
with a composition of 0.5 at%. There is about 0.15 atom
Bi per formula in the as-annealed films, which transferred
from the pellet. Reproducibility over more than 20 samples
was very good.
The XRD θ –2θ diffraction spectrum of a good-quality
Cr-doped (Tl,Bi)-1212 film on an LaAlO3 substrate is shown
in figure 1. All the major reflections are indexed as (00l)
peaks of (Tl,Bi)-1212 phase and (001), (002) peaks of the
LaAlO3 substrate. The strong (00l) peaks of the (Tl,Bi)1212 phase indicate the 1212 phase to be dominant with a
large degree of uniaxial alignment of the c-axis normal to the
substrate. Some weak minor impurity peaks are also found
in the spectrum. Figure 2 shows the XRD ω scan of the (005)
peak of the (Tl,Bi)-1212 phase. The measured FWHM of
this peak is only 0.58◦ , which shows the good out-of-plane
alignment of the (Tl,Bi)-1212 phase. The in-plane alignment
was measured by a φ scan of the (103) peak of (Tl,Bi)-1212,
as shown in figure 3. The four strong equally separated peaks,
with an FWHM value of 0.6◦ , indicate the excellent a- and
b-axis alignment of the (Tl,Bi)-1212 phase.
Tl-based 1212 films on LaAlO3
50
100
(005)
45
90
LaAlO 3 (002)
70
Re lative Resistivity (%)
(003)
30
LaAlO 3 (001)
80
35
(004)
(006)
25
20
10
(002)
15
(001)
XRD Intensity (Arbitrary Unit)
40
60
50
40
30
5
20
0
0
5
10
15
20
25
30
35
40
45
50
10
2-Theta (deg)
0
0
Figure 1. XRD θ–2θ pattern of Tl-1212 film on LaAlO3 substrate.
30000
150
200
250
300
Te mperature (K )
20000
1.00E+07
15000
1.00E+06
10000
H//C
77 K
2
J c (A/cm )
Intensity (Arbitray Units)
100
Figure 4. The temperature dependence of the resistivity, showing
the zero-resistance superconducting transition temperature of
96.5 K.
T l-1 21 2 (0 05 )
F W HM = 0 .5 8
25000
50
5000
1.00E+05
0
18
19
20
21
22
O me ga (deg )
1.00E+04
Figure 2. XRD ω scan of (005) peak of the Tl-1212 phase.
1.00E+03
0
40
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
H (T)
35
Tl-1212 (103)
FWHM = 0.6
Figure 5. The magnetic field dependence of Jc for the field
Intensity (Arbitray Units)
30
oriented perpendicular to the film plane (H kc) at 77 K.
25
20
15
10
5
0
0
90
180
270
360
Phi (deg)
Figure 3. φ scan of (103) peak of the Tl-1212 phase.
Transport measurement of the (Tl,Bi)-1212 film showed
that the zero-resistance Tc of the films is in the range
94–100 K. Figure 4 shows the typical transport-temperaturedependent resistivity curve of a (Tl,Bi)-1212 film with
zero-resistance Tc of 96.5 K The near-linear temperature
dependence above the transition temperature and the
extrapolation of resistivity to the origin suggest a highquality film. The highest transport Jc at self-field and
77 K was 1.5 × 106 A cm−2 , with reproducible Jc values
of over 1.0 × 106 A cm−2 . Figure 5 shows the typical
magnetic field dependence of the transport Jc at 77 K with
the field applied parallel to the c-axis. This curve shows
that the irreversibility line is ∼1.6 T, which is smaller than
the 2.5 T of the (Tl,Bi)-1223 film grown on the LaAlO3
substrate.
Figure 6 shows the surface appearance of the as-annealed
films observed at 2000× and 10 000× respectively. The film
is smooth, well connected and plate like. Some pinholes
can be found on the film surface. The small rods on the
surface could be the minor-phase impurities shown in the
XRD θ–2θ scan of figure 1. Also, many small balls can
be seen on the surface. The size and distribution of these
correspond to the small balls on the as-deposited film surface,
and this feature is typical of PLD films. The superconducting
properties should be further improved by optimizing the
stoichiometry, eliminating features such as pinholes, rods
and balls.
A section of the film was analysed under cross-sectional
TEM. Figure 7(a) shows a typical bright field TEM image
of the cross section. The black spots in the film are possibly
impurity precipitates. This is not surprising considering the
multi-element nature of the system, high volatility of thallium
and particulates in the films due to the PLD method. For
some areas, a very thin amorphous layer was found at the
interface between the substrate and the film. However, the
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J Y Lao et al
(a)
(a)
(b)
Figure 7. (a) Typical bright field TEM image of Tl-1212 film on
LaAlO3 substrate. (b) Bright field TEM image of a good area of
Tl-1212 film.
(b)
Figure 6. (a) Surface microstructure of as-annealed Tl-1212 film
at 2000×. (b) Surface microstructure of as-annealed Tl-1212 film
at 10 000×.
film on top of this amorphous layer retains good epitaxy.
Figure 7(b) shows a bright field TEM image of a good
area at higher magnification. Except for a few stacking
faults caused by the number variation of CuO2 planes in
the Tl-1212 film, this film exhibits perfect crystallinity.
The film thickness measured at this cross section is about
0.41 µm, which is in good agreement with surface profile
measurement. Figure 8 shows a high-resolution electron
microscopy image of the interface between the film and the
substrate. The film is highly epitaxial with the substrate. This
is expected since the lattice parameters of LaAlO3 substrate
and high-Tc superconducting materials are well matched.
However, three layers of intergrown 1201 phase were found
at the interface between the film and substrate. This kind
of phase with lower n of the Can−1 Cu2 O2n perovskite units
is observed at the interface of film/substrate and bulk (such
as 2212)/Ag tape [21, 22]. The measured c-axis of the
(Tl,Bi)-1212 phase is 1.190–1.195 nm, and the a-axis is
0.382 ± 0.0005 nm according to figure 8. Compared with
the data from Sheng et al [19], the a-axis of the film
is slightly longer and the c-axis is slightly shorter. This
difference could be due to the Bi addition or film strain, or
both.
176
Figure 8. High-resolution electron microscopy image of Tl-1212
film on LaAlO3 substrate.
4. Conclusions
Epitaxial Cr-doped (Tl,Bi)-1212 films have been successfully
grown on LaAlO3 substrates with the highest transport Jc
of over 1.5 × 106 A cm−2 at 77 K and self-field. The
Tc values of the films are in the range 94–100 K. The
superconductive properties could be further improved by
eliminating the defects found by SEM and TEM analysis.
The high Jc and good reproducibility of the Cr-doped (Tl,Bi)1212 system provide another alternative to (Tl,Bi)-1223 for
potential electric power device applications.
Tl-based 1212 films on LaAlO3
Acknowledgments
The work performed at Boston College is sponsored in part
by DOE under a grant DE-FG02-98ER45719, in part by
Oak Ridge National Laboratory and in part by National
Renewable Energy Laboratory. The work performed at
Oak Ridge National Laboratory (ORNL) was co-sponsored
by the DOE Division of Materials Sciences, Office of
Basic Energy Sciences, Office of Power Technology,
and by the DOE Office of Electric Utility Concepts,
Superconductivity for Electric Energy Systems, both under
Contract No DE-AC05-96OR22464 with Lockheed Martin
Energy Research.
References
[1] Bednorz J G and Muller K A 1986 Z. Phys. B 64 189
[2] Heine K, Tenbrink J and Thoner M 1989 Appl. Phys. Lett. 55
2441
[3] Ren Z F and Wang J H 1992 Physica C 192 55
[4] Ren Z F and Wang J H 1993 Appl. Phys. Lett. 62 (23) 3025
[5] Iijima Y, Tanabe N, Kohno O and Ikeno Y 1992 Appl. Phys.
Lett. 60 (6) 769
[6] Reade R P, Berdahl P, Russo R E and Garrison S M 1992
Appl. Phys. Lett. 61 2231
[7] Norton D P et al 1996 Science 274 755
Goyal A et al 1996 Appl. Phys. Lett. 69 1795
[8] Ren Z F, Li W, Wang D Z, Lao J Y, Wang J H,
Paranthaman M, Verebelyi D T and Christen D K 1999
Physica C 313 241
[9] Kim D H, Gray K E, Kampwirth R T and Smith J C 1991
Physica C 177 431
[10] Parkin S S, Lee V Y, Engler E M, Nazzal A I, Huang T C,
Gorman G, Savoy R and Beyers R 1988 Phys. Rev. Lett.
60 2539
[11] Siegal M P, Venturini E L, Newcomer P P, Overmyer D L,
Dominguez F and Dunn R 1995 J. Appl. Phys. 78 (12)
7186
[12] Maysuda S, Takeuchi S, Soeta A, Suzuki T, Alihara K and
Kamo T 1988 Japan. J. Appl. Phys. 27 2062
[13] Subramanian M A, Torardi C C, Gopalakrishnan J, Gai P L,
Calabrese J C, Askew T R, Flippen R B and Sleight A M
1988 Science 242 249
[14] Haldar P, Sridhar S, Roig-Janiki A, Kennedy W, Wu D H,
Zahopoulos C and Giessen B C 1988 J. Supercond. 1 211
[15] Li S and Greenblaat M 1989 Physica C 157 365
[16] Sheng Z Z, Sheng L, Fei X and Hermann A M 1989 Phys.
Rev. B 39 2918n
[17] Liu R S, Liang J M, Huang Y T, Wang W N, Wu S F,
Koo H S, Wu P T and Chen L J 1989 Physica C 162–164
869
[18] Myers K E, Face D W, Kountz D J and Nestleerode J P 1994
Appl. Phys. Lett. 65 (4) 490
[19] Sheng Z Z, Gu D X, Xin Y, Pederson D O, Finger L W,
Hadidiacos C G and Hazen R M 1991 Mod. Phys. Lett. B
5 635
[20] Tang Y Q, Chen K Y, Chan I N, Chen Z Y, Shi Y J,
Salamo G J, Chan F T and Sheng Z Z 1993 J. Appl. Phys.
74 (6) 4259
[21] Wen J G, Morishita T, Koshizuka N, Traeholt C and
Zandbergen H W 1995 Appl. Phys. Lett. 66 (14) 1830
[22] Feng Y, Larbalestier D C, Babcock S E and
Vander Sande J B 1992 Appl. Phys. Lett. 61 (10) 1234
177