Physica C 298 Ž1998. 166–172
Tl 2 Ba 2 CaCu 2 O 8yx superconducting films deposited by aerosol
and their hysteretic ac losses
A. Conde-Gallardo
a
a,b
, T. Klein a,) , C. Escribe-Filippini a , J. Marcus a ,
T. Lopez-Rios a , M. Jergel b
Laboratoire d’Etudes des proprietes
´ ´ Electroniques des Solides, Centre National de la Recherche Scientifique, BP166,
38042 Grenoble Cedex 9, France
b
Departamento de F’sica, CINVESTAV-IPN, Apdo. Postal 14-740, Mexico
D.F. 07300, Mexico
´
Received 2 August 1997; revised 20 November 1997; accepted 8 December 1997
Abstract
ac susceptibility measurements have been performed on Tl 2 Ba 2 CaCu 2 O 8yx superconducting films. The films were
grown in two steps: first Ba–Ca–Cu–Ag–O precursor films have been prepared by spraying an aerosol on MgO substrates
and the Tl was then subsequently introduced by diffusion into the precursor films. The most favourable preparation
conditions have been determined by comparing the structural and magnetic properties of the films. A detailed study of the ac
susceptibility shows that the ac losses are due to intragrain bulk hysteretic pinning eventhough small geometrical barriers
contributions could also be observed. q 1998 Elsevier Science B.V.
PACS: 74.75; 74.60G; 74.30G
Keywords: Tl-2212 films; ac susceptibility; Flux pinning; ac losses
1. Introduction
High critical temperature ŽHTc. superconducting
tapes Ži.e., films. have been proposed as an alternative to long wires for high-current, high-fields applications w1,2x. In this context, thallium-based films
ŽTl–Ba–Ca–Cu–O. are of great interest since they
present at least four stable phases with Tc ) 100 K. It
is thus important to perform a detailed characterization of their ac response in order to get a better
insight of the ac losses due to weak link junctions,
)
Corresponding author. Fax: q33-4-76-88-76-88; e-mail:
klein@lepes.polycnrs-gre.fr.
flux flow losses andror hysteretic losses by flux
pinning. Several techniques have been used to grow
HTc superconducting films however, one of the most
promising one is the spray pyrolysis Žaerosol. technique w1,2x. Indeed this technique is the simplest and
less expensive way to get films. In the present paper,
we present a detailed study of Tl 2 Ba 2 CaCu 2 O 8yx
films grown by a combined aerosol deposition and
thallium diffusion process. We have determined the
best technological conditions for preparing the samples by comparing their morphology, structural and
magnetic properties. We show that those films present very sharp transitions in ac susceptibility which
are broadening up as the amplitude of the ac modula-
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 8 8 8 - 1
A. Conde-Gallardo et al.r Physica C 298 (1998) 166–172
Table 1
Annealing conditions of the different samples
Sample
Temperature
ŽTa . Ž8C.
Time Ž ta .
Žmin.
S1
S2
S3
S4
S5
854
854
854
854
854
y2
2
5
8
30
S6 sS2
S7
S8
854
859
870
2
2
2
167
mechanism is predominant in our samples. Finally,
geometrical barrier pinning is known to play an
important role in the BiSrCaCuO w9,10x family and
we will thus discuss the origin of the intragrain
pinning in the Tl-2212 compound.
2. Sample preparation and experiments
The annealing time is counted after the stabilization of the furnace
temperature. y2 min means that for this sample the annealing
temperature Ž8548C. was not yet reached.
tion field is increased as expected for weak bulk
pinning.
In ac susceptibility measurements, the real Ž x X .
and imaginary Ž x Y . part reflect the superconducting
shielding capabilities and the ac losses respectively.
Some particular features in x X and x Y can be used
for detecting intergranular as well as intragranular
properties w3,4x. Usually, in granular superconductors, the intragrain shielding currents are larger than
the intergranular ones which have to cross the junction boundaries. If those two mechanisms of shielding are present, x X presents two steps corresponding
to the intragrain superconducting transition temperature and the temperature below which the current is
able to flow through the junctions respectively. Correspondingly, x Y shows two peaks which are related
to the energy losses due to the intragrain vortex
pinning and the junction resistance w3–8x respectively. We will show that the intragrain pinning
We report here only a short description of the
sample preparation, more details can be found elsewhere w11x. Due to the thallium volatility and toxicity, the films have been prepared in a two-step
procedure. First, a Ba–Ca–Cu–Ag–O precursor film
is deposited on a ; 5 = 3 mm2 MgO substrate using
an ultrasonically generated aerosol. This aerosol has
been obtained from a solution of BaŽNO 3 . 2 ,
CaŽNO 3 . 2 P H 2 O, CuŽNO 3 . 2 P 3H 2 O and AgŽNO 3 . 2
dissolved at 0.03 M in a 2:2:3:0.3 ratio in order to
synthesize the Tl 2 Ba 2 Ca 2 Cu 3 O 10 phase which has
the highest Tc . Silver has been added because it is
known to contribute to a better nucleation and intergrain coupling in the case of thallium-based superconducting films w12x. This precursor film has been
subsequently preannealed at 8508C in air for 30 min
in order to complete the homogenization of the
starting materials. The thallium is then introduced in
the film through a diffusion process by annealing the
precursor film in the presence of a Tl 2 O source
under oxygen flow. The film and the thallium source
were kept at the same temperature ŽTa . in a one zone
reactor chamber. Under these conditions, the thallium is diffusing into the film and the superconducting structure is developed simultaneously. The annealing temperatures ŽTa . and times Ž ta . are reported
Table 2
Transition temperature Tc , EDAX analysis, lattice parameters and relative orientation determined as the ratio of the Ž0012. and the Ž107.
X-ray intensity peaks
Sample
Tc
Composition
Lattice parameters
Ž0012.rŽ107.
X-ray intensity
Tl
Ba
Ca
Cu
˚.
aŽA
˚.
cŽA
S1
S2
S3
S4
S5
101.1
99.9
101.8
100.9
103.3
28.6
22.9
22.5
18.3
20.3
21.3
28.9
24.1
22.1
24.1
32.8
34.1
16.7
26.8
23.3
33.2
3.84
3.84
3.84
3.84
3.84
29.17
29.18
29.15
29.15
29.16
6
16
20
37
37
S6
S7
S8
101.8
100.0
95.3
22.9
26.1
26.3
20.3
21.1
16.4
24.1
23.3
29.9
32.8
29.5
27.5
3.84
3.84
3.84
29.18
29.14
29.13
16
30
168
A. Conde-Gallardo et al.r Physica C 298 (1998) 166–172
in Table 1. The resulting film thickness is around 5
m m.
The films were then characterized by various
techniques: Ži. the cell parameters and the grain
orientation of the different phases have been determined by X-ray diffraction using the Cu K a radia˚ .; Žii. the morphology and chemtion Ž l s 1.54051 A
ical composition have been determined by scanning
electron microscopy and EDAX analysis; Žiii. the
real and imaginary parts of the ac susceptibility have
been measured with a home made spectrometer. The
modulation field Ž10 kHz. was perpendicular to the
film surface and its amplitude has been changed
from 0.05 to 2.5 Oe. The ac susceptibility has also
been measured in a constant magnetic field up to 5 T
parallel to the c-axis of the films in order to get an
estimation of the position of the irreversibility line.
3. Characterization
The critical temperatures measured by ac susceptibility, the lattice parameters, as well as the average
chemical composition measured on a 1 mm2 surface
are reported on Table 2. The transition temperatures
of the films Žonset of x X . are ranging between 95
and 103 K which is close to the Tc of the best films
obtained by laser ablation or sputtering Ž106 K.
w12–14x.
From the EDAX analysis, it can be seen that the
Tl:Ba:Ca:Cu average ratio is close to 2:2:2:3 for
samples S 2 and S 3 as expected from the initial
composition of the precursor film. However a more
punctual analysis Ž1 m m2 . suggests that the film is
actually constituted of 2212 grains and a small
amount of Ca andror Cu rich extra phases. This is in
good agreement with the X-ray diffraction spectra
which can be well indexed using the 2212 indexation
scheme ŽFig. 1. as well as with Raman spectroscopy
which presents the characteristic vibrational modes
of this phase. Some of the 2223 peaks however
appear on the X-ray spectra ŽFig. 1. of sample S 5
Žlong annealing time. suggesting that the growing
kinetic of this phase is probably slower than the one
of the 2212 phase Ža similar result has been observed
in Refs. w13,14x.. As shown on Fig. 2, under those
conditions, the diffusion time has a great influence
on the morphology of the films. Indeed it is leading
to a progressive destruction of the smooth morphol-
Fig. 1. X-ray diffraction patterns for films S3 and S5. The c-axis
orientation is indicated by the dominant presence of Ž001. peaks.
Similar results were obtained for the other samples Žsee tables..
Some small 2223 peaks are present in sample S5 Žsee the inset..
ogy of the precursor film as the growth of the 2212
crystallites takes place. Moreover, rather high Tl
average ratio and extra phases become visible on the
X-ray spectra for samples S 1 , S 7 and S 8 which
suggests that the best annealing conditions for Tl
diffusion in order to get the 2212 phase are Ta ;
8548C and ta ; 5 to 8 min. Table 2 also shows that
those annealing conditions lead to highly c-axis oriented polycrystalline grains. Indeed the intensity ratio between the Ž00 l . Ždata are given for l s 12 in
Table 2. and the Ž107. peaks reaches its maximum
after about 8 min. The Ž107. peak has been chosen
for this comparison because it has a 100% intensity
for random orientation.
The renormalized real part of the first harmonic of
the ac susceptibility as a function of temperature is
A. Conde-Gallardo et al.r Physica C 298 (1998) 166–172
169
later, this enlargement is related to the weakness of
pinning and the width of the transitions actually
becomes small for all samples when the ac modulation field is decreased Žsee Fig. 3b.. This shows that
the distribution in Tc is rather small in all films
indicating that they are quite homogeneous as far as
their superconducting properties are concerned. The
width of the transition is increasing much slower in
S 2 and S 3 suggesting that pinning is more efficient
in those films which makes them more favourable
for applications. Those samples have also a rather
good chemical composition and crystalline orientation, as we have seen in the previous paragraph. We
Fig. 2. SEM photographs for samples annealed at 8548C for
ta s 2, 5 and 30 min. The effect of the annealing time on the
morphology is evident. Similar behaviour is obtained if ta is fixed
and the annealing temperature is changed Žsamples S6, S7 and S8
of Table 1..
presented on Fig. 3a for some characteristic samples
Žsee Table 2.. Samples S 5 and S 1 apparently present
a rather large transition. However, as we will see
X
Fig. 3. Ža. Renormalized real part of the ac susceptibility x vs T
at h ac s 0.05 Oe. Žb. Width of the transition vs. h ac . The narrowest transitions are obtained for sample S3 but, in any case, DTc
goes to zero as h ac tends towards 0 indicating that the enlargement is related to the weakness of pinning rather than to sample
inhomogeneities.
A. Conde-Gallardo et al.r Physica C 298 (1998) 166–172
170
X
Fig. 4. Onset of x vs. dc field for the samples S2. As usually
observed in the Tl 2 Ba 2 CaCu 2 O 8yx , this line is rather low.
tion field. As can be seen, the peak in x Y is very
close to the y0.4 value as predicted w16,17x and
observed in YBaCuO films w18x for intragranular
contributions. For intergranular coupling we would
have expected the x Y peak to lie close to the 0.9
value, i.e., in the tail of the x X transition w5x. Note
however that x Y becomes quite asymmetric for large
h ac values suggesting that a second contribution,
characteristic of small intergranular effect, may appear for large h ac .
The origin of pinning in high Tc is still an open
question; it has been shown in BiSrCaCuO that this
pinning is due to potential barriers of geometrical
origin w9,10x whereas the ac response of YBaCuO
films has been interpreted recently in term of bulk
pinning in the critical state w18x. The plot of x X and
x Y as function of modulation field h ac , at a given
will thus present in Section 4 a systemic study of the
ac susceptibility in those films ŽS 2 will be taken as
an example but very similar results have been obtained in S 3 ..
As shown in Fig. 4 for sample S 2 , the position of
the onset in x X is shifting very rapidly towards
lower temperature as a dc magnetic field is applied.
This is in agreement with measurements performed
in bulk Tl-based samples which have shown that the
irreversibility line of this highly anisotropic compound is very low w15x.
4. Magnetic susceptibility and ac losses
The real and imaginary parts of the ac susceptibility as a function of temperature for various amplitudes of the modulation field are presented on Figs.
5a and 5b respectively. As already mentioned, the x X
transition is broadening as h o is increased and the
peak in the x Y becomes broader and shifts towards
lower temperatures. But even for our highest h ac
values there is no evidence for two steps in x X or
two peaks in x Y as would be expected in the presence of both intra and intergranular effects Žsee
Section 1.. To distinguish between those two contributions we have plotted on Fig. 6, the temperatures
where x X reaches y0.1, y0.4 and y0.9 as well as
the temperature corresponding to the maximum of
x Y as a function of the amplitude of the ac modula-
Fig. 5. Ža. Real and Žb. imaginary parts of ac susceptibility for
sample S2 for different amplitudes Ž h ac . of the modulation field.
Similar results were obtained for the others samples in Table 1.
Only one transition can be observed.
A. Conde-Gallardo et al.r Physica C 298 (1998) 166–172
X
Fig. 6. Temperatures where x is equal to y0.1, y0.4 and y0.9
Y
values and temperature Tma x where x gets its maximum Žsee
Fig. 4., as a function of the amplitude Ž h ac . of modulation field.
As expected for intrinsic granular effect w14,15x, Tma x is close to
X
the y0.4 x values. The lines are only guides to the eyes.
171
tal data, as expected for bulk pinning the maximum
value for x Y is , 0.24.
However, as seen on Fig. 7, a clear deviation can
be seen for the low h acrh m values. Indeed both x X
and x Y present a rather sharp onset, characteristic of
geometrical barriers w9,10x. Geometrical barriers are
known to play an important role in the BiSrCaCuO
compound w9,10x in which bulk pinning only governs
the irreversibility properties below 22 K. It is thus
not surprising to find geometrical barrier contributions in our Tl-2212 system of very similar structure.
Those contributions are expected to become negligible in very thin films Žthicknessrwidth - 10y3 . and
are thus expected to give some minor contribution in
our films. However any quantitative analysis has to
be taken with caution given the granular morphology
of our films. We can thus conclude that the ac
response of our thallium films is related to intragran-
temperature, can help to distinguish the pinning
mechanism. x X and x Y as a function of h ac rh m ,
where h m is the field of maximum dissipation for a
given temperature, are presented on Fig. 6. As can
be seen, all the curves collapse on a single one for a
very large temperature range indicating that thermally activated processes do not contribute to the ac
response.
Several theoretical models for the response of
superconducting films under a perpendicular Žto the
surface. ac applied magnetic field have been published w16,17,19–22x for both geometrical barriers
and bulk pinning. Brandt w17x has predicted that, for
a thin strip Žit actually depends only very weakly on
the shape of the film., the ac response for bulk
pinning is given by:
xYs
4
2
px x
ln cosh Ž x . y tanh Ž x .
Ž 1.
and
xXsy
4
p
H cos Ž a . tanh
px 0
x sin2 Ž ar2 . d a
Ž 2.
with x s h ac rh c where h c s h m r2.46. The continuous lines in Fig. 7 are fits to the data using the above
equations without any free parameter. As shown the
fits are in rather good agreement with the experimen-
X
Y
Fig. 7. x and x as function of the ratio h ac r h max , where h max
Y
correspond to the maximum in x . Independently of the temperature, all the curves collapse on the same one. The continuous lines
are fits to data using the bulk pinning models w15x.
172
A. Conde-Gallardo et al.r Physica C 298 (1998) 166–172
ular pinning in the Bean modified model, even though
small geometrical contribution can also be observed.
According to Brandt’s calculations, the maximum
of x Y should occur at h m s 0.78 Jc d, where d is the
thickness of the film. Taking d s 5 m m and h m s 1.4
Oe at 80 K, one gets Jc ; 3 = 10 4 Arcm2 which is
much smaller than the Jc values measured in
Tl 2 Ba 2 CaCu 2 O 8 single crystals and epitaxial films
w12,23,24x.
5. Conclusions
We have established the best technological conditions to prepare Tl 2 Ba 2 CaCu 2 O 8 superconducting
films, deposited by a combined aerosol precursor
growth and Tl diffusion process, by correlating crystallographic and morphologic properties with ac susceptibility measurements. We have shown that the
optimal conditions are around ta ; 5–8 min when
the annealing temperature is Ta ; 8548C. The development of the 2212 Tl-based phase, is governed by
the thermodynamics of the thallium diffusion rather
than by the technological process of the precursor
film. Indeed, the annealing conditions do not depend
on the technique used to prepare the precursor w13–
15x. The behaviour of the real and imaginary parts of
the ac susceptibility as a function of the amplitude of
the applied magnetic field, has been used to establish
which kind of the energy losses are present in our
films; we have shown that the intragranular hysteretic process is predominant and, that both bulk
and geometrical barriers pinning are present.
Acknowledgements
A. Conde-Gallardo is in post-doctoral position,
under the economical support of CONACyT-Mexico.
´
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