Download Critical current, magnetic irreversibility line and relaxation in a single

Critical current, magnetic irreversibility line
and relaxation in a single TI- 0 layer 1223
superconductor*
D.N. Zheng, A . M . Campbell, R.S. Liu and P.P. Edwards t
IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge
CB30HE, UK
tSchool of Chemistry, University of Birmingham, Edgbaston, Birmingham, UK
We have carried out systematic studies on the critical current, magnetic irreversibility line
and magnetic relaxation for a c-axis orientated powdered (TIo.sPbo.s)Sr2Ca2Cu309 (TI1223) sample. This single T I - O layer compound, in comparison with the double T I - O
layer compound TI2Ba2Ca2Cu3Olo(TI-2223), exhibited a high irreversibility line, less
temperature and magnetic field dependent critical currents and a smaller flux creep rate.
The substantially greater flux pinning ability of TI-1223 can be understood in terms of its
structural characteristics; in T1-1223, there is only one insulating T I - O layer between
superconducting CuO2 blocks while there are t w o in TI-2223. The increase in the
number (and distance) of insulating layers weakens the Josephson coupling between
CuO2 blocks along the c-axis and the results in a decrease in the value of Jc. This
decoupling between blocks causes the vortex lattice to break into pancake-like vortices
which are easily thermally activated.
Keywords: critical currents; flux creep; irreversibility
Among high Tc superconductors, the thallium based
high T~ compounds are thought to be attractive for
applications because they show the highest To. In terms
of structural characteristics, the thallium based
superconductors can be distinguished into two series
with chemical formulae of T12Ba2Ca,_~CunO2,+4
and TlBa2Ca,_lCU,O2,+3 (n = 1, 2, 3 and 4)1'2; in the
latter, Ba can be replaced by Sr 3'4. The major
distinction between these two types of superconductors
is that in the former compounds the copper perovskitelike blocks containing 1, 2, 3 or 4 CuO2 planes (which
are believed to be responsible for the superconductivity)
are separated by two insulating T I - O layers, while the
latter have only one T I - O layer separating the CuO2
blocks.
The Tl2Ba2Ca:Cu30~0 (TI-2223) compound, which
shows the highest T~ at 125 K among high T~ superconductors, has been studied extensively. At 4.2 K J~ is
high and comparable to other high J~ superconductors.
However, with increasing temperature J~ drops rapidly
and becomes even worse when a magnetic field is
applied. This has been explained in terms of the very
pronounced thermally activated flux flow s or decoupling of pancake-like vortices 6 existing in the highly
anisotropic superconductors.
It has been shown recently that at 77 K
*Paper presented at the conference 'Critical Currents in High "/'c
Superconductors', 2 2 - 2 4 April 1992, Vienna, Austria
O011 - 2 2 7 5 / 9 3 / 0 1 0 0 4 6 - 0 4
© 1993 B u t t e r w o r t h - Heinemann Ltd
46
Cryogenics 1993 Vol 33, No 1
(T1,Pb)Sr2Ca2Cu309 (T1-1223) or compounds with the
same structure but slightly different compositions (T11223) has a much higher Jc in the presence of a
magnetic field 7-9. The short separation between
superconducting CuO2 blocks along the c-axis has been
noticed and was believed to result in an enhancement of
the CuO2 inter-block coupling, leading to the high
value of Jc at 77 K. However, the previous work was
carried out on multiphase and polycrystalline films 7 or
multiphase partially melted samples 8. Here we present
a systematic study on the irreversibility line, the
temperature and magnetic field dependence of J~, and
the flux creep based on the aligned monophase
(Tl0.sPb05)Sr:Ca2Cu309 compound and compare them
with the results of T1-2223. The goal is to show that the
T1-1223 has an intrinsically good supercurrent carrying
capability.
Experimental
The samples were made through a solid state reaction.
For the T1-1223 sample, the introduction of Pb in the T1
site is found to be helpful for the formation of the T11223 phase 3'4. In order to prevent loss of thallium at
elevated temperatures, the samples were sealed in Au
foil during synthesis. The sintering temperatures were
910°C and 950°C for TI-2223 and T1-1223, respectively.
Based on X-ray diffraction (XRD) and energy dispersive X-ray spectrometry (EDS) analyses, single phase
Critical current, magnetic irreversibility line and relaxation: D.N. Zheng et al.
samples of both materials were obtained. Bar-shaped
samples (1.5 x 2 x 10 mm) were cut from the sintered
pellets and used for resistivity and a.c. susceptibility
measurements to define T~. Resistivity measurements
were made using a four-probe a.c. method at 77 Hz.
In order to focus on the intrinsic intragrain properties
and not to be confused by the intergrain contribution to
the magnetization, the sintered bulk specimens were
carefully ground into fine grains. The grains were then
mixed with liquid wax and placed in a magnetic field of
6.5 T. The liquid was hardened and held grains aligned
with their c-axis parallel to the field. The average grain
size was around 2 and 4/zm as measured by scanning
electron microscopy (SEM) for the T1-1223 and T1-2223
samples, respectively.
Magnetization data were taken with a vibrating sample
magnetometer (VSM3001, Oxford Instruments) at different fixed temperatures. The applied magnetic fields
were parallel to the c-axis in all measurements. The
irreversible field Hi, was defined as the field over
which the magnetic hysteresis became undetectable.
Jo was determined from magnetization hysteresis loops
using the Bean critical state model. The time relation
(flux creep) measurements were performed at H = 1 T
and various temperatures. Careful procedures were
made to avoid overshoot of the applied field which is
small but sometimes can give rise to rather confused
results. The magnetization M(t) was then recorded over
a period of 10 to 3600 s.
Results and discussion
In Figure 1 we show the temperature dependence of the
resistivity of the T1-1223 and T1-2223 samples. The
transition is fairy sharp. T¢ is 115 and 118 K for T11223 and T1-2223 respectively, corresponding to zero
resistances.
The temperature dependence of Hirr (irreversibility
line) of T1-1223 and T1-2223 is shown in Figure 2. The
dashed line roughly represents the upper critical field
H¢2. It is clear that T1-1223 has a much steeper irreversibility line and, therefore, a much larger irreversible
region in the H - T plane than does T1-2223, although
0.8
¢~
~
0.6
TI-2223
~
O, 0.4
0.2
0
i
50
i
i
,
i
i
100
150
i
i
,
i
i
200
,
i
i
i
250
T e m p e r a t u r e (K)
Figure 1 Temperature dependences of the normalized resistivity
of the TI-1223 and TI-2223 samples. The zero resistivity point of
Tl-1223 is shifted up to show both curves clearly
14
12
~-~
10
¢/3
~J
[.
8
~ 6
(3
4
Ij
0
I
0
I
20
40
60
Temperature
80
100
120
(K)
Figure 2 The irreversibility lines of TI-1223 (open circle) and TI2223 (solid square). The solid lines are a guide for the eye. The
broken line represents the upper critical field Hc2
their Tes are very close. This means that the T1-1223
material can still carry supercurrent under high
temperature and magnetic field conditions and the T12223 material, in contrast, can carry no current under
the same conditions, so becoming useless for practical
applications under these conditions. In fact the rather
low irreversibility line of T1-2223 may set very severe
limits to the applications of T1-2223 at liquid nitrogen
temperature.
As mentioned previously, we determined Hir, by
choosing the field over which the magnetization
hysteresis disappears. In this case the value of Hi,,
might rely on the criteria that we used in the experiment
and the sensitivity of the instruments. However, we
found that the scatter of the experimental data had little
effect on our conclusions about the T1-1223 and TI-2223
samples. Also for YBa2Cu307 (YBCO), the result
obtained on the aligned powdered sample seems in
agreement with the results in the literature. The actual
criterion we used was equivalent to the value of Jc of
100 A cm -2.
The difference between T1-1223 and T1-2223 can be
understood according to their structural characteristics ~
and the thermally activated flux flow 5 or pancake
vortex decoupling 6. In T1-1223 and T1-2223
compounds, the superconducting CuO2 blocks are
separated by one or two insulating T 1 - O layers along
the c-axis. The coupling between adjacent CuO2 blocks
is Josephson coupling whose strength is exponentially
reduced with increasing separation distance\Thus an
increase in the number (and distance) of insulating layers
greatly weakens the Josephson coupling between superconducting CuO2 blocks along the c-axis. When the
coupling energy is comparable to ksT, the CuO2 blocks
will be decoupled. This decoupling causes the 3D vortex
lattice to break into 2D pancake vortices which are
easily thermally activated or melted.
Jc has been estimated from the Bean critical state
model 1°. Assuming the grains are spherical, then
J~ (A c m - 2 ) = 30AM/d
where AM (emu cm -3) is the difference in magnetization for field ramping up and down, and d (cm) is the
Cryogenics 1993 Vol 33, No 1
47
Critical current, magnetic irreversibility line and relaxation: D.N. Zheng et al.
average diameter of the grains. In Figure 3, we show the
magnetic field dependence of J¢ at 4.2 K and 77 K. At
low temperature J¢ for both samples is very similar.
This is due to the enhancement of the inter-block coupling and the reduction of thermal activation energy kBT.
However, on increasing the temperature to 77 K, J~ for
T1-2223 decreases quickly and becomes strongly field
dependent; the T1-1223 sample, however, still shows a
large value of J¢. At T = 7 7 K and H = 1T, the
values of Jc for T1-1223 and T1-2223 samples are
estimated to be 1.24 x 105 A cm -2 and zero, respectively. J~ for both samples roughly decreases exponentially as the temperature increases, as shown in Figure
4. It drops far more quickly for the T1-2223 material.
The time relaxation of magnetization at 1 T and 30 K
is shown in Figure 5. The equilibrium magnetization,
which is the average of the magnetization for increasing
and decreasing field, has been backed off. Within the
period of data recording, magnetizations showed a
logarithmic decay in most cases, as expected by Anderson theory u. An effective pinning potential Uaf was
obtained by using the formula
M(t) = M(0)(I - kBT/U¢ff In t)
108
E
¢J
I
I
I
106
104
102
0
I
I
I
I
20
40
60
80
Figure 4 Temperature dependence of de at 1 T for the T1-1223
(open circle) and TI-2223 (solid square) samples
0.6
0.4
0.2
i
i
i i i iii
i
i
i
100
Conclusions
Time
In summary, we have studied the irreversibility line,
the critical current and the flux creep for a single T 1 - O
layer (T1-1223) superconductor. All the results show
'
'
'
I
'
'
'
I
'
~ '
I
'
'
'
I
'
'
'
I
'
'
'
1 02
E
%
101
1 00
10 -1
~ ~ ~ I ~ ~ L I ~ ~ h I ~ ~ ~ I ~ ~ ~ I
10 -2
0
2
4
6
8
10
12
Field (Tesla)
Figure
3
circle)
and
48
Magnetic
TI-2223
field
(solid
dependence
square)
O0
T e m p e r a t u r e (K)
0.8
Although Uaf may not represent the true height of the
pinning well 12, comparison of the results obtained
under similar conditions can still be meaningful. For the
T1-1223 and T1-2223 samples, the values of Uefewere
13 and 14 meV respectively at 4.2 K and 1 T. The
values increased to 70 and 43 meV at 30 K and the same
field. The values of Uerrare very close to each other for
the two samples at 4.2 K. However, with increasing
temperature the difference becomes significant. This
seems to be consistent with the results for Jc.
1 03
I
of dc for the TI-1223
samples
(open
at 4.2 K and 77 K
Cryogenics 1993 Vol 33, No 1
i J iiii
1000
(sec)
Figure 5
Time dependence
of magnetization
at 1 T and 30 K of
the T1-1223 (open circle) and TI-2223
(solid square)
samples
that in comparison with the double T1-O layer (T12223) superconductor, the single T 1 - O layer (T1-1223)
superconductor exhibits a higher irreversibility line, a
higher Jc and a larger pinning energy at high fields and
temperatures. An explanation based on the thermally
activated decoupling of pancake-like vortices in different CuO2 blocks in combination with their structural
characteristics has been given.
Apart from this explanation based on their intrinsic
structural characterics, the possible introduction of pinning centres by non-superconducting phases, for instance
(Ca,Sr)2CuO3 and BaPbO3, has also been proposed in
connection with the multiphase partially melted
sample 6. However, since similar results have been
obtained for single phase grains, it is unlikely that the
non-superconducting grains can provide extra pinning
centres that are substantially stronger. Additionally, the
size of these impurity grains seems too large in comparison with the coherence length of the material;
although they may make some contribution to the pinning it is not essential.
Critical current, magnetic irreversibility line and relaxation: D.N. Zheng et al.
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