~i
Flux pinning in BizSr2Caa_xYxCU208+
~
Journalof
magnetism
and
magnetic
materials
and Y B a z C u 3 0 7
c.P. Dhard, S.N. Bhatia *
Department of Physics, Indian Institute of Technology, Bombay 400 076, India
Abstract
We have measured the magnetoresistivity of polycrystalline Bi2Sr2Cal_xYxCu2Os+~ (BSCCO) with x = 0, 0.05 and
0.20 and YBa2Cu307 (YBCO) in magnetic fields of 4 T. We find Inui et al's model to describe the data in a wide
temperature range within the flux creep and flux flow regions. The pinning behaviour in BSCCO due to Y doping is found to
be similar to the one with application of magnetic field, with the pinning potentials Uo around 2000 K. Uo further decreases
to around 1300 K for x = 0.20 in the field of 4 T. The pinnings in YBCO system are found to be stronger compared to
BSCCO.
Low critical current density in the bulk form has
been the major barrier to restrict the applications of
high T~ superconducting materials. It is observed that
pinning centers in their various forms improve the
current carrying capacity. The broadening of resistivity transition below the mean field critical temperature T~ f is an important tool to study the pinning
behaviour in these systems. Various theoretical models involving fluctuation effects, flux creep and flux
flow behaviour, paracoherence effects, etc. have been
proposed in order to interpret the origin of this
broadening. The fluctuation effects are present above
Tmf and can be successfully explained in terms of
Aslamazov-Larkin (AL) and Maki-Thompson (MT)
contributions with the Zeeman and orbital correc-
Proceedings, this paper has not been published therein because it
was not presented at the Conference.
* Corresponding author. Fax: + 91-22-578 3480; email: [email protected].
tions [1]. Below Tcme, Anderson and Kim (AK) [2]
have proposed the flux creep model involving thermally activated depinning (with potential U0) of
coherent flux bundles. The resistivity is proposed to
follow p = po e-u,'/kr. Adopting a similar analogy
Tinkham [3] has derived the expression within
Ginzburg-Landau theory modifying the pinning potential U0. Inui et al. [4] have explained this behaviour assuming independent vortex depinning in
the homogeneous and densely distributed pinning
centers.
The explanation of the resistivity broadening has
been attempted in terms of the above models but still
a coherent picture is lacking. The question regarding
the nature of depinning, whether the flux bundles
move individually or coherently, remains open. Almost all the studies have been performed on single
crystals or thin films and the behaviour in the polycrystalline samples with additional pinning centres at
the grain boundaries remains unclear. The effect of
broadening due to doping on the pinning behaviour
is also not understood. In order to understand some
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSD1 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 0 3 4 - 8
C.P. Dhard, S.N. Bhatia/Journal of Magnetism and Magnetic Materials 146 (1995) 198-200
-10.0
BSCCO x=O
-12.0
Flux flow regime
"--'-14.0
Flux creep
regime
-16.0
i i ii
- 180.000"~ '
iiiii11
0.008
ii
i i I
J iii
0M12
iii
i [[
~ 1 i i ii i
0.016
1/T
i i [ i
~.02~
i t t
i
0.024
Fig. 1. In(p) vs. 1 / T plot for BSCCO with x = 0 in B = 0, 2.5
and 4 T. Theoretical fit for the Inui et al. model within the flux
creep/flow regime is shown by solid lines.
of these points we have measured the p of
Bi2Sr2Cal_xYxCU208+ ~ (BSCCO) with x = 0, 0.05
and 0,20 and YBa2Cu30 7 (YBCO) systems in the
polycrystalline forms in external magnetic field.
Following the A K model we plotted In(p(T))
against 1 / T for all concentrations of BSCCO samples within the transition region (Fig. 1). A thermally
activated behaviour was observed over a fairly large
temperature range where the resistivity changed by
more than two orders of magnitude i.e. - 18 < I n ( p )
< - 1 4 for all the samples. The upper limit of the
resistivity forms almost 2% of its value at room
temperature. The pinning potential U0 obtained from
the slope of these plots is found to be large ( = 5000
K) for undoped BSCCO in the absence of magnetic
field (B = 0), and reduces to = 2500 K for x --- 0.05
and -~ 800 K for x = 0.20. U0 is further affected by
B. It reduces to = 800 K for x = 0 and = 480 K for
x = 0.20 in B = 4 T. The value of U0--- 800 K for
x = 0 in B = 4 T is near the value observed for
single crystals, = 700 K (for B II a - b plane) by
Palstra et al. [5]. Polycrystallinity reflects itself in the
199
preexponential factor P0. P0 is found to be independent of B and show a slight increase with Y doping.
Plots of I n ( p ) vs. U o / T at various fields merge
together for each x. And they shift remaining parallel to each other for different concentrations at the
same B. The pinning potential U0 further shows a
power law dependence, Uoct B-1/2 over one decade
change in B ( = 0 . 4 to 4 T) for all the three Y
concentrations, similar to the behaviour obtained by
Palstra et al. [6] in single crystals of BSCCO for
B_<3T.
Though the AK model fits to resistivity over a
fairly wide range - 18 _< In(p(T)) <_ - 14 giving
reasonable values of the parameters also, a range of
resistivity - 14 _ In(p(T)) <_ - 12 remains unexplained. Essentially the AK model explains the
behaviour in the flux creep region and fails when
flux filaments depin and start moving in the flux
flow region. Inui et al. have incorporated this contribution and derived the expression as p / p , =
p o [ I o ( U i / 2 k B T ) ] - 2 using a modified pinning potential U i = U o - i x k B T where I o is a modified Bessel
function and /x a constant and Pn the linear part of
the normal state resistivity. We find the undoped
BSCCO sample to follow this dependence in the
extended resistivity range - 18 _< In(p(T)) <_ - 12
below Tmf. The fittings are found to be equally good
for the zero field as well as for B _< 4 T. Po remains
around 6 p~l~ cm for all concentrations of Y at all
fields. This is comparable to the value found by Inui
et al. for the single crystal BSCCO. They find P0 to
vary with B. We could not varify this dependence
because of the restricted range of B used here.
However this value of P0 is in accordance with the
Bardeen-Stephen proposition Po = P n B / H c 2 • T h e
width of the fits decrease both with x and B and the
In(p) vs. 1 / T plots develop a shoulder with decreasing slope above I n ( p ) _> - 1 4 . 0 . Inui et al.'s
single crystal data also show such a shoulder which
has been interpreted as a cross over from the creep to
flow regime.
The YBCO samples show a similar behaviour.
Within the AK model Uo is almost a factor of three
larger than for the BSCCO samples. Uo is = 15000
K i n B = 0 and reduces to = 2 0 0 0 K i n B = 4 T .
Values of such high order were also observed by
Palstra et al. for the single crystals of YBCO. This
model fits in the range - 18.0 _< I n ( p ) _< - 14.5. As
200
C.P. Dhard, S.N. Bhatia /Journal of Magnetism and Magnetic Materials 146 (1995) 198-200
seen in the undoped BSCCO samples also, the fitting
improves drastically within the Inui model, which
fits almost in the whole temperature range below
T~ f with U o = 5400 K for B = 2.5 T and = 4200 K
for 4 T. The flux is observed to creep within the
temperature interval 71-82 K which is much shorter
than for the BSCCO sample (47-64 K). Thus it
appears that the activation process is stronger in this
system and the flux flow process starts comparatively at lower temperatures with Uo becoming temperature dependent.
The model proposed by Tinkham derived within
the Ginzberg-Landau model, incorporating the coherent motion of flux bundles, does not appear to be
adequate also: the rms deviations are high and the
value of fitted parameter J~o(0) is found to be around
1.2 X 105 A / c m 2 which is very large for the polycrystalline samples and the fitted value of Tc to be
much larger than the measured To( p = 0). For the
doped samples fittings further worsen.
To conclude, the temperature and magnetic field
dependences of the resistivity of BSCCO and YBCO
in the polycrystalline form remains similar to that of
the single crystals within the flux creep and flow
regions. The resistivity of polycrystalline samples
contains two contributions, the intragrain resistivity
(single crystal) and the resistivity due to grain
boundaries and the later usually dominates the total
in the phase transition regions. Thus it appears that
the grain boundary resistivity shows a temperature
dependence similar to that of a single crystal. The
pinnings in the undoped samples follow flux
creep/flow behaviour in a considerably large temperature range and in the doped samples the range is
reduced. The pinning is found to be stronger in
YBCO compared to BSCCO.
Acknowledgements
The authors are grateful to Dr. J.V. Yakhmi,
Chemistry Division, BARC, Bombay for the samples
and Dr. A.K. Nigam, Low Temperature Division,
TIFR, Bombay for the help in magnetoconductivity
measurements. The work was supported by a grant
(No. BR/91004) from the Board of Research in
Nuclear Sciences, DAE, Govt. of India, which is also
thankfully acknowledged.
References
[1] S.N. Bhatia and C.P. Dhard, Phys. Rev. B 49 (1994) 12206.
[2] P.W. Anderson, Phys. Rev. Letts. 9 (1962) 309.
Y.B. Kim, C.F. Hempstead and A.R. Strnad, Phys. Rev. 131
(1963) 2486.
[3] M. Tinkham, Phys. Rev. Lett. 61 (1988) 1658.
[4] M. Inui, P.B. Littlewood and S.N. Coppersmith, Phys. Rev.
Letts. 63 (1989) 2421.
[5] C.P. Dhard and S.N. Bhatia, J. Appl. Phys. 76 (1994) 6944.
[6] T.T.M. Palstra, B. Batlogg, R.B. van Dover, L.F. Schneemeyer
and J.V. Waszczak, Phys. Rev. B 41 (1990) 6621.