INTRINSIC GEOMETRICAL BOUNDARIES FOR THE CRITICAL CURRENTS INSIDE YBa2Cu3O7-x SINGLE CRYSTALS

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INTRINSIC GEOMETRICAL BOUNDARIES FOR

THE CRITICAL CURRENTS INSIDE YBa2Cu3O7-x

SINGLE CRYSTALS

André Sulpice, P. Lejay, R. Tournier, J. Chaussy

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, Supplement au no 12, Tome 49, decembre 1988

INTRINSIC GEOMETRICAL BOUNDARIES FOR THE CRITICAL CURRENTS

INSIDE YBa2Cu307-, SINGLE CRYSTALS

A. Sulpice, P. Lejay, R. Tournier and J. Chaussy

Centre de Recherches sur les Trbs Basses Tempiratures, C.N.R.S., BP 166X, 38042 Grenoble Cedex, Friznce Abstract.

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Magnetization curves habe been measured on YBa2Cu307-, single crystals of various size, and compared

with other results published in the literature. The irreversible magnetization does not follow the scaling law predicted by the Bean model. We conclude that even the superconducting crystals contain normal barriers or weak links which limit the spatial scale of supercurrents.

1. Introduction

The applicability of the Bean model to calculate the critical current of the superconductors involves the proportionality of the irreversible magnetization measured along a cycle of magnetic field to the diameter d of the sample:

d

M T ( H ) - M l ( H ) = % J c

For YBazCu307-,, the scaling law has been care- fully verified for powders containing different grain di- ameters [2], then the critical current flows on at least a 20 microns size. The law was also verified on a thin film which has been divided in 4 and 16 equal parts

131.

High critical currents such as 3

x

lo6

~ , / c m ~ have been calculated for small crystals [4]. Nevertheless

J, seems to become smaller when the crystal size in-

creases [5]. Consequently, it is important t o determine if the scale of the currents corresponds to the size of crystals or to a smaller size determined by possible weak links of an unknown nature. For this purpose, several single crystals of various sizes have been pre- pared in the same manner and we can expect the same value of J, for all the samples.

2. Results

The field cooled magnetization in 10 Oe and 100 Oe as well as the zero-field cooled magnetization measured

1

CRYSTAL 1 H//c

I I

Fig. I. - The zero-field-cooled and field-cooled magnetization of crystal 1 have been measured in 10 Oe and 100 Oe external fields applied along the c-axis.

in the same fields applied at 2 K are plotted in figure 1

for a crystal representative of our samples. The size of this crystal is about 1

x

1 x 0.052 mm3 and the field is applied along the c-axis.

The magnetization at 4 K is linear with the field. In 10 Oe, the transition appears to be very sharp up to a critical temperature of 93 K. In a 100 Oe field applied along the c-axis, the flux penetrates into the sample above 20 K because the strong demagnetizing coefficient enhances the internal field: in 100 Oe at

2 K the internal field is equal to 1 040 Oe, value which becomes larger than the first critical field H,I above

20 K. The Meissner effect is small due t o a strong pinning of vortices.

We studied the same crystal with the field applied along the (a, b) plane. Owing to the presence of a

small demagnetizing field, the zero-field cooled magnetization at 2 K is 5 % larger han (-H

/

47rI

.

Then the crystals appear as bulk superconductors.

We report in figure 2, the magnetization curves at 4 K for different crystals of the following sizes: 1

x

1

x

0.052 mm3, 0.4 x 0.33 x 0.019 mm3 and 0.17

x

0.17 x 0.007 mm3. The rernanent magnetization weakly de- pends on the dimensions of the crystal. This magnetization in zero external field is in fact measured in a internal field of about 20 kOe. The perimeter of

-2000

7-4K CRYSTALS

111

(2) .!?.!.

0 2 4 6 8 H (kOel

Fig. 2.

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The magnetization of crystals 1, 2, 3, having the following sizes 1 x 1 x 0.052 mm3, 0.4 x 0.33 x 0.019 mm3, 0.17 x 0.17 x 0.007 mm3 has been measured in fields a p

plied along the c-axis. The maximum applied field attains 40 kOe.

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C8 - 2110 JOURNAL DE PHYSIQUE

the slabs varies from 4 rnm t o 0.68 mm. We expect in the Bean model a remanent magnetization varying by a factor 6 instead of a factor 2 as experimentally observed, then the scaling law is not followed.

ing and normal. Then the possible existence of a two phases microstructure should weaken the critical currents.

4. Conclusions 3. Discussion

We infer that the apparent limitation of critical currents when the crystal size increases could be due t o intrinsic geometrical barriers inside the crystals. We have plotted in figure 3 the value of the remanent magnetization found in the literature [48] to- gether with our own results in function of the crystal perimeters. We can see that the crystals having the highest irreversible magnetization are approx- imatively on the same curve. A saturation value of about 2 300 emu/cm3 is obtained. A typical dimen- sion of 300 microns for the geometrical scale of supercurrents can be evaluated assuming that our smaller crystal obeys the Bean scaling law and that the saturation of the remanent magnetization corresponds to the geometrical limit. We calculate a critical current of 4 x

lo6

~ / c m ~ . A larger remanent magnetization equal to

lo4

emu/cm3 has been obtained [9] for thin films showing that such a limitation can disap- pear. Such boundaries can be normal regions or weak links between superconducting regions which could be due t o a local deficit in oxygen. These concen- tration fluctuations could be related to the existence at low temperature of a secondary decomposition of the off-stoichiometric orthorhombic phase into a mixture of ordered orthorhombic and disordered tetragonal phases

[lo].

Such a decomposition seems to have been observed experimentally in transmission electron microscopy [ll]. The tetragonal phase is semiconduct-

0 , p e r i p e t e r

Fig. 3.

-

The remanent magnetization measured after ap- plying a large field along the c-axis has been plotted versus

the perimeter of different crystals: (+) our crystals 1, 2,

3. The other points come from the references [49]. The remanent magnetization induces an internal field of about

20 kOe which permits the application of the Bean model.

By studying the irreversible magnetization of different size crystals and by comparing with results published in the literature, we infer that the superconducting crystals could contain normal barriers or weak links which would strongly limit the spatial scale of supercurrents. These phenomena could be due to a microstructure resulting from a possible secondary decomposition of the off-stoichiometric orthorhombic phase into a mixture of ordered orthorhombic and disordered tetragonal phases. The absence of such geometrical barriers in some thin films suggests that such a decomposition can be absent depending on the procedure of preparation of the off-stoichiometric orthorhombic phase.

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Male, S., Ashby, M. F., Evetts, J. E. (to be published).

[3] Oh, B., Naito, M., Amason, S., Rosenthal, P., Barton, R., Beasley, M; R., Geballe, T. H., Ham- mond, R. H., Kapitulnik, A., Appl. Phys. Lett. 51 (11) 852.

[4] Dinger, T. R., Worthington, T. K., Gallagher, W. J., Sandstrom, R. L., Phys. Rev. Lett. 58

(1987)' 2687.

[5] Tholence, J . L., Noel, H., Levet, J. C., Potel,

M., Gougeon, P., Solid State Commun. 65 (1988) 1131.

[6] McGuire, T. R., Freitas, P. J. P., Gallagher, W. J., Plaskett, T. S., Sandstrom, R. L., Shaw, T. M., Phys. Rev. B 36 (1987) 4032.

[7] Senoussi, S., Oussena, M., Collin, G., Orsay, High

T, Superconductors preprints, vol. I11 (Dec. 87).

[8] Crabtree, G. W., Liu, J. Z., Umezawa, A., Kwok, W. K., Sowers, C. H., Malik, S. K., Veal, B. W., Lam, D. J., Brodsky, S. K., Downey, J. W., Phys.

Rev. B 36 (1987) 4021.

[9] Chaudari, P., Koch, R. H., Laibowitz, R. B.,

McGuire, T. R., Gambino, R. J., Phys. Rev. Lett. 58 (1987) 2684.

[lo]

Khachaturyan, A. G., Morris, Jr J. W., Phys.

Rev. Lett. 59 (1987) 2776.

[ l l ] Hiroi, Z., Takano, M., Ikeda, Y., Takeda, Y.,