Magnetic flux dynamics in inhomogeneous high-Tc superconductors in weak fields

PHYSICA

Physica B 194-196 (1994) 1873-1874 North-Holland

Magnetic flux dynamics in inhomogeneous high-Tc superconductors in weak fields Eugene V. II'ichev*, Claus S. Jacobsen, and Jesper Mygind Physics Department, Technical University of Denmark, DK-2800 Lyngby, Denmark Features of magnetic flux dynamics in samples of the high temperature superconductor YBa2Cu30 x have been experimentally investigated in weak magnetic fields (10"3-10e) and at 77 K. The experiments were carried out in a simple arrangement including a field coil, a flat sample perpendicular to the field, and an RF-SQUID along a common axis. Samples of both high and low quality thin films, as well as polycrystaUine ceramics were investigated. Attention is focused on the inhomogeneous thin film, which show unusual "flux jumps" for small field changes. We find strong evidence for abrupt penetration of flux to the center of the sample. Such behavior, which would be expected within "spin-glass" like models, is also found in other inhomogeneous type-II superconductors.

1. I N T R O D U C T I O N High-Tc superconductors usually have some level of inhomogeneity. In the mixed state, where flux penetrates the superconductor, this leads to descriptions in terms of either effective medium models based on Josephson coupling between grains, or continuum models with distributions of pinning energies [1]. In this paper we report on some experimental studies of non-equilibrium flux dynamics at very low fields ( < 1 0 e ) , which might help distinguishing between such models.

films with Jc > 10~ A/cm2 were used. For one film or two stacked films the response was reduced by a factor of 15. The response was reversible in the 1 Oe field span within the accuracy of the set up. Thus we observe field lines around the sample. Two films separated by 1.6 mm reduced the response by a factor of 48. This arrangement modelled perfect shielding for a ceramic sample.

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2. E X P E R I M E N T A L The schematics of the experiments are shown in Fig. 1. The sample is positioned in between a small field coil and a ceramic YBa2Cu30 x RF-SQUID. The space is shielded by a YBa2Cu30 x cylinder, and the assembly is operated at 77 K. The primary experimental results consist of SQUID output versus coil current. Typical applied field changes at the sample center are smaller than 1 0 e . The set up was calibrated as follows: Without sample the response is 5.9 q~0/mA. Here ®0 is the flux quantum. To model perfect shielding, thin

~mr Figure 1. Sketch of experimental arrangement.

3. R E S U L T S Fig. 2 presents data obtained with a ceramic sample. The arrows indicate the directions of the current sweep. After a reversal the response is about

* Permanent address: Institute of Problems of the Technologyof Microelectronics and Highly Pure Materials, Chernogolovka,Moscow district 142432, Russia. 0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)1568-7

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Figure 2. SQUID voltage versus coil current for a ceramic YBa~Cu30 x sample. Size: 5 x 5 x 1.6 mm 3.

0.15 ®0/mA, gradually increasing to 1.3 ®0/mA with the current changing about 10 mA. Just 2 mA changes the response markedly. Thus a field change of c. 0.01 Oe is sufficient to change the flux distribution in the center of the sample. Fig. 3 presents data obtained with a defect rich thin film. This film contains many holes and particles of foreign phases. Its measured Jc at 77 K is only 2,500 A/cm 2. Thus this sample is believed to be representative for strongly inhomogeneous superconducting thin films. The top trace in Fig. 3 is taken for increasing currents and is fully reproducible. At bottom the SQUID response is plotted as the period in current corresponding to 4['0 at the SQUID and normalized to the period in the absence of a sample. The remarkable feature of these data is the jump observed. Similar jumps are observed in other field ranges. The behavior in decreasing currents depends on the point of reversal. At the high level reversible behavior is observed. Reversal at the low level results in an immediate return to the long period in response.

4. DISCUSSION AND CONCLUSIONS For the ceramic sample the observed hysteretic behavior is reminiscent to what is expected from

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A I (mA) Figure 3. SQUID voltage versus coil current for an inhomogeneous film of YBa~Cu30 x (upper part), and period of the SQUID signal vs. current normalized to the period without sample (lower part).

using the critical state model of Bean [2] on the intergranular medium. The ability of a very small field change to redistribute the flux in the sample is consistent with non-hysteretic behavior of SQUIDs made from similar material [3]. The behavior of the thin film is attributed to the strongly position and field dependent Jc for an inhomogeneous sample. When Jc is reached in the weakest part of the sample flux enters and the changes spread deeply into the film through the interrelated fields and currents. For a decreasing field, the weak part immediately closes. Thus flux is trapped. We believe this behavior is typical for inhomogeneous type-II superconductors. Indeed experimental results on both low-Tc and high-Tc materials may be re-interpreted along these lines [3].

REFERENCES 1. See, for example, M.Tinkham and C.J.Lobb, Solid State Phys. 42 (1989) 91. 2. C.P.Bean, Phys.Rev.Lett. 8 (1962) 250. 3. E.V.II'ichev and C.S.Jacobsen, to be published.