Magnetic field and temperature dependence of the transport critical current density and flux pinning in anisotropic (Bi,Pb)2Sr2Ca2Cu3Ox high Tc superconductor

Magnetic field and temperature dependence of the transport critical current density and flux pinning in anisotropic (Bi,Pb)zSr2Ca2Cu30x high Tc superconductor* P. Kottman t.t, H. Jones, V. Plechd(:ek § and M. Pol&kt ~University of Oxford, Clarendon Laboratory, Oxford OXl 3PU, UK tlnstitute of Electrical Engineering, SIovak Academy of Sciences, 842 39 Bratislava, CSFR hnstitute of Physics, Czechoslovak Academy of Sciences, 180 40 Prague, CSFR Voltage-current characteristics (V-I) in the temperature range 5 0 - 8 0 K and in magnetic fields up to 3 T have been measured on uniaxially pressed bulk (Bi,Pb)2Sr2Ca2Cu30x ( = 2223) samples in both parallel and perpendicular orientation of the external magnetic field Bext with respect to the direction of pressing. The critical current density Jc, determined from V - I , decreased in magnetic field exponentially with the characteristic field Bs(T). Large anisotropy of the temperature dependences of Bs in the two orientations of Bext was observed. B,(T) from presented measurements were compared with B,(T) in Bi-based 2212 samples. It was found that Bs(T) in 2223 samples were significantly shifted towards higher temperatures. As for 2212 material, the reduced flux-pinning force density in 2223 samples scaled with Bs; the shape of the scaling function was different in the two orientations of Bext. This result indicates that different mechanisms are responsible for flux pinning in various orientations of the external magnetic field. The shape of the voltage-current characteristics will be discussed.

Keywords: high Tc superconductors; critical currents; flux pinning The possiblity of practical applications of high Tc superconductors (HTS) is closer to reality after recent progress in technology. The range of magnetic fields and temperatures in which applications of HTS could be envisaged will be determined, among other factors, by the character of the J~(B, 70 dependence. The conductor technology research has been concentrated mainly on Bi-based HTS (BSCCO) because this type of material is more easily prepared in the form of the composite HTS/metal-clad conductor ~. The excellent currentcarrying properties of BSCCO at low temperatures below 20 K in high magnetic fields up to 30 T are now well established 2-5. However, there has been some controversy concerning the behaviour of BSCCO in magnetic fields at higher temperatures. A strong decrease of J~ above 30 K has often been observed 3"5, but there are indications that this is not an intrinsic property of BSCCO and that the form of J~(B, T) depends on the type of BSCCO, microstructure, orientation of the external magnetic field and other factors 6. It is obvious that the J~(B, T) dependence is closely related to the flux-pinning behaviour and properties of the flux line lattice.

*Paper presented at the conference 'Critical Currents in High Tc Superconductors' 2 2 - 2 4 April 1992, Vienna, Austria

In this work we present the magnetic field and temperature dependences of the transport Jc and the flux-pinning force density Fo in uniaxially pressed polycrystalline (Bi,Pb)2Sr2Ca2Cu3Ox (= 2223) samples in magnetic fields up to 3 T at temperatures 5 0 - 8 0 K. The temperature dependence of the characteristic scaling field Bs, defined by extrapolation of Jc(B) to Jc -- 0, is presented and compared with Bs(T) in Bi-based 2212 samples. Scaling of the reduced flux-pinning force density with B, is presented and discussed. The influence of the orientation of the external field with respect to the direction of pressing is clearly shown in all dependences studied. In the last section, the shape of the electric field versus current density characteristics is presented and discussed.

Experimental Polycrystalline (Bi,Pb)2Sr2Ca2Cu30x samples were prepared by conventional powder technology 7. Briefly, Bi, Pb and Cu oxides and Sr and Ca carbonates were mixed, ground and calcined at 770 °C for 80 h in air and annealed at 840 °C for 400 h in flowing gas at Po: = 7 kPa. The reacted powders were uniaxially pressed (1 GPa) into pellets and finally heat treated at 830 °C for 128 h in flowing oxygen. Partial orientation of grains has been achieved by uniaxial pressing, as was

O011 - 2 2 7 5 / 9 2 / 1 1 1 0 0 5 - 0 5 © 1992 B u t t e r w o r t h - H e i n e m a n n Ltd

Cryogenics 1992 Vol 32, No 11

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Magnetic field and temperature dependence of transport critical current density: P. Kottman et al. confirmed by X-ray diffraction patterns. The orientation of the c axis in large portions of grains was parallel with the direction of pressing (in the following text denoted by z). A bar sample with 1 mm 2 cross-section in the central part was cut from the pressed pellet. Measurements of voltage-current ( V - / ) characteristics were made by the four-probe method. Contacts were made by ultrasonic soldering of pure In. The experimental setup and measuring procedure were the same as described in References 8 and 9. The sample, mounted on a sample holder, was inserted into a continuous-flow cryostat. The external magnetic field was produced by a superconducting magnet. The construction of the sample holder allowed measurements in both parallel and perpendicular orientations of the applied field with respect to the direction of pressing z. The 0.1 /.iV cm - t electric field criterion was used for determination of the critical current density.

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Results and discussion Magnetic field dependences of J~ for B .L z and B II z orientations at various temperatures are shown in Figure la and b, respectively. For lower temperatures, the lowfield Jc values were not measurable because of the problem with heating in contacts. The anisotropy of J~ in the two orientations is apparent. For example, J~ in B _L z orientation is almost an order of magnitude higher than in B II z in 1 T at 70 K. It can also be seen that the anisotropy of Jc increases with both magnetic field and temperature. In the lower regions of J~(B, 7) (i.e. for higher fields) the critical current density decreases approximately exponentially with increasing magnetic field. If we define the characteristic field B~ in a similar way as in Reference 9, i.e. as the value of the magnetic field at which extrapolated J~(B) decreases to some level near zero (for example 1 A cm-2), J~(B, 7") can be written in the form

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J~(B, 7) = J0(7) exp[ -cz(7) B/B~(7)] where J0(T) ~ Jc(B = 0, 7) in our case and the value of the parameter tx depends on the definition of B, (in fact, = In J0 + In Jder where Jdef is the level of Jc used in the definition of Bs). This type of Jc(B, 7) dependence was also observed in our experiments on Bi-based 2212 samples 9 and by several other groups ~0-t2 The temperature dependence of Bs is crucial for applications because it represents practical limits of operation of HTS. In Figure 2, Bs(7) for both orientations of Bext are shown and compared with B,(T) found in our previous experiments with 2212 samples 9. It is evident that Bs(7) in the uniaxially pressed 2223 sample is strongly anisotropic and significantly shifted towards higher temperatures. Even in the worse case, i.e. the B II z orientation, Bs(7) values in the 2223 sample are higher than in the 2212 sample. This is evidence that the rapid decrease of Jc in magnetic field above 30 K is not an intrinsic property of BSCCO material. The actual shape of Bs(7) (and thus the applications limits of HTS) depends strongly on the nominal composition of BSCCO and other factors. Hikata et al. L3 showed that even in the samples prepared with the same nominal composition the temperature dependence of/z0H~rr (defined in a

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Cryogenics 1992 Vol 32, No 11

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Magnetic field and temperature dependence of transport critical current density: P. Kottman et al. different manner from B, in our paper but in fact having similar meaning) was different for samples with different values of Jc(B = 0 ) and thus with different morphology. This means that B,(T) could be improved by improving the microstructure and by preparing pure high T¢ phase ( - 110 K) BSCCO samples. Flux-pinning force densities Fp = J~ x B in the two orientations of B e x t at various temperatures are presented in Figure 3a and b, respectively. By comparing F2(B) curves for a given temperature one can see that nux pinning is significantly greater in the B _L z orientation with the peak Fpmax(T) shifted to higher magnetic fields. This result indicates that different mechanisms might be responsible for flux pinning in various orientations of the external magnetic field. This is also confirmed by the different scaling behaviour of the reduced flux-pinning force density fp = Fp/Fpmax, plotted against b = B/B~ in Figure 4. It has already been found that scaling offp, similar to that observed in low-temperature superconductors ~4.~, can also be observed in HTS ~6-t8. However, in HTS the scaling magnetic field in b = B/B~ is not identical to the upper critical field, B~2, as was the case for LTS. According to Kramer ~9, the form of the scaling function depends

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on structure: strong pinning results in a high peak at low reduced fields b and weaker pinning centres produce a low peak at hi~gher values of b. The scaling relation f p - hi/2(1 - b ) " with maximum at b = 0.2 is usually observed in HTS 9"t6 and seems to be appropriate also for our samples in the B .L z orientation. This type of scaling function is characteristic for core surface or planar defects pinning tS. For our samples, plate-like grains with the flat surface perpendicular to z may effectively pin flux lines when B _L z, i.e. flux lines are parallel with the flat surface of the grains. In the B IIz orientation (Figure 4b), the scaling relation fp b ( 1 - b ) 2 with maximum at b = 0 . 3 3 is more appropriate for our experimental data. The function of this type characterizes core point pinning t~. Because of the plate-like nature of the oriented grains, the individual grains will interact only with very short lengths of flux lines when B IIz, thus approaching the point-like type of interaction. Deviations of experimental data from the exact scaling functions could be explained by the fact that the orientation of the grains in the sample is not perfect and we can only suppose that a certain type of pinning interaction is prevailing over the other one, not excluding it. Another reason for the

Cryogenics 1992 Vol 32, No 11

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Magnetic field and temperature dependence of transport critical current density: P. Kottman et al. slightly shifted peaks of our data could be the method of defining Bs. The exact position of the peak obviously depends on the definition of B~. The electric field vs. current density characteristics ( E - J ) at 80 K for the B I z orientation in l o g - l o g and log-lin scales are presented in Figure 5a and b, respectively. It is clear that the curves cannot be described unambiguously as either a power-law or an exponential relation between E and J over the whole range of electric fields. For the higher electric fields, E > 0.1 #V cm - :, the power-law behaviour is apparent whereas for the lower electric fields, E < 0.1 tcV c m - ' , the E - J relation can be better described by an exponential dependence. The first type of E - J characteristics, E ~ aJ", is well known for conventional inhomogeneous superconductors, where it is connected with the distribution of critical currents 2°-22. This type of E - J form with similar interpretation, which seems to be quite reasonable for granular inhomogeneous materials, has also been found in HTS 8"23-25. However, in HTS there exist several other approaches to powerlaw behaviour of E - J dependence. It is quite difficult to determine which of the mechanisms suggested is responsible for the observed dependence. E - J curves of e~,itaxial YBa2Cu3Ox thin films obtained by Koch et al. -6 and magnetron-sputtered BSCCO (2212) thin films obtained by Myoren et a/. 27 have been interpreted within the model of a transition into a vortexglass state, This model does not seem to be applicable to our experimental data because in our temperature and magnetic field range relaxation effects have been observed 28 which are in contradiction with the glass state with 'frozen' flux lines. Another model is based on a modified flux-line shear mechanism 29. Good agreement with experiments on epitaxial YBa2CuaOx thin films has been obtained. However, the model seems to be applicable only to YBCO material because it does not account for thermal activation, which is known to play an important role in Bi-based HTS. Thermal activation of flux lines is incorporated in the modified flux-creep model by Zeldov et al. 30 According to this model, the power-law characteristics are consistent with the conventional flux-creep model assuming the logarithmic current dependence of the activation energy U. Chen et al. 3~ showed that the power-law E - J relation can be obtained without this modified current dependence of U and without the superconducting-glass-state model. The model of Chen et al. is based on a dynamical equation for thermally assisted vortex motion and reproduces a crossover from the flux-creep regime to the flux-flow regime, This crossover is included also in the model proposed by Griessen 3:, which incorporates flux creep, flux flow and a distribution of activation energies. These last two models seem to be the most relevant for our experimental data in the higher electric field region (E > 0.1 /zV cm-:) because they do not contradict the observed exponential shape of E - J curves in the lower electric fields region ( E < 0 . 1 /zV cm-~). In this region, classical flux creep is the dominant mechanism of dissipation, as was confirmed also by relaxation measurements and by evaluation of E - J curves from hysteresis loops measured at different magnetic field sweep rates on samples cut from the same pellet as the bar samples presented in this work 28.

1008

Cryogenics 1992 Vol 32, No 11

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The exponential relation between E and J was observed in conventional superconductors 33 and explained by the model of thermally activated flux creep (FC) 34. In HTS, exponential E - J curves have been also observed 3"~-37. The corresponding theory of FC in HTS has been discussed in Reference 38. According to this model, flux creep results in the exponential increase of E with increasing J in the case that U ~ ksT. At 77 K, the value of kBT is 6.66 meV, which is much less than the activation energies observed in HTS. Therefore, using the above-mentioned model seems to be justified.

Conclusions The critical current densities in uniaxially pressed (Bi,Pb)2Sr2Ca2Cu30, samples were determined from voltage-current characteristics measured by the fourprobe method at temperatures 5 0 - 8 0 K in an external magnetic field applied perpendicular to and parallel to the direction of pressing. The characteristic field B~ was obtained by extrapolation of Jc(B) to Jc -- 0. The

Magnetic field and temperature dependence of transport critical current density: P. Kottman et al. temperature dependence of B~ was compared with B,(T) in Bi-based 2212 samples A significant shift o f B,(T) towards higher temperatures in the 2223 samples discussed here was observed. Moreover, a large anisotropy of the Bs(T) in 2223 samples in the two orientations of applied field was found. These results confirm that the actual shape of Bs(T) is not an intrinsic property of HTS but depends strongly on the type of HTS material, nominal composition, microstructure and orientation of the external magnetic field. (In fact, Bs(T) is by its nature similar to the 'irreversibility lines' observed in other types of experiments and defined in other ways.) Anisotropy of properties in uniaxially pressed samples with respect to the orientation of Bext was clearly shown also by flux-pinning behaviour. Scaling of the reduced flux-pinning force density with b = B/B~ was found, with different scaling functions in the two orientations of Bext, implying that different mechanisms are responsible for flux pinning. The study of the shape of the electric f i e l d - c u r r e n t density characteristics showed that they cannot be described by one type of dependence over the whole range of electric fields. For E > 0 . 1 tzV cm-1 power-law behaviour was observed, which could be described either by a distribution of critical currents or by the model of thermally activated flux motion incorporating flux flow, flux creep and a distribution of activation energies. In the lower E region, E < 0.1 #V cm -1, the exponential E - J relation, corresponding to the classical flux creep model, was observed in accordance with E - J curves determined from inductive measurements

Acknowledgements PK was in receipt of a Soros/FCO scholarship at University College, Oxford, during his stay in Oxford. The authors would like to thank Mr R.O. Storey for technical assistance.

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37 Pohik, M., Windte, V., Schauer, W., Reiner, J., Gurevich, A. and Wiihl, H. Physica C (1991) 174 14 38 Dew-Hughes,D. Cryogenics (1988) 28 674

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