Flux jumps in the magnetization of QMG processed Y1Ba2Cu3O7

Flux jumps in the magnetization of QMG processed YIBa2Cu307* K. Watanabe, S. Awaji, N. Kobayashi, S. Nimori, G. Kido, K. Kimura t and M. Hashimoto t Institute for Materials Research, Tohoku University, Sendai 980, Japan t Advanced Materials & Technology Research Labs., Nippon Steel Corp., Kawasaki 211, Japan The magnetization behaviour of q u e n c h - m e l t - g r o w t h (QMG) Y1Ba2Cu307 bulk materials has been measured at 77.3 and 4.2 K in high fields. The flux jump phenomena presented by means of sample extraction, vibrating sample and SQUID magnetometers were essentially due to the high J¢ and the low specific heat at low temperature in the QMG-Y1Ba2Cu307 bulk. It was found that the irreversibility field measured magnetically is in practical agreement with that obtained resistively. The anisotropic J¢ vs B properties at 4.2 K in the QMG-Y~Ba2Cu307 bulk were very different from those in Y1Ba2Cu307 films. It was considered that QMG-Y1Ba2Cu307 bulk materials include weak links due to imperfect c-axis alignment.

Keywords: high Tc superconductors; critical current density; magnetization

One of the notable characteristics of high T~ superconducting oxides is the high transport critical current density in high fields at 77.3 K'. The Jc properties in superconductors are important enough for their applicable value to be decided. The critical current needs to be evaluated by direct transport measurement for practical use. However, polycrystalline superconducting oxides include weak links at grain boundaries 2, and consequently the transport critical current properties as a function of applied magnetic field are too poor to be utilized for power applications in fields. Such a weak link is an awkward problem to be overcome, and still remains unsettled for high T~ superconducting oxides. The superior intragrain current properties are negated by the inferior intergrain ones in resistive measurement. Magnetic measurement enables us to investigate both intergrain and intragrain Jc 3. Moreover, since it is difficult to measure the Jc value of a YIBa2Cu307 bulk material owing both to a large transport current and to a large contact resistance 4, magnetization measurement has occasionally been employed to elucidate J~. Magnetization measurement has also proved to be a useful experimental tool for obtaining information on the flux-pinning mechanism of high T¢ superconducting oxides. The basic physical phenomena such as flux creep 5 and dimensionality 6, which are related to flux pinning, are successfully presented through the magnetization behaviour. We have performed the magnetization measurement for a q u e n c h - m e l t - g r o w t h (QMG) YtBa2Cu307

*Paper presented at the conference 'Critical Currents in High Tc Superconductors' 2 2 - 24 April 1992, Vienna, Austria

bulk 78 in high fields up to 23 T at 4.2 K by means of a sample extraction magnetometer. Many flux jumps were observed in the magnetization curve at 4.2 K for the QMG-Yt Ba2Cu307 bulk material. The flux jump phenomena depend strongly on the surroundings of magnetic instability. In the present work, the behaviour of magnetic instability in the bulk sample of QMGYtBa2Cu307 was compared by using sample extraction, vibrating sample and SQUID magnetometers. The magnetization hysteresis loops were examined in relation to the Jc evaluation.

Experimental details

Sample preparation YtBa2Cu307 bulk materials were produced by the QMG process 9. The important characteristic of these bulk materials is the presence of Y2BaCuO5 (211) phase inclusions which are residues of the peritectic reaction between the Y203 and liquid phases before crystallization of Y~Ba2Cu307. The finely dispersed 211 inclusions prevent cracking in the YIBa2Cu307 matrix, and also improve the Jc properties in fields. Two kinds of plate-like samples with different alignments (c axis perpendicular to and parallel to the plate surface) were prepared for the magnetization measurements. The samples were typically about 4 x 4 x 1 mm 3 in size.

Magnetization measurements The magnetization measurements using a sample extraction magnetometer were performed in combination with a hybrid magnet (23 T; 0.3% homogeneity in 20 mm

O011 - 2 2 7 5 / 9 2 / 1 1 0 9 5 9 - 0 5 © 1992 Butterworth - Heinemann Ltd

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Flux jumps in the magnetization of QMG processed YTBa2CLI307: K. Watanabe et al. DSV). In this method, the sample was moved by an air piston through the two search coils set up at intervals of 20 mm and was extracted at a distance of 100 mm. The magnetization was calculated by the integration of the induction voltages which were detected by a transient recorder with a converting time of 1 /~s. A vibrating sample magnetometer (VSM) was equipped with a water-cooled resistive magnet (15 T; 0.2% homogeneity in 20 mm DSV). The sample was vibrated in two opposite series pick-up coils with a frequency of 25 Hz and an amplitude of 3 mm. The magnetization of the sample was obtained by the a.c. signal detected with a lock-in amplifier. Accurate magnetization measurements were also made by using a SQUID magnetometer and a superconducting magnet (5.5 T; 10 -4 homogeneity in 20 mm DSV). The sample was traversed through a set of pickup coils which were 30 mm apart for a measurement. The sample was directly immersed in liquid helium or liquid nitrogen in utilizing a sample extraction magnetometer and VSM, while for the SQUID magnetometer the temperature was controlled by He gas flow. The magnetic fields were always applied parallel to the plate surface of the sample, and the sweep rates were typically 0 . 5 - 1 T min -t for a sample extraction magnetometer and VSM, and 0.3 T min -t for a SQUID magnetometer.

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Results and discussion

Figure l a - d shows magnetization hysteresis loops of the QMG-Y~Ba2Cu307 bulk at 77.3 and 4.2 K for B perpendicular to the c axis measured by both the sample extraction magnetometer and VSM. The magnetization behaviour in the sample extraction magnetometer indicated that the loop width at 77.3 K became smaller than that in VSM and apparently disappeared at around 8 T (Figure lb), whereas the hysteresis at 15 T and 77.3 K can be largely observed in VSM as seen in Figure ld. However, at 4.2 K the flux jump phenomena occurred in both magnetometers. In particular numerous flux jumps appeared in the sample extraction magnetometer and the loop width abruptly decreased around 10 T (Figure la). The sample extraction magnetometer seems to cause magnetic instability, because the sample is subjected to the large field change due to the long extraction length. The field change AB amounts to 8 T at 23 T at the maximum. In order to consider the field distribution of a bulk sample, the Maxwell equation in the mixed state for a slab having thickness 2d is introduced as follows

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Figure 1 Magnetization m e a s u r e m e n t s on the QMG-Y1Ba2Cu307 bulk material at (a) 4.2 K and (b) 7 7 . 3 K in the sample e x t r a c t i o n m a g n e t o m e t e r , and at (c) 4.2 K and (d) 7 7 . 3 K in the V S M m a g n e t o m e t e r . The external magnetic fields w e r e applied perpendicular to the c axis and the s w e e p rate w a s O . 5 - 1 T m i n - 1.

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Flux jumps in the magnetization of QMG processed Y~Ba2Cu307: K. Watanabe et al. where an external magnetic field B is applied parallel to the z axis of the slab with the thickness along the x axis, and E is the induced electric field. For aB/at = 10 T s -t and 2 d = 1 mm, as in the sample extraction magnetometer, a large induced electric field of E = 5 mV cm-~ is generated and as a result flux flow appears inside the slab of QMG_Y~Ba2CU3OTm. The usual sweep rate of a magnet is OB/Ot ~ 0.01 T s-~ as mentioned above and this results in an electric field of E = 5/~V cm- ~. The level of the vibrating field change in the VSM is smaller than this sweep rate. It is considered that the flux flow state determined by a resistive J~ measurement corresponds to the order of 1 #V cm -~. Therefore the magnetic flux enters the QMG-Y~Ba2Cu307 bulk during the process of sample extraction. If weak links exist in the QMGYtBa2Cu307 bulk, then the effective size responsible for the magnetization will be changed owing to the magnetic flux invasion. This is why the magnetization hysteresis in the sample extraction magnetometer apparently disappeared at 77.3 K and decreased significantly at 4.2 K. Figure 2 a - d shows the results of the magnetization loops at 77.3 and 4 . 2 - 5 K for B parallel to the c axis measured by both VSM and the SQUID magnetometer. At 4 . 2 - 5 K flux jumps in the magnetization curves were obtained also in both magnetometers, while at 77.3 K no flux jump behaviour was observed. Predictions of magnetic instability for high T¢ superconduct'

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ing oxides with high J~ properties in fields can lead into the regime of adiabatic stabilization similar to that for conventional high J~ superconductorsT: the flux jump phenomena in bulk superconductors come from the high critical current density and the low specific heat at low temperature. The sample extraction magnetometer induced magnetic instability for the QMG-Y,Ba2Cu307 bulk material, but neither VSM nor the SQUID magnetometer can suppress the flux jump at low temperature. The proposed method for preventing flux instability in practice is to use a fine multistructured superconductor at low temperature. The irreversibility magnetic field B~,, at 77.3 K for QMG-YIBa2Cu307 can be derived from Figure 2b and d. In fields higher than B~,, the magnetization is reversible during a cycle of increasing and decreasing field. The irreversibility lines have been addressed in work on thermally assisted flux creep 5 as well as flux-line lattice melting ~. Although the irreversibility in high T¢ superconducting oxides has not been basically interpreted yet, it is clear that the irreversibility line represents J¢ = 0. The obtained results w e r e B i r r = 6 T at 77.3 K for B parallel to the c axis. The slight difference observed between the VSM and the SQUID magnetometer was considered to be due to degradation of the sample caused by repetition of the measurements. A point of interest was a comparison of the magnetic and resistive measurements of irreversibility. This value of B~, was very close to the field of J~ = 0 measured resistively at

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Flux jumps in the magnetization of QMG processed YTBa2Cu307: K. Watanabe et al. 77.3 K for B parallel to the c axis in the QMGY~Ba2Cu307 bulk ~2. Thus one can argue that the parameter B*, which in practice denotes J~ = 0 , derived from transport critical current measurement corresponds to B~, detected in magnetization measurement. Figure 3 shows the magnetization hysteresis at 77.3 K for B both parallel to and perpendicular to the c axis in QMG-YtBa2Cu3OT. The data obtained by VSM are presented. The hysteresis width normalized by that at 1 T is shown as a function of the magnetic field. The magnetization hysteresis, which is proportional to J~, indicates similar profiles to results for YtBa2Cu307 films t3. This suggests that the anisotropic behaviour of the magnetization hysteresis is basically associated with the anisotropic upper critical field B~2. We found that in principle the J~ vs B properties of YIBa2Cu307 at 77.3 K for B perpendicular to the c axis are excellent at high field even in bulk materials, because of the high Be2 value. Figure 4 shows the normalized magnetization hysteresis at 4.2 K. In the field Bp at which the magnetic field just reaches the centre of the slab with thickness of 2d, the following relations are applicable.

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B(T) Figure 3 C o m p a r i s o n of the normalized m a g n e t i z a t i o n hysteresis measured in the V S M at 7 7 . 3 K for B p e r p e n d i c u l a r to the c axis ( ~ ) and B parallel t o the c axis ( [3 )

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order to study the origin of the anisotropic behaviour of J~. It is important to notice that the dimensionality will significantly affect the J¢ character. In our previous study j4, the YIBa2CuaO 7 film with a preferred c axis orientation revealed the field-insensitive J~ properties for B perpendicular to the c axis at 4.2 K. In contrast with the above field dependence of J¢ in a film, the dependence in the bulk material indicated a gradual decrease of Jc with increase of magnetic field. We must consider two different values of Jc encountered experimentally. Resistive measurement gives jab for current flow in the a - b plane, while magnetization hysteresis for B perpendicular to the c axis denotes j~bc including current flow along the c axis. A distinction between j~,b and j~,b, might be responsible for the different anisotropic behaviours in fields, but JgO~-jgbc also can be expected tS. Therefore, we presume that there is imperfection of the c axis alignment in the QMG-Y,Ba2Cu307 bulk. It was considered that this imperfect microstructure might also be related to weak links in high fields.

Summary Flux jump behaviour in YIBa2Cu307 bulk material prepared by the q u e n c h - m e l t - g r o w t h process commonly appears at 4.2 K in magnetization measurements, independent of the experimental methods used, including sample extraction, vibrating sample and SQUID magnetometers. The flux jumps of high Tc bulk materials are essentially due to the high Jc and low specific heat at low temperature. It was found that the irreversibility field B~rr ~ 6 T at 77.3 K measured in the VSM is in good agreement with the field B* at which the Jc value obtained in a resistive measurement practically goes to zero. The anisotropic Jc behaviour of the QMGYtBa2Cu307 bulk for B perpendicular to and parallel to the c axis exhibited points of similarity at 77.3 K in comparison with films having a strong c-axis orientation, whereas at 4.2 K they were very different from those of the films. It was considered that the QMGY~Ba2Cu307 bulk includes some imperfect c-axis alignment.

Flux jumps in the magnetization of QMG processed YIBa2Cu307: K. Watanabe et al. Acknowledgement This w o r k is supported by G r a n t - i n - A i d for Scientific Research on Priority A r e a from the M i n i s t r y o f E d u c a tion, Science and Culture, Japan.

References 1 Watanabe, K., Yamane, H., Kurosawa, H., Hirai, T., Kobayashi, N., Iwasaki, H., Noto, K. and Muto, Y. Appl Phys Len (1989) 54 575-577 2 Watanabe, K., Noto, K., Morita, H., Fujimori, H., Mizuno, K., Aomine, T., Ni, B., Matsushita, T., Yamafuji, K. and Muto, Y. Cryogenics (1989) 29 263-267 3 Matsushita, T., Ni, B., Murakami, M., Morita, M., Miyamoto, K., Saga, M., Matsuda, S. and Tanino, M. Jpn JAppl Phys (1989) 28 L1545-LI548 4 Ekin, J.W., Braginski, A.I., Panson, A.J., Janocko, M.A., Capone If, D.W., Zaluzec, N.J., Flandermeyer, B., de Lima, O.F., Hong, M., Kwo, J. and Liou, S.H. J Appl Phys (1987) 62 4821-4828 5 Yeshurun, Y. and Malozemoff, A.P. Phys Rev Lett (1988) 60 2202 -2205 6 Kes, P.H., karts, J., Vinokur, V.M. and van der Beek, C.J. Phys Rev Left (1990) 64 1063-1067

7 Watanabe, K., Kobayashi, N., Awaji, S., Kido, G., Nimori, S., Kimura, K., Sawano, K. and Muto, Y. Jpn JApp! Phys (1991) 30 LI638-LI640 8 Watanabe, K., Awaji, S., Kido, G., Kobayashi, N., Muto, Y., Kimura, K. and Sawano, K. Supercond Sci Technol (1992) 5 $288-$291 9 Sawano, K., Morita, M., Tanaka, M., Kimura, K., Takebayashi, S., Kimura, M. and Miyamoto, K. in Advances in Superconductivity (ed Kajimura, K. and Hayakawa, H.) Springer-Verlag, Tokyo, Japan (1991) 715-720 10 Yamafuji, K. and Mawatari, Y. Proc 7th US-Japan Workshopon High-Field Superconducting Materials, Wires and Conductors and Standard Procedures for HTSC Wires Testing, Fukuoka (1991) 184-189 l I Gammel, P.L., Schneemeyer, L.F., Waszczak, J.V. and Bishop, D.J. Phys Rev Lett (1988) 61 1666- i669 12 Kimura, K., Morita, M., Tanaka, M., Takebayashi, S., Trouilleux, L., Miyamoto, K., Hashimoto, M., Watanabe, K., Awaji, S. and Kobayashi, N. Cryogenics to be published 13 Watanabe, K., Awaji, S., Kobayashi, N., Yamane, H., Hirai, T., Muto, Y. and Yamashita, T. in: Advances in Superconductivity (Ed Kajimura, K. and Hayakawa, H.) Springer-Verlag, Tokyo, Japan (1991) 473-478 14 Awaji, S., Watanabe, K., Kobayashi, N., Yamane, H. and Hirai, T. Submitted to Jpn J Appl Phys 15 Gyorgy, E.M., van Dover, R.B., Jackson, K.A., Schneemeyer, L.F. and Waszczak, J.V. Appl Phys Lett (1989) 55 283-285

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