Anisotropic magnetization and weak links in melt textured YBa2Cu3O7

Anisotropic magnetization and weak links in melt textured YBazCu307* V. Gomis, I. Catalan, F. P6rez, B. Martfnez, J. Fontcuberta, A. Fuertes and X. Obradors Institut de Ci~ncia de Materials de Barcelona, Barcelona, CSIC, 08193 Bellaterra, Spain

Campus Universitat Aut6noma de

Textured YBa2Cu307 superconducting samples have been prepared by melt texturing growth (MTG) under variable conditions. We have investigated low field magnetic susceptibility (d.c. and a.c.), high field hysteresis loops and magnetic irreversibility through ZFCFC temperature scans. We show that low field flux penetration effects, measured by screening capability and dissipative behaviour, may be convenient to detect residual weak links and to characterize the degree of texturation when their anisotropic behaviour is analysed. Critical currents up to 2 x 105 A cm -2 at 1 T and 25 K and for H II c have been obtained and their temperature and field dependence determined. We find that enhanced critical currents in the H II c configuration also lead to an increase in the magnetic anisotropy, which probably reflects reduced misorientation of the crystallites within a domain.

Keywords: YBCO; critical currents; magnetization

The development of practical applications of bulk high temperature superconductors has a serious drawback in the small critical currents observed in materials prepared by conventional ceramic processing. The most promising advance to overcome the limitation associated with sintered YBa2Cu307 ceramics is the melt-texturing process developed by several authors in different forms ~, but always involving the preparation of YBa2Cu307through a peritectic reaction of the Y2BaCuO5 (211) phase and a liquid based on BaCuO2-CuO mixtures. The main goal of this technique is to decrease, or even eliminate, the weak links existing at the grain boundaries of sintered ceramics. Critical current densities in excess of 104 A cm-2 in magnetic fields of a few Tesla at 77 K have been achieved in these melt growth materials and so they may be considered as very promising for practical applications. It is important to stress, however, that these materials have a complex microstructure which needs to be investigated in connection with its transport properties to understand the mechanisms limiting their critical currents, which up to now are still much lower than those observed in thin films. One of the methods to investigate the superconducting properties of these high Jc superconducting materials is through magnetic measurements, which avoid all the contact heating problems appearing in transport critical current measurements-', and allow easy investigation of their anisotropic character. *Paper presented at the c o n f e r e n c e 'Critical Currents in High Tc S u p e r c o n d u c t o r s ' , 2 2 - 24 April 1992, Vienna, Austria

In this work we report an investigation of the magnetic properties of several melt-textured YBa2Cu307 ceramics, giving special emphasis on the relationship between their anisotropic magnetic character, the presence of the residual weak links in the sample and the critical currents. The complex microstructure of melt-textured YBa2Cu307 ceramics has been investigated by several authors 3. First a domain structure develops with high angle grain boundaries which finally behave as weak links. There also exists a complex subdomain structure with planar defects (twin boundaries, cracks, stacking faults, low angle grain boundaries, dislocations, etc.) and non-planar defects (211 phase precipitates, etc.) A detailed analysis of the influence of these defects on the superconducting properties of melt textured ceramics is still lacking, although if it seems clear that an enhancement of flux pinning is achieved by an increase in the concentration of the 211 phase precipitates. Our present work will show that a clear correlation seems to exist between the increase in the critical current density, the enhanced magnetic anisotropy and the reduction of weak-link effects evidenced through flux screening and flux penetration measurements.

Experimental The samples used in this work were prepared through a melt texturing process previously described 4. Essentially the, method involves a slow cooling step (1 °C h -l) through [~e peritectic line using a furnace which has a variable temperature gradient (up to 25°C cm -~) and no

0011 - 2 2 7 5 / 9 3 / 0 1 0 0 3 9 - 0 7 © 1993 B u t t e r w o r t h - Heinemann Ltd

Cryogenics 1993 Vol 33, No 1

39

Anisotropic magnetization and weak links: V. Gomis et al.

moving parts. Typical bar dimensions were 3 x 1 x 0.4 cm 3 and they were sustained horizontally with a configuration minimizing contamination from the sample holder. The starting ceramics were either prepared by conventional solid state reaction or obtained commercially, but they had in all cases a YBa2Cu307 stoichiometry. The ceramics were characterized by X-ray diffraction and optical and electron microscopy. Semiquantitative chemical analysis was performed by EDX measurements in a scanning electron microscope. A complete report of the microstructure of these samples will be published separately ~, but we may mention that they contain quasi-spherical diluted 211 precipitates with diameters ranging from 4/~m to 12 ~m and a maximum concentration of about 5 - 1 0 % volume. Optical microscopy also showed that a domain structure develops with sizes up to 1 cm. In this work four different samples will be investigated which essentially differ in the concentration of high angle grain boundaries (i.e. the size of the quasi-single crystalline domains) and the degree of texture. X-ray diffraction pole figures of samples S1 and J4 were previously reported (samples A and B respectively in Reference 4) where it was shown that S1 has a quasi-isotropic distribution of domains while J4 has a very strong c axis texture. In Table 1 we list the samples investigated in this work together with their basic microstructural characteristics (i.e. polydomain or single domain). To allow comparison of magnetization data for different sample orientations with respect to the magnetic field, the bars were cut to a 1 x 1 x 1 mm 3 size. A.c. susceptibility measurements (X' and X") were performed at low fields (1 /zT-1 mT) with a Lake-Shore susceptometer at a fixed frequency ( v = 111 Hz). D.c. magnetic susceptibility and isothermal magnetization measurements were carried out using a SQUID magnetometer (Quantum Design) in fields up to 5.5 T. Field inhomogeneity effects were minimized by using a scan length of 2 cm. The critical currents jab (H II c, currents flowing within the ab plane) were evaluated from the hysteresis loops using the Bean J~ =20AM/ (t(1 - t/31)) expression, where Jc is in A cm -2 and AM in Gauss and t and I are the dimensions of the basal plane of the parallelepiped 6 (t < I). The irreversibility line was determined from d.c. magnetization measurements obtained at constant field and scanning the temperature under ZFC-FC conditions.

Results Low field d.c. and a.c. susceptibility

Temperature-dependent low field susceptibility measurements are very useful to identify whether a textured sample has a simple domain character or not (i.e. if it contains any residual high-angle grain boundary) and if a high degree of texturation has been achieved. In Figure 1 we show the temperature dependence of diamagnetic shielding (x'(T)) and dissipation (X"(T)) measured at several excitation fields, for the different samples in Table 1. A clear difference exists among the single domain (I1C, I1F) (Figures la and/d) and the polydomain samples (S1, J4) (Figures lb and lc). In the first case no double peak is observed in the out-of-phase

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C r y o g e n i c s 1 9 9 3 Vol 33, No 1

Table 1

Microstructure of the samples

Sample

Microstructure

Jc (Acm 2) 77 K, 0 T

$1 J4 I1C I1F

Polydomain Quasi-single Single domain Single domain

500 3 X 10 3 7 × 10 3 6 × 10 3

component, complete diamagnetic shielding is obtained at much higher temperatures and there is a weaker dependence of both components of the susceptibility on the amplitude of the ripple field. The minimum transition width is found for the IIC sample, having A T = 0 . 1 K at 10/~T and A T = I . S K at 10roT. No double steps may be observed in the in-phase component of the a.c. susceptibility when they are single domain. The strong dependence of x'(T) and X"(T) on the measuring a.c. field observed for the S1 sample reveals a weakly connected multidomain structure. The J4 sample appears to have either a stronger connectivity or a less important multidomain microstructure. These measurements confirm our microstructural observations (Table 1) and can be used as a convenient reference of the quality of the melt textured samples. A further step in the magnetic characterization of the textured superconductors may be achieved by looking at the anisotropic behaviour of the a.c. susceptibility. When the magnetic field is applied along the c axis the shielding currents flow within the ab plane, while in the H II ab configuration there exist components within the ab plane and along the c axis. The existence of planar defects (cracks, low angle grain boundaries) could have a strong influence over these shielding currents and hence it is worthwhile to compare the magnetic anisotropy in melt textured ceramics and single crystals. In Figures 1 and 2 we report x'(T) and X"(T) components of the complex susceptibility for the single domain samples when the magnetic field is either along the c axis (O = 0 °) or perpendicular to it (O = 90°). Data from Figures 1 and 2 reveal the more anisotropic character for the I1C sample with respect to the I1F one, which probably marks a different degree of texturation of both samples. A quantitative estimate of their anisotropy may be achieved by evaluating the irreversibility line as defined experimentally by Neumann et al. 7, through the extrapolation of X"(T) to zero (Figure 2). Whereas in sample I1C the irreversibility line in both directions is separated by 1.5 K, in I1F the separation reduces to 0.5 K. As we will see in our high field magnetization data, the enhanced anisotropy of the irreversibility line matches an increase in the irreversible magnetization and finally also leads to an enhancement of the critical current of the IIC sample when H Uc. Note that the higher anisotropy occurs for the sample having the weaker ripple field dependence of the in-plane component of x'(T). It is also straightforward to note that such a simple a.c. susceptibility measurement may also show how different a sample is from a single crystal by looking at the first penetration field (either in x'(T) or X"(T)) which in the absence of weak links effects and surface pinning should measure the lower critical field Hc~. In the I1C sample,

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for instance, we may observe (Figure 2) that for H II c, the penetration of magnetic flux at 1 mT starts at T -= 87.2 K. This value is very near to the lower critical field measured in YBa2Cu307 single crystals 8 and a similar pattern is found for the H IIab configuration. In the I1F sample the dissipation always starts before this. The flux penetration in the single domain samples (I1C and I1F) was also investigated by means of d.c. susceptibility at several magnetic fields. As may be observed in Figure 3, a clear difference exists concerning the temperature where complete magnetic screening (ZFC curves) is achieved in these samples in a field of 12 mT. This experimental method was actually used by Krusin-Elbaun et al. 8 to determine the lower critical field Hc~ in single crystalline samples. When used in textured ceramics it makes it possible to estimate when the penetration of the magnetic flux starts and hence give

clear indications of the electrical connectivity between the grains through the different crystalline defects. Our results (Figure 3) give further evidence of a weaker crystallite coupling in I1F samples, probably due to an increased misorientation of the crystallites which form every single domain 3. As a final remark, concerning our low field susceptibility investigation of a melt textured YBazCu307 superconductor we would like to stress a surprising result, i.e. the non-observance of a strongly anisotropic weak link behaviour in the single domain samples, which one should expect in view of the presence of a considerable amount of cracks perpendicular to the c axis 3'5. These cracks should behave as current barriers or weak links when current flow perpendicular to the ab plane and should be more apparent in X,c(T) than actually observed.

Cryogenics 1 9 9 3 Vol 33, No 1

41

Anisotropic magnetization and weak links: V. Gomis et al. 0.20

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A plausible explanation may be found if one imagines that these currents have a percolative character around the cracks, thus allowin~g them to avoid all the nonsuperconducting barriersfl

Critical currents and irreversibility line In Figure 4 we show the experimental hysteresis loops for the four samples previously characterized. The critical currents were obtained from the hysteresis loops assuming that the induced supercurrents flow along all the specimen. Actually when the critical currents are calculated with the Bean model it is implicitly assumed that there are no granular effects in the sample for the reported field ranges. We can check the assumption that

42

-1.00

C r y o g e n i c s 1 9 9 3 Vol 33, No 1

the characteristic length where currents are flowing is given by the sample dimensions by fitting all the M(H) curve to the predictions of the Bean model. It can be seen 9 that the downward part of the M(H) curve after the maximum field has been reached can be explicitly written in terms of the critical current and the effective sample dimension (D) where currents flow. In Figure 5 we show the high field part of the decreasing M(/-/) cycle measured for the J4 sample at 5 K. In single-domain samples, the fit of the M(H) curve of Figure 5 can be successfully made by using a single set of parameters (Jc and D) thus showing that, indeed, the Bean model can be used. Moreover, by using the Jc value extracted through the conventional Bean formula from the lower field region of the hysteresis loop, it turns out that D ~ 1 ram, in good agreement with the geometrical size of the sample. In this way, the internal consistency of our determination of Jc is stressed. Similar good fits are obtained for the single domain I 1C and I 1F samples. For the $4 samples a similar procedure does not allow the data to be fitted by using a single set of (Jc, D) parameters, thus showing that for this sample the effective size changes with field. This is further evidence of the multidomain character of this sample. The temperature and field dependences of the critical currents evaluated through the Bean model are represented in Figure 6, which shows that our best sample (I1C) has values of J~ = 2 x 105 A cm -2 at 1 T and 25 K. The field dependence of Jc is very weak at low temperatures but becomes much higher, even with anomalous 'peak effects', at higher temperatures (Figure 7). At 77 K we get critical current values approaching 104 A cm -2. The low temperature critical currents are very similar to those reported for YBa2Cu307 samples prepared by the QMG method, even when additional 211 phase precipitates are included ~°, but they become slightly lower when increasing temperature to T = 77 K. This is probably a consequence of a reduced concentration of 211

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precipitates which act as flux pinning centres and hence thermal activation processes become more important. This is, however, a point that deserves further investigation. It is important to stress also that the same trend revealed through the low field susceptibility is followed when the critical currents of the different samples are compared (Table 1 and Figure 4). The critical currents calculated through the critical state model become higher when no granularity effects are observed in the low field susceptibility measurements and when the anisotropy of the susceptibility is larger. We may further test this correlation between anisotropy and critical currents by recording high field hysteresis loops with the magnetic field along the c axis and perpendicular to it in the single domain samples (J1C and J1F). As described by Gyorgy et al. 6 the

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extended Bean model must be used to evaluate the critical currents in such a magnetic field orientation to obtain J,."h.b (H IJ ab, currents within the ab plane) and J,.'.,,h (H II ab, currents along the c axis). It is often ,.., are the same when J,.~.ab has to assumed that J,,. 1,,hoh and j~b be evaluated, however, as has been shown by Fan et al.~l, these values may differ considerably and hence we prefer to compare in this paper only the irreversible magnetization, instead of the calculated critical currents. In Figure 7 we show the hysteresis loops measured at 77 K for O = 0 ° and O = 90 ° . From this figure it is clear that the I1C sample has an enhanced magnetic irreversibility with respect to I1F. It is straightforward to note

Cryogenics 1 9 9 3 Vol 33, No 1

43

Anisotropic magnetization and weak links: V. Gomis et al. 0.020 8 c==== (3=0 ¢¢00¢ (3=90

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also that these figures display very clearly the anisotropy of the irreversibility lines in melt textured YBa2Cu307, while the irreversibility vanishes for H II c at our highest field. AM remains fairly constant for H II ab and hence a critical current quasi-independent of the field might be inferred from the measurements. Moreover, the increased critical current in the I1C sample for H II ab suggests that the intrinsic pinning effects 12 are more effective in this sample, and this is indeed what must be expected in a sample having a stronger texturation ~3. What is more remarkable, however, is that the M(/-/) cycle for the I1F sample (O = 90 °) becomes reversible at much lower fields than the corresponding one for the IIC sample. In addition, for the IIF sample both M(H) loops close almost at the same field. Therefore, we can again conclude that the I1C sample is more anisotropic

44

Cryogenics 1993 Vol 33, No 1

and that magnetic anisotropy goes in parallel with higher measured critical currents. It is to be noted there is an anomalous field dependence of the irreversibility magnetization in the I1C single domain sample, where a well pronounced maximum is observed at H = 1 T. This anomalous peak structure is also observed at intermediate temperatures (45 K). Similar current enhancement has been reported by Daeumling et al. ~t in YBa2Cu307 single crystals. These authors suggested that this enhanced flux pinning at higher fields arises from field-induced granular regions which become pinning centres. The necessary inhomogeneity of the samples could arise from small oxygen deficiencies, typically observed in YBa2Cu307 single crystals, where the oxygenation process is very slow. At the present stage of our experiments it seems premature to ascertain if this is indeed the origin of the magnetization anomalies in our melt-textured superconductors. Although other authors have also shown ~5 that there is a close correlation among the anomalies and the oxygen contents in single crystals, it is also important to consider that the melt-textured ceramics contain several defects which could also contribute to this enhanced pinning at intermediate fields. For instance, we show in Figure 7a that this effect is nearly absent in a textured sample having a lower critical current (I1F) which probably has the same degree of oxygen homogeneity. We feel that further analysis of these anomalies in the melt-textured ceramics is necessary to determine their true microstructural origin. Finally it is also instructive to characterize our melt texture ceramic by looking at the irreversibility line which, as has been shown by several authors 1°'16, may be strongly displaced when flux pinning centres (211 precipitates, or ion or neutron irradiation induced defects) are introduced. We have measured the irreversibility line in the I1C sample when the magnetic field is applied along the c axis by looking at the beginning of the magnetic irreversibility in constant field temperature scans. We show in Figure 8 the values obtained in our I1C sample together with those reported by Murakami et al. l0 in a sample prepared by QMG processing and containing 10% in volume of additional 211 phase. As may be observed, our irreversibility line does appear slightly displaced at lower fields when compared with that of Murakamil°. Our irreversibility line approaches, however, that measured by Wacenovsky et al. l° in a sample also prepared through the QMG technique, but in this case the irreversibility line was measured through a.c. susceptibility. In conclusion, we may say that our simpler sample preparation procedure may lead to irreversibility lines very similar to those associated with QMHG processing techniques. In summary, we have investigated several melt textured YBa2Cu307 samples having different critical currents. We have shown that the primary limitations in the critical currents arise from the presence of a multidomain structure which may be easily detected through low field susceptibility measurements. We have also shown that the textured samples having the highest critical currents have an enhanced magnetic anisotropy and the weak link effects, detected through low field susceptibility, have been completely eliminated. This is an interesting correlation which may be useful in fully

Anisotropic magnetization and weak links: V. Gomis et al.

H//c axis

10

~5

7850; Murakami, M., Morita, M., Doi, K., and Miyamoto, K. Jpn J Appl Phys (1989) 7 1189; Salama, K., Selvamenickam, V., Gao, L. and Sun, K. Appl Phys Lett (1989) 54 5 2 Ekin, J.W., Salama, K. and Selvamanickam, V. Appl Phys Lett (1991) 59 360 3 Alexander, K.B., Goyal, A., Kroeger, D.M., Selvamaniekam, V. and Salama, K. Phys Rev B (1992) 45 5622 Kimnra, M., Tanaka, M., Horiuchi, H., Morita, M., Matsuo, M., Morikana, H. and Sawano, K. Physica C (1991) 174 263

4 Cataldn, I., Obradors, X., Fuertes, A., P~rez, F., Martinez, B., Gomis, V., Fontcuberta, J., Germi, P., Pernet, M. and

tn

:==== I1F

"~,~_

~

I

80

85

90

Temperature (K) Figure 8 Irreversibility line of the I1F sample for H U c. The data from Murakami's sample non-doped with additional 211 phase 1° is shown for comparison

Chateigner, D. Proc lnt Conf Mod Aspects Supercond: 1. C.M.A.S. 91 (Eds Raveau, B., Wasa, K. and Suryanaryanan, R.) I.I.T.T. International, Paris (1991) 223 5 Catahin, I., Obradors, X. and Fuertes, A., to be published 6 Gyorgy, E.M., van Dover, R.B., Jackson, K.A., Schneemeyer, L.F. and Waszczak, J.V. Appl Phys Lett (1989) 55 283 7 Neumann, Ch., Ziemann, P. and Geerk, J. Europhys Len (1989) 10 771; Heinzel, Ch., Neumann, Ch., Ziemann, P. and Geerk, J. Europhys Lett (1990) 13 531

8 Krusin-EIbaun,

L.,

Maiozemoff,

A.P.,

Yeshurun,

Y.,

Cronemeyer, D.C. and Holtzberg, F. Phys Rev B (1989) 39 2936; Umezawa, A., Crabtree, G.W., Welp, U., Kwok, W.K. and Vanderwoort, K.G. Phys Rev B (1990) 42 8744 9 Cave, J.R. and Critchlow, P.R. IEEE Trans Magn Mater (1991) 27 1379; Cave... J.R. Supercond Sci Technol (1992) 5 5399

10 Murakami. M., Fujimoto, H., Gotoh, S., Yamaguehi, K.,

understanding the role played by the different defects in the transport properties of these superconductors. Further analysis of this point is now in progress.

ll

Acknowledgements

12

This work has been supported by the Spanish MIDAS program, CICYT (MAT90-1020) and the ECC. VG and IC have also been supported by a DGICYT programme of doctoral fellowship.

13 14 15

References

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Koshizuka, N. and Tanaka, S., Physica C (1991) 185-189 321; Wacenovski, M., Milerch, R., Weber, H.W. and Murakami, M. Physica C (1991) 185-189 2501 Fan, H.C., Zhang, Y.T., Jin, X., Lu, M., Ji, H.L., Xu, X.N., Yao, X.X., Ren, H.T., Xiao, L. and He, Q. Physica C (1991) 185189 2331 Tachiki, M. and Takahashi, S. Solid State Comm (1989) 70 291; Kwok, W.K., Welp, U., Vinokur, V.M., Fleshier, S., Downey, J. and Crabtree, G.W. Phys Rev Lett (1991) 67 390 Selvamanickam, V., Fosster, K. and Salama, K. Physica C (1991) 178 147 Daeumling, M., Seuntjens, J.M. and Larbalestier, D.C. Nature (1990) 346 332 Osofsky,M.S., Cohn, J.L., Skelton, E.F., Miller, M.M., Sonlen, Jr., R.J. and Wolf, S.A. Phys Rev B (1992) preprint Ciballe, L., Marwick, A.D., Worthington, T.K., Kirk, M.A., Thompson, J.R., Krusin-Elbaun, L., Sun, Y., Clem, J.R. and Hoitzberg, F. Phys Rev Lett (1991) 67 648; Weber, H.W. Physica C (1991) 185-189 309

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