MO-IMAGING OF GRANULAR AND STRUCTURED HIGH-T C SUPERCONDUCTORS

Transcription

1 MO-IMAGING OF GRANULAR AND STRUCTURED HIGH-T C SUPERCONDUCTORS Michael R. Koblischka and Anjela Koblischka-Veneva 1 Institute of Experimental Physics, University of the Saarland, P.O. Box , D Saarbrücken, Germany. 2 Institute of Functional Materials, University of the Saarland, P.O. Box , D Saarbrücken, Germany. Abstract: Pecularities observed during the magnetic studies of silver-sheathed (Pb,Bi) tapes revived the interest in the properties of granular superconductors. In this contribution, several observations on granular superconducting materials will be presented, showing also the unique advantages of magnetooptic imaging (MOI) over most other methods for field distribution imaging, and, of course, over integral measurements. The MOI technique enables the direct measurements of critical current densitieswithin individual grains of a granular superconductor. The properties of various high-t c superconductors are discussed. Several different regimes of flux penetration into granular superconductors can be distinguished. The (Pb,Bi)-2223 tapes are found to be an unique system due to the specific temperature dependence of the inter- and intragranular current densities, being "granular" at low temperatures. This is the reason for most of the pecularities observed in integral magnetic measurements. In order to model a granular superconductor, model samples were prepared starting from YBCO thin films, where a hexagonally closepacked structure was patterned. MOI of this type of samples allows to distinguish between the inter- and intragranular properties, and hence, this sample exhibited the desired properties in integral magnetization measurements. Key words: Magneto-optic imaging, granular superconductors 1 1. INTRODUCTION It is very important to understand the properties of granular samples in detail. For many future applications, especially large-scale power T.H.Johansen and D. V. Shantsev (eds.), Magneto-Optical Imaging, Kluwer academic Publishers. Printed in the Netherlands. 71

2 1 applications, this is an important issue as the materials involved will always contain a certain amount of granularity, may it be a bulk piece of meltprocessed material or a (Pb,Bi)-2223 powder-in-tube tape or a YBa 2 Cu 3 O x (YBCO) thick film produced by electrophoretic deposition or by laserablation on a non-single crystalline substrate. Granular high-t c superconductors are characterized by two distinct current densitites [1], the intergranular or transport current, j c J (j inter or j trans ), and the intragranular current density, j c G or j intra. The superscript J indicates that the transport current flows as Josephson current through the grain boundaries within the sample; the superscript G denotes the properties of the grains. Both these currents have different dependencies on the external magnetic field. Furthermore, several experiments are only sensitive to one of these current densities; transport (four-point) resistance measurements measure by definition only j inter ; magnetic measurements are sensitive to the shielding currents, which may change their length scale on increasing external field. By means of AC susceptibility measurements, it is possible to distinguish between j inter and j intra, but only in a narrow temperature range around T c [2]. Several different samples are described in the literature, characterized by the ratio Γ = j intra / j inter = H c1 G / H c1 J [3]. Polycrystalline YBCO samples have a very high j intra, but only a very limited j inter, leading to Γ p 1. Granular Bi samples produced by melt-casting or partial melt-processing show considerably improved transport properties, but still Γ [ 1. Bi-2223 tapes have proven to be the other extreme case with dominating j inter, i. e. Γ > 1. Mechanical deformation (bending) leads to a gradual degradation of the intergranular properties, depending on the bending diameter. In this way, Γ may change from Γ > 1 to Γ < 1 [3,4]. In this contribution, we discuss the features observed in various experiments on granular superconductors, and present the attempt to model the specific properties of (Pb,Bi)-2223 tapes by means of a model sample. 2. MO PATTERNS OF GRANULAR SUPERCONDUCTORS Figure 1 presents MO images obtained on a partially melt-processed Bi sample at 18 K. The details of preparation of this sample are given elsewhere; here it is important to note that the mean grain size of the Bi grains is approx. 60 µm [5]. I a first step, flux enters the sample preferentially through some weak channels. Due to well shielded grains (i.e. with a high intragranular current density) flux amplification between them can be clearly observed. Then in (b) and (c), flux begins to enter also the remaining bulk. Note that the well-shielded areas are larger than the grain size determined by SEM. This is an indication of a well-developed grain 72

3 coupling in these partial-melt processed Bi-2212 ceramics. Finally, in (d) the remanent state corresponding to (c) is shown. Now, the vortices have left from the weak channels firstly, and some flux remains being trapped at well developed pinning sites. This observation makes the remanent states very important to analyse the flux pinning behavior [6]. Figure1. Flux patterns of the partial melt-processed Bi-2212 ceramic at applied fields of 21 (a), 30 (b) and 60 mt (c) at T = 18 K. (d) presents the corresponding remanent state to (c). The marker is 200 µm long. Figure 2 presents another example of a granular superconductor, a polycrystalline YBCO sample. These samples are characterized by elongated grains (a) and the presence of well-defined high-angle grain boundaries, as visualized in the inverse pole figure map by EBSD mapping [7]. As a result, there is only a very weak coupling between the grains and j inter is only small. The MO image (c), therefore, reveals only flux penetrated within some individual grains; the other grains will only be either penetrated by flux at higher fields depending on their respective crystallographic orientation (b), or, are even non-superconducting. MO imaging enables the measurement of the individual values of j intra for each grain. This enables a statistical analysis, e.g. depending on the grain size. This was carried out in Refs. [3,8], where a reciprocal dependence of j intra on the grain size was found. 73

4 1 Figure 2. (a) presents an SEM image of a typical microstructure of a granular, polycrystalline YBCO sample. (b) shows an EBSD mapping of the crystallographic orientations, as indicated in the stereographic triangle. (c) shows the MO image (remanent state, after applying 0.23 T at T = 5 K). Figure 3 shows a different situation, also recorded on a polycrystalline YBCO sample, but doped with a certain amount of K. Even though the microstructure of this sample looks in the SEM quite similar to that of fig. 2, the flux penetrates the sample only through some weak channels; the remainder of the sample is well shielded. In higher applied fields and temperatures, even a penetrating flux front could be observed [9]. 74

5 Figure 3. MO images of a K-doped granular YBCO sample. A field of 100 mt is applied in all cases, but the temperature is raised from 10 K (left) via 30 K (middle) to 77 K (right). The marker is 500 µm long. This observed behavior was only recently explained by means of a careful EBSD analysis [10]. It was found that at a certain concentration of K-doping, a texture [(0 0 1) and (1 0 0)] is introduced during the processing. As a result, a good coupling between most of the superconducting grains is generated, leading to the appearance of a flux-front in the MO images. Silver-sheathed (Pb,Bi)-tapes represent another different species of granular superconductors, showing again some unique properties. As j inter is the matter of optimization and therefore rather high, the parameter Γ is close to 1. However, when observing the flux patterns of tapes at different temperatures, there is a clear change of behavior as illustrated in fig. 4. The left side of the image shows flux patterns in decreasing external magnetic field starting from 150 mt at T = 12 K (see caption), the right column shows the same experiment at T = 77 K. Whereas the flux patterns at T = 77 K reveal a quite homogeneous flux penetration into the sample, the flux patterns obtained at T = 12 K show clear signs of granularity as the flux trapping is quite inhomogeneously. As a result, the tape appears to be granular at low temperatures [4]. This behavior can be explained regarding the temperature dependencies of j intra and j inter. In Bi-2223, j intra is raising considerably at temperatures below 25 K, and reaches values similar to that of YBCO at T = 5 K. In contrast, j inter is only weakly dependent of temperature. As a consequence, the corresponding Γ values are changing from about 100 to about 10, with the result that the flux patterns reveal granularity at low temperatures. The coupling between the grains and also between the filaments in multifilamentary tapes can be quite strong depending on the processing conditions and geometry, so that even the flux patterns of a multifilamentary tape may appear as uniform. Therefore, the (Pb,Bi)-2223 tapes are a specific type of granular superconductors, offering many interesting phenomena for basic studies. 75

6 1 Figure 4. MO experiments on (Pb,Bi)-2223 tapes. Left: T = 12 K. The field is reduced from 150 mt to 60 mt (a), 30 mt (b), 18 mt (c), 6 mt (d) to 0 mt (e). The right column shows the same experiment at T = 77 K. Images (a,b) are reworked by image processing in order to enable a direct comparison the left part of the image is the original image. At 12 K, signatures of granularity are well visible, whereas at 77 K, the MO patterns appear to be homogeneous. The arrows mark the edge of the tape. 76

7 3. MODEL SAMPLE To achieve a better understanding of these rather complex material systems, it is essential to prepare and study appropriate model systems. For such a purpose, patterned YBCO thin films are an ideal starting point as Figure 5. Flux patterns of the model sample. Left: Observation on the entire sample, revealing the flux pattern in the "effective medium", (a) 6 mt, (b) 18 mt. Two defects with the according d - -lines are also visible. (c) shows a schematic drawing of a rectangular superconductor with the corresponding d-lines. Right: MO images with higher resolution showing the flux penetration into the individual disks. 77

8 1 (i) the use of YBCO allows studies in a wide temperature range, almost identical to the materials modelled, (ii) YBCO thin films on single crystalline substrates can be prepared and patterned in excellent quality without any defects disturbing the flux distributions, and (iii) the presence of a substrate yields model samples which can be easily handled in the experiments [11]. The resulting model sample could prove that it is capable to reproduce e.g. the anomalous position of the central peak in the magnetization loops, which was the original reason for creating this type of sample [12]. Furthermore, it became possible to study the flux patterns in an "effective medium" as revealed by the appearance of the so-called d-lines in fig. 5. By means of variation of the contact widths between the "grains" respective disks, the coupling strength can be varied. Furthermore, it was found that this sample also exhibits the paramagnetic Meissner effect due to flux trapping [13]. In summary, MO imaging represents an unique tool to study the properties of granular superconductors in great detail. The MO results can easily be linked with other data e.g. obtained from EBSD [7,14], thus enabling a complete picture to be formed. REFERENCES 1. J. R. Clem, Physica C , 50 (1988). 2. K.-H. Müller, Physica C 159, 717 (1989). 3. M. R. Koblischka, Th. Schuster and H. Kronmüller, Physica C 219, 205 (1994). 4. M. R. Koblischka, T. H. Johansen, H. Bratsberg and P. Vase, Supercond. Sci. Technol. 12, 113 (1999). 5. M. R. Koblischka, S. L. Huang, K. Fossheim, T. H. Johansen, and H. Bratsberg, Physica C 300, 207 (1998). 6. M. R. Koblischka, A. Das, M. Muralidhar, N. Sakai, and M. Murakami, Jpn. J. Appl. Phys. 37, L1227 (1998). 7. A. Koblischka-Veneva and M. R. Koblischka, in: "Studies of high-temperature superconductors", ed. A. V. Narlikar (Nova Science, New York, 2002) Vol. 41, p N. Moser, M. R. Koblischka, and H. Kronmüller, J. Less Common Metals , 1308 (1990). 9. A. Koblischka-Veneva, M. R. Koblischka, and M. Murakami, Physica C , 201 (2001). 10. A. Koblischka-Veneva, M. R. Koblischka, P. Simon, F. Mücklich and M. Murakami, Physica C 382, 311 (2002). 11. M. R. Koblischka, L. Pust, A. Galkin, P. Nalevka, M. Jirsa, T. H. Johansen, H. Bratsberg, B. Nilsson, and T. Claeson, Phys. Rev. B 59, (1999). 12. M. R. Koblischka, L. Pust, A. Galkin, and P. Nalevka, Appl. Phys. Lett. 70, 514 (1997). 13. M. R. Koblischka, L. Pust, N. Chikumoto, M. Murakami, B. Nilsson, and T. Claeson, Physica B , 599 (2000). 14. M. R. Koblischka and A. Koblischka-Veneva, Physica C , 545 (2003). 78