- Email: [email protected]
Observation of the flux line lattice and crystal defects in Bi–Sr–Ca–Cu–O single crystals
Physica C 392–396 (2003) 353–358 www.elsevier.com/locate/physc
Observation of the flux line lattice and crystal defects in Bi–Sr–Ca–Cu–O single crystals K. Furusawa
a,b
, Y. Zhao c, N. Chikumoto b,*, K. Kishio d, T. Nagatomo a, M. Murakami b
a
d
Shibaura Institute of Technology, 3-9-14, Shibaura, Minato-ku, Tokyo 108-8548, Japan b SRL-ISTEC, 1-16-25, Shibaura, Minato-ku, Tokyo 105-0023, Japan c Superconductivity Research Group, School of Materials Science and Engineering, University of New South Wales, P.O. Box Sydney, NSW 2052, Australia Department of Superconductivity, Faculty of Engineering, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113, Japan Received 13 November 2002; accepted 25 December 2002
Abstract We have observed flux line lattice (FLL) structure in two different types of Bi2 Sr2 CaCu2 O8þd (Bi-2212) single crystals with different pinning by using a Bitter decoration technique. We have also observed the microstructure and analyzed local chemical composition with wave dispersive spectroscopic analyses. In a low Jc sample, we found two types of line defects, one with Ca-rich and the other with Ca-deficient. These defects extended almost parallel to the growth direction, which caused a distortion in the FLL structure. In contrast, no such defects were observed in the matrix in a high-Jc sample, and a relatively ordered FLL was observed. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.60.Ge; 74.62.Dh; 74.72.Hs Keywords: Line defect; Bitter decoration technique; Magneto-optical imaging; Bi-2212
1. Introduction In ideal type II superconductors, ordered hexagonal flux line lattice (FLL) are formed. In real superconductors, however, the FLL is disordered due to the presence of various crystal defects that act as pinning centers of quantized fluxoids. Since flux pinning directly determines the critical current
*
Corresponding author. Address: SRL-ISTEC, 10-13 Shinonome 1-chome, Koto-ku, Tokyo 135-0062, Japan. E-mail address: [email protected] (N. Chikumoto).
density, the observation of FLL togeher with the microstructural characterization has been performed to reveal the pinning mechanism [1–3]. Recently, several groups discovered [4–7] the presence of line defects extending parallel to the growth direction in Bi2 Sr2 CaCu2 O8þd (Bi-2212) single crystals grown by a traveling solvent floating zone (TSFZ) method. The pinning-induced disorders of the FLL at such defects have been observed with a Bitter decoration technique [6,7]. Magnetooptical (MO) imaging and energy dispersive spectroscopy (EDS) have revealed that similar defects with Ca deficiency act as weak-links [4]. However,
0921-4534/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-4534(03)01230-9
354
K. Furusawa et al. / Physica C 392–396 (2003) 353–358
the correlation between the microstructure of these defects and magnetic properties has not yet been clarified. In this report, we present a more detailed investigation of such line defects in Bi2 Sr2 CaCu2 O8þd (Bi-2212) single crystals grown by the TSFZ method by means of a Bitter decoration technique. It was found that flux distributions in such defects are different, which depends on the localized chemical compositions. We also found that the defect morphology in the crystal is largely affected by the starting composition. We compare the FLL structure and crystal defect structures in two kinds of crystals and discuss an observed difference in Jc –B performance.
2. Experimental Two kinds of Bi-2212 single crystals, samples A and B, grown with different starting compositions were prepared with a TSFZ method. Details of the crystal growth conditions are described elsewhere [8,9]. Starting compositions and chemical compositions of the crystals are presented in Table 1. The chemical compositions of the grown crystals were determined by inductively coupled plasma (ICP) analyses. All the data were normalized by the chemical composition of Cu. After cleaving along the (0 0 1) planes to a final thickness of several tens of micrometers, the samples were mounted on a Cu-plate using Ag-paste. The morphology and composition of samples were investigated by mean of an electron probe micro analyzer (EPMA) equipped with a wave dispersive spectroscopy (WDS) analyzer (JEOL, JXL-8800R/RL). Magneto-optical (MO) imaging technique was employed to study macroscopic flux line distributions [10]. In this technique, a Bidoped yttrium iron garnet film with in-plane
magnetization was used as an indicator. Bitter decoration experiments have been carried out using Ni particles at 4.2 K after field cooling in the presence of a magnetic field (60 G) perpendicular to the a–b plane. Flux line patterns were observed with a scanning electron microscope (SEM).
3. Results and discussion Fig. 1(a) and (b) show compositional images of two different types of line defects that are observed as bright (denoted as L1) or dark contrast (denoted as L2) in sample A. EMPA analyses confirmed that these are non-stoichiometric line defects extending parallel to the growth direction. From characteristic X-ray images of each element using Bi-Ma, Sr-La, Ca-Ka, and Cu-Ka intensities in WDS, we found Bi-enrichment and Ca-deficiency in the L1 region and Ca-enrichment in the L2 region. It was also found that these two types of line defects were often observed together and were not annealed out. In Fig. 2, we show the decoration pattern of sample A at 4.2 K field cooled in the presence of 60 G. The flux line positions are indicated as bright spots (Ni deposits) on the black background. Insets are the fast Fourier transformation (FFT) images that the vector basis is associated with the average lattice spacing. Fig. 2(a) shows the image of the FLL in the defect-free region. One can observe an ordered hexagonal FLL in this region. In Fig. 2(b) and (c), we display the FLL observed in the respective defect regions shown in Fig. 1(a) and (b). Due to the presence of line defects, one sees that the FLL structures in both L1 and L2 regions are totally different from that in the defect-free region. The FFT image is not ordered and has an amorphous structure in the L1 region, while the FLL in the L2 region has a hexagonal order with
Table 1 Starting composition and chemical composition for Bi-2212 crystals used in this study Sample ID (Tc ) A (85 K) B (89 K)
Starting composition
Analyzed composition
Bi:Sr:Ca:Cu (feed)
Bi:Sr:Ca:Cu (solvent)
Bi:Sr:Ca:Cu
2.1:1.8:1.0:2.0 2.1:1.9:1.0:2.0
– 2.5:1.9:1.0:2.6
2.21:1.91:1.01:2.00 2.03:2.08:0.85:2.00
K. Furusawa et al. / Physica C 392–396 (2003) 353–358
355
Fig. 1. Compositional image of (a) Ca-deficient line defect that corresponds to the bright contrasted region and (b) Ca-rich line defect that corresponds to the dark contrasted region.
small scattering. From FFT images, the average spacing of the FLL was determined to be 0.571 and 0.629 lm for the defect-free region and the L2 region, respectively. (The FLL spacing for 60 G is 0.631 lm.) It should be noted that the average spacing in the L2 region is larger than that in the defect-free region, which suggests that the FLL spacing changes in the Ca-rich region. Fig. 3 present the field distribution of sample B at 10 and 20 K observed with an MO imaging technique. The sample is zero-field cooled to the target temperature, and the external field was applied parallel to the c-axis. The optical micrograph of the sample shown in Fig. 3(a) reveals that the sample surface is completely flat and no observable defects are present. Fig. 3(b) is the remnant state after applying a maximum field of 200 mT at
Fig. 2. SEM images of the decoration pattern at (a) defect-free, (b) Ca-deficient and (c) Ca-rich regions after field cooling to 4.2 K in Bex ¼ 60 G. Inset is the corresponding FFT image, showing (a) an ordered hexagonal lattice, (b) amorphous and (c) overlapping of double ordered hexagonal lattices. White arrows indicate the growth direction of the sample.
10 K, while Fig. 3(c) is the remnant state after applying a maximum field of 200 mT at 20 K.
356
K. Furusawa et al. / Physica C 392–396 (2003) 353–358
Fig. 3. (a) The optical micrograph of sample B. (b) Flux distribution in the remnant state after applying a maximum field of 200 mT at 10 K. (c) Flux distribution in the remnant state after applying a maximum field of 200 mT at 20 K (c). The scale bar is 0.25 mm.
Bright regions correspond to high field areas. The scale bar in the figure is 0.25 mm. One may note that flux distribution is rather uniform at 10 K. In contrast, it is quite interesting to note that the flux distribution at 20 K is obviously inhomogeneous: sheet-like flux penetrated regions and sheet-like flux free regions also appear roughly perpendicular to the growth direction, which is parallel to the longer edge of the sample. In spite of a homogeneous surface morphology observed in an optical
micrograph (Fig. 3(a)), this result suggests that two kinds of sheet-like regions with different pinning properties lie alternately perpendicular to the growth direction. In our experiment, these lamellae-like structures of flux pattern were observed over wide temperature range of 15–70 K. Fig. 4(a) shows the FLL pattern at 4.2 K field cooled in 60 G applied perpendicularly to the a–b plane. One can find that the observed flux line pattern is in good agreement with the results of MO imaging. Sheet-like structures appear as undulation pointing to the growth direction in the image. We observed the ordered FLL in the whole area of the image. We also observed line-like structures with size comparable to the FLL distance that extend toward almost parallel to the line-like structures observed in an MO image presented in Fig. 4(b). We performed FTT in two regions with (i) and without (ii) line-like flux structure. As seen from Fig. 4(i), the average spacing of FLL for bright and dark contrasted regions is slightly different. In addition, the distance between line structures that lie parallel to the growth direction is approximately twice the average spacing of FLL in Fig. 4(ii). WDS revealed that a bright region corresponds to a Bi- and Srrich phase where the average spacing of FLL is small, while the dark region is Cu- and Ca-rich phase where the average spacing of FLL is large. We also found that Ca-deficient and Ca-rich phases correspond to a strongly pinned region and a weakly pinned region, respectively. From these observations, we can deduce the temperature dependence of superconducting properties in sheet-like flux structures observed in an MO image. The superconducting condensation energy between sheets is not so different at 10 K. Hence we observed almost uniform distribution of flux lines in an MO image (Fig. 3(b)). At 20 K, however, the difference in the condensation energy between sheets become large, and flux pattern starts to show sheet-like structures (Fig. 3(c)). In the decoration pattern, we can see the difference in the pinning strength in the region with different chemical compositions even at 4.2 K. Flux lines were frozen in strongly pinned region where Ca and Cu are rich, and accumulated at those regions.
K. Furusawa et al. / Physica C 392–396 (2003) 353–358
357
Fig. 4. (a) SEM photograph of the decorated sample after field cooling to 4.2 K in Bex ¼ 60 G. Note that closely packed Ni deposits formed a sheet-like structure. A white arrow indicates the crystal growth direction. (b) Enlarged SEM photograph of the region surrounded by white lines in (a). (i) FFT image and line profiles obtained from the region (i) surrounded by white straight lines in (b) indicating double ordered hexagonal lattices. (ii) FFT image and line profiles obtained from the region (ii) in (b) surrounded by white dashed line indicating double ordered hexagonal lattices. An average spacing of 2 spots is about half of the average spacing of six main spots in the reciprocal space.
Fig. 5 shows the Jc –B curve of the samples A and B at 20 K. Sample B exhibits a high Jc value of about 2.3106 A/cm2 with Hirr exceeding 7 T. This
value is extraordinarily high and exceeded that of Pb-doped Bi-2212 [11]. At present, the origin of such a high pinning performance in sample B is
358
K. Furusawa et al. / Physica C 392–396 (2003) 353–358
composition exhibited extraordinarily high Jc and Hirr values. In such sample, Ca-rich sheet-like region acted as weakly pinned region, while Cadeficient sheet-like region as a strongly pinned region.
Acknowledgements
Fig. 5. Field dependence of the critical current densities at 20 K of the sample A, the sample B and Pb-doped Bi-2212 [11]. The external field was applied parallel to the c-axis.
This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as collaborative Research and Development of Fundamental Technologies for Superconductivity Applications under the New Sunshine Program administered by the Agency of Industrial Science and Technology (AIST) of the Ministry of International Trade and Industry (MITI) of Japan.
References not clear. We think that the observed fluctuation in the chemical composition and a resulting change in the FLL structure can be correlated to this large Jc value.
4. Conclusions Detailed investigations of flux line structure were carried out for Bi-2212 single crystals grown by a TSFZ method by combining Bitter decoration technique and MO imaging. Compositional analyses were also performed with WDS. We found that the local structure of FLL is strongly affected by the variation of the local chemical composition. It was found that FLL of Bi-rich and Ca-deficient line defect is amorphous, while that of Ca-rich line defect has a hexagonal ordered lattice. Sr-rich and Ca-deficient sample grown from a Bi-rich starting
[1] F. Weiss, J.P. Senatueur, A. Rouault, M. Audier, Y. Brechet, S. Blais, R. Blum, Cryogenics 33 (1993) 50. [2] I.V. Grigorieva, Supercon. Sci. Technol. 7 (1994) 161. [3] L.Ya. Vinnikov, T.L. Barkov, B. Irmer, K. Kragler, G. Saemann-Ischenko, Physica C 308 (1998) 99. [4] I.F. Tsu, J.L. Wang, S.E Babcock, A.A. Polyanskii, D.C. Larbalestier, K.E. Sickafus, Physica C 349 (2001) 8. [5] D.J. Drost, Ph.D. Thesis, Leiden University. [6] E. Yasuda, N. Chikumoto, Y. Nakayama, K. Kishio, T. Nagatomo, M. Murakami, Physica C 357–360 (2001) 564. [7] L.Ya. Vinninkov, L.A. Gurevich, Yu.I. Latyshev, A.M. Nikitina, A.V. Antokhina, M.P. Lisitskii, N.P. Kukhta, JEPT Lett. 57 (1993) 259. [8] N. Motohira, K. Kuwahara, T. Hasegawa, K. Kishio, K. Kitazawa, J. Ceram. Soc. Jpn. Inter. Ed. 97 (1989) 994. [9] Y. Zhao, H. Zhang, D.P. Feng, Y.G. Liu, X.F. Hou, S.H. Han, Physica C 337 (2000) 322. [10] M.R. Koblischka, R.J. Wijngaarden, Supercond. Sci. Technol. 8 (1995) 199. [11] I. Chong, Z. Hiroi, M. Izumi, J. Shimoyama, Y. Nakayama, K. Kishio, T. Terashima, Y. Bando, M. Takano, Science 276 (1997) 770.
Observation of the flux line lattice and crystal defects in Bi–Sr–Ca–Cu–O single crystals K. Furusawa
a,b
, Y. Zhao c, N. Chikumoto b,*, K. Kishio d, T. Nagatomo a, M. Murakami b
a
d
Shibaura Institute of Technology, 3-9-14, Shibaura, Minato-ku, Tokyo 108-8548, Japan b SRL-ISTEC, 1-16-25, Shibaura, Minato-ku, Tokyo 105-0023, Japan c Superconductivity Research Group, School of Materials Science and Engineering, University of New South Wales, P.O. Box Sydney, NSW 2052, Australia Department of Superconductivity, Faculty of Engineering, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113, Japan Received 13 November 2002; accepted 25 December 2002
Abstract We have observed flux line lattice (FLL) structure in two different types of Bi2 Sr2 CaCu2 O8þd (Bi-2212) single crystals with different pinning by using a Bitter decoration technique. We have also observed the microstructure and analyzed local chemical composition with wave dispersive spectroscopic analyses. In a low Jc sample, we found two types of line defects, one with Ca-rich and the other with Ca-deficient. These defects extended almost parallel to the growth direction, which caused a distortion in the FLL structure. In contrast, no such defects were observed in the matrix in a high-Jc sample, and a relatively ordered FLL was observed. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.60.Ge; 74.62.Dh; 74.72.Hs Keywords: Line defect; Bitter decoration technique; Magneto-optical imaging; Bi-2212
1. Introduction In ideal type II superconductors, ordered hexagonal flux line lattice (FLL) are formed. In real superconductors, however, the FLL is disordered due to the presence of various crystal defects that act as pinning centers of quantized fluxoids. Since flux pinning directly determines the critical current
*
Corresponding author. Address: SRL-ISTEC, 10-13 Shinonome 1-chome, Koto-ku, Tokyo 135-0062, Japan. E-mail address: [email protected] (N. Chikumoto).
density, the observation of FLL togeher with the microstructural characterization has been performed to reveal the pinning mechanism [1–3]. Recently, several groups discovered [4–7] the presence of line defects extending parallel to the growth direction in Bi2 Sr2 CaCu2 O8þd (Bi-2212) single crystals grown by a traveling solvent floating zone (TSFZ) method. The pinning-induced disorders of the FLL at such defects have been observed with a Bitter decoration technique [6,7]. Magnetooptical (MO) imaging and energy dispersive spectroscopy (EDS) have revealed that similar defects with Ca deficiency act as weak-links [4]. However,
0921-4534/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-4534(03)01230-9
354
K. Furusawa et al. / Physica C 392–396 (2003) 353–358
the correlation between the microstructure of these defects and magnetic properties has not yet been clarified. In this report, we present a more detailed investigation of such line defects in Bi2 Sr2 CaCu2 O8þd (Bi-2212) single crystals grown by the TSFZ method by means of a Bitter decoration technique. It was found that flux distributions in such defects are different, which depends on the localized chemical compositions. We also found that the defect morphology in the crystal is largely affected by the starting composition. We compare the FLL structure and crystal defect structures in two kinds of crystals and discuss an observed difference in Jc –B performance.
2. Experimental Two kinds of Bi-2212 single crystals, samples A and B, grown with different starting compositions were prepared with a TSFZ method. Details of the crystal growth conditions are described elsewhere [8,9]. Starting compositions and chemical compositions of the crystals are presented in Table 1. The chemical compositions of the grown crystals were determined by inductively coupled plasma (ICP) analyses. All the data were normalized by the chemical composition of Cu. After cleaving along the (0 0 1) planes to a final thickness of several tens of micrometers, the samples were mounted on a Cu-plate using Ag-paste. The morphology and composition of samples were investigated by mean of an electron probe micro analyzer (EPMA) equipped with a wave dispersive spectroscopy (WDS) analyzer (JEOL, JXL-8800R/RL). Magneto-optical (MO) imaging technique was employed to study macroscopic flux line distributions [10]. In this technique, a Bidoped yttrium iron garnet film with in-plane
magnetization was used as an indicator. Bitter decoration experiments have been carried out using Ni particles at 4.2 K after field cooling in the presence of a magnetic field (60 G) perpendicular to the a–b plane. Flux line patterns were observed with a scanning electron microscope (SEM).
3. Results and discussion Fig. 1(a) and (b) show compositional images of two different types of line defects that are observed as bright (denoted as L1) or dark contrast (denoted as L2) in sample A. EMPA analyses confirmed that these are non-stoichiometric line defects extending parallel to the growth direction. From characteristic X-ray images of each element using Bi-Ma, Sr-La, Ca-Ka, and Cu-Ka intensities in WDS, we found Bi-enrichment and Ca-deficiency in the L1 region and Ca-enrichment in the L2 region. It was also found that these two types of line defects were often observed together and were not annealed out. In Fig. 2, we show the decoration pattern of sample A at 4.2 K field cooled in the presence of 60 G. The flux line positions are indicated as bright spots (Ni deposits) on the black background. Insets are the fast Fourier transformation (FFT) images that the vector basis is associated with the average lattice spacing. Fig. 2(a) shows the image of the FLL in the defect-free region. One can observe an ordered hexagonal FLL in this region. In Fig. 2(b) and (c), we display the FLL observed in the respective defect regions shown in Fig. 1(a) and (b). Due to the presence of line defects, one sees that the FLL structures in both L1 and L2 regions are totally different from that in the defect-free region. The FFT image is not ordered and has an amorphous structure in the L1 region, while the FLL in the L2 region has a hexagonal order with
Table 1 Starting composition and chemical composition for Bi-2212 crystals used in this study Sample ID (Tc ) A (85 K) B (89 K)
Starting composition
Analyzed composition
Bi:Sr:Ca:Cu (feed)
Bi:Sr:Ca:Cu (solvent)
Bi:Sr:Ca:Cu
2.1:1.8:1.0:2.0 2.1:1.9:1.0:2.0
– 2.5:1.9:1.0:2.6
2.21:1.91:1.01:2.00 2.03:2.08:0.85:2.00
K. Furusawa et al. / Physica C 392–396 (2003) 353–358
355
Fig. 1. Compositional image of (a) Ca-deficient line defect that corresponds to the bright contrasted region and (b) Ca-rich line defect that corresponds to the dark contrasted region.
small scattering. From FFT images, the average spacing of the FLL was determined to be 0.571 and 0.629 lm for the defect-free region and the L2 region, respectively. (The FLL spacing for 60 G is 0.631 lm.) It should be noted that the average spacing in the L2 region is larger than that in the defect-free region, which suggests that the FLL spacing changes in the Ca-rich region. Fig. 3 present the field distribution of sample B at 10 and 20 K observed with an MO imaging technique. The sample is zero-field cooled to the target temperature, and the external field was applied parallel to the c-axis. The optical micrograph of the sample shown in Fig. 3(a) reveals that the sample surface is completely flat and no observable defects are present. Fig. 3(b) is the remnant state after applying a maximum field of 200 mT at
Fig. 2. SEM images of the decoration pattern at (a) defect-free, (b) Ca-deficient and (c) Ca-rich regions after field cooling to 4.2 K in Bex ¼ 60 G. Inset is the corresponding FFT image, showing (a) an ordered hexagonal lattice, (b) amorphous and (c) overlapping of double ordered hexagonal lattices. White arrows indicate the growth direction of the sample.
10 K, while Fig. 3(c) is the remnant state after applying a maximum field of 200 mT at 20 K.
356
K. Furusawa et al. / Physica C 392–396 (2003) 353–358
Fig. 3. (a) The optical micrograph of sample B. (b) Flux distribution in the remnant state after applying a maximum field of 200 mT at 10 K. (c) Flux distribution in the remnant state after applying a maximum field of 200 mT at 20 K (c). The scale bar is 0.25 mm.
Bright regions correspond to high field areas. The scale bar in the figure is 0.25 mm. One may note that flux distribution is rather uniform at 10 K. In contrast, it is quite interesting to note that the flux distribution at 20 K is obviously inhomogeneous: sheet-like flux penetrated regions and sheet-like flux free regions also appear roughly perpendicular to the growth direction, which is parallel to the longer edge of the sample. In spite of a homogeneous surface morphology observed in an optical
micrograph (Fig. 3(a)), this result suggests that two kinds of sheet-like regions with different pinning properties lie alternately perpendicular to the growth direction. In our experiment, these lamellae-like structures of flux pattern were observed over wide temperature range of 15–70 K. Fig. 4(a) shows the FLL pattern at 4.2 K field cooled in 60 G applied perpendicularly to the a–b plane. One can find that the observed flux line pattern is in good agreement with the results of MO imaging. Sheet-like structures appear as undulation pointing to the growth direction in the image. We observed the ordered FLL in the whole area of the image. We also observed line-like structures with size comparable to the FLL distance that extend toward almost parallel to the line-like structures observed in an MO image presented in Fig. 4(b). We performed FTT in two regions with (i) and without (ii) line-like flux structure. As seen from Fig. 4(i), the average spacing of FLL for bright and dark contrasted regions is slightly different. In addition, the distance between line structures that lie parallel to the growth direction is approximately twice the average spacing of FLL in Fig. 4(ii). WDS revealed that a bright region corresponds to a Bi- and Srrich phase where the average spacing of FLL is small, while the dark region is Cu- and Ca-rich phase where the average spacing of FLL is large. We also found that Ca-deficient and Ca-rich phases correspond to a strongly pinned region and a weakly pinned region, respectively. From these observations, we can deduce the temperature dependence of superconducting properties in sheet-like flux structures observed in an MO image. The superconducting condensation energy between sheets is not so different at 10 K. Hence we observed almost uniform distribution of flux lines in an MO image (Fig. 3(b)). At 20 K, however, the difference in the condensation energy between sheets become large, and flux pattern starts to show sheet-like structures (Fig. 3(c)). In the decoration pattern, we can see the difference in the pinning strength in the region with different chemical compositions even at 4.2 K. Flux lines were frozen in strongly pinned region where Ca and Cu are rich, and accumulated at those regions.
K. Furusawa et al. / Physica C 392–396 (2003) 353–358
357
Fig. 4. (a) SEM photograph of the decorated sample after field cooling to 4.2 K in Bex ¼ 60 G. Note that closely packed Ni deposits formed a sheet-like structure. A white arrow indicates the crystal growth direction. (b) Enlarged SEM photograph of the region surrounded by white lines in (a). (i) FFT image and line profiles obtained from the region (i) surrounded by white straight lines in (b) indicating double ordered hexagonal lattices. (ii) FFT image and line profiles obtained from the region (ii) in (b) surrounded by white dashed line indicating double ordered hexagonal lattices. An average spacing of 2 spots is about half of the average spacing of six main spots in the reciprocal space.
Fig. 5 shows the Jc –B curve of the samples A and B at 20 K. Sample B exhibits a high Jc value of about 2.3106 A/cm2 with Hirr exceeding 7 T. This
value is extraordinarily high and exceeded that of Pb-doped Bi-2212 [11]. At present, the origin of such a high pinning performance in sample B is
358
K. Furusawa et al. / Physica C 392–396 (2003) 353–358
composition exhibited extraordinarily high Jc and Hirr values. In such sample, Ca-rich sheet-like region acted as weakly pinned region, while Cadeficient sheet-like region as a strongly pinned region.
Acknowledgements
Fig. 5. Field dependence of the critical current densities at 20 K of the sample A, the sample B and Pb-doped Bi-2212 [11]. The external field was applied parallel to the c-axis.
This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as collaborative Research and Development of Fundamental Technologies for Superconductivity Applications under the New Sunshine Program administered by the Agency of Industrial Science and Technology (AIST) of the Ministry of International Trade and Industry (MITI) of Japan.
References not clear. We think that the observed fluctuation in the chemical composition and a resulting change in the FLL structure can be correlated to this large Jc value.
4. Conclusions Detailed investigations of flux line structure were carried out for Bi-2212 single crystals grown by a TSFZ method by combining Bitter decoration technique and MO imaging. Compositional analyses were also performed with WDS. We found that the local structure of FLL is strongly affected by the variation of the local chemical composition. It was found that FLL of Bi-rich and Ca-deficient line defect is amorphous, while that of Ca-rich line defect has a hexagonal ordered lattice. Sr-rich and Ca-deficient sample grown from a Bi-rich starting
[1] F. Weiss, J.P. Senatueur, A. Rouault, M. Audier, Y. Brechet, S. Blais, R. Blum, Cryogenics 33 (1993) 50. [2] I.V. Grigorieva, Supercon. Sci. Technol. 7 (1994) 161. [3] L.Ya. Vinnikov, T.L. Barkov, B. Irmer, K. Kragler, G. Saemann-Ischenko, Physica C 308 (1998) 99. [4] I.F. Tsu, J.L. Wang, S.E Babcock, A.A. Polyanskii, D.C. Larbalestier, K.E. Sickafus, Physica C 349 (2001) 8. [5] D.J. Drost, Ph.D. Thesis, Leiden University. [6] E. Yasuda, N. Chikumoto, Y. Nakayama, K. Kishio, T. Nagatomo, M. Murakami, Physica C 357–360 (2001) 564. [7] L.Ya. Vinninkov, L.A. Gurevich, Yu.I. Latyshev, A.M. Nikitina, A.V. Antokhina, M.P. Lisitskii, N.P. Kukhta, JEPT Lett. 57 (1993) 259. [8] N. Motohira, K. Kuwahara, T. Hasegawa, K. Kishio, K. Kitazawa, J. Ceram. Soc. Jpn. Inter. Ed. 97 (1989) 994. [9] Y. Zhao, H. Zhang, D.P. Feng, Y.G. Liu, X.F. Hou, S.H. Han, Physica C 337 (2000) 322. [10] M.R. Koblischka, R.J. Wijngaarden, Supercond. Sci. Technol. 8 (1995) 199. [11] I. Chong, Z. Hiroi, M. Izumi, J. Shimoyama, Y. Nakayama, K. Kishio, T. Terashima, Y. Bando, M. Takano, Science 276 (1997) 770.