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Peak effect in Jc of the cation-stoichiometric SmBa2Cu3Oy superconductor
PII:
Applied Superconductivity Vol. 5, Nos 1±6, pp. 119±125, 1997 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0964-1807(98)00014-3 0964-1807/98 $19.00 + 0.00
PEAK EFFECT IN Jc OF THE CATION-STOICHIOMETRIC SmBa2Cu3Oy SUPERCONDUCTOR M. KAWANO*$, H. SUEMATSU*, T. ONDA%, M. HAYAKAWA%, H. OGIWARA$, M. KARPPINEN*} and H. YAMAUCHI*} *Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan $Department of Electrical Engineering, Shonan Institute of Technology, Tujido, Fujisawa 251-0046, Japan %Department of Mechanical Engineering, Tottori University, Koyama, Tottori 680-0945, Japan }Laboratory of lnorganic and Analytical Chemistry, Helsinki University of Technology, FIN-02150 Espoo, Finland AbstractÐCation-stoichometic SmBa2Cu3O7 ÿ d samples were prepared by a solid-state reaction method and exhibited a peak eect in their Jc vs H characteristics. Oxygen contents of the samples were determined by a coulometric titration method. A sample annealed at 3508C for 40 h showed the largest peak among those studied in this work. The oxygen content of this sample was 7 ÿ d = 6.84, which was lower than those of other samples. Tetragonal regions were revealed in the orthorhombic matrix of this sample by a transmission electron microscope (TEM) observation. On the other hand, a sample annealed at the same temperature (3508C) for a long period (200 h) showed a small peak. The former sample was thought to contain some oxygen de®ciency which would play a role of pinning centers to cause a peak eect in the Jc vs H characteristics. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
It is known that melt-grown Y-Ba-Cu-O superconducting bulks possess high critical current densities (Jc) even at liquid nitrogen temperature [1±3]. REBa2Cu3O7 ÿ d (RE: re-earth, RE-123) are superconductors isomorphous to YBa2Cu3O7 ÿ d (Y-123) with rare earth (RE) ions occupying the Y site. When RE-123 samples were synthesized in a reducing atmosphere, Tc rose to 93 K for La-123 [4] and 96 K in Nd-123 [5]. Moreover such RE-123 superconducting samples showed a peak in the Jc- vs -H (magnetic ®eld applied) characteristics. This characteristic is called a peak eect. The eect has commonly been observed in superconducting bulks containing an RE-123 superconducting phase prepared by a melt-growth process [5], a top-seeded solution-growth method [6], and a solid-state reaction method [7]. Recently, such a peak eect was observed not only in melt-grown samples containing RE-123 but also in Y-123 single crystals [8]. Several cases have been reported for the formation of ¯ux pinning centers. For example, a phase separation due to spinodal decomposition of an unstable solid solution such as Nd1 + xBa2 ÿ xCu3Oy caused spatially periodic variation in the cation ratio [6]. The region where Nd substitutes for the Ba site, showing the lower Tc, alternates with one with zero Nd substitution, having the higher Tc. As the magnitude of applied magnetic ®eld increases, the region with the lower Tc becomes non-superconducting and would then contribute to ¯ux pinning [5]. Tiny Nd4Ba2Cu2O10 crystals were found to form in the Nd-123 phase when prepared by a meltgrowth technique, and were suspected to work as a pinning center [6]. However the eectiveness of such a second phase in ¯ux pinning is usually limited to relatively low magnetic ®elds [9]. Twin boundaries in Y-123 were reported to work as a pinning center [10]. Previously we related the inhomogeneously distributed oxygen vacancies to ¯ux pinning centers [8]. In previous works, however, oxygen contents were not measured quantitatively, but just estimated using a sort of phase diagram. In this work, oxygen contents of the samples with dierent Jc- vs -H }Corresponding author. 119
M. KAWANO et al.
120
Table 1. Tc, oxygen content and lattice parameters of samples annealed at 3508C for dierent lengths of period
Sample 1 2 3
Sample annealing temperature and duration in ¯owing O2 gas 3508C, 40 h 3508C, 100 h 3508C, 200 h
Tc (K) 94 95 94
Oxygen content Lattice Lattice parameter b Lattice 7ÿd parameter a (AÊ) (AÊ) parameter c (AÊ) 6.84 6.89 6.95
3.90620.002 3.90720.001 3.90920.002
3.84620.001 3.84920.001 3.84720.001
11.71920.004 11.72020.002 11.72320.005
characteristics are determined by means of a coulometric titration method. Then, the eect of the oxygen content on the peak eect is discussed. EXPERIMENTAL
Cation-stoichiometric SmBa2Cu3O7 ÿ d (Sm-123) samples were synthesized by a solid-state reaction method from a mixture of powders of Sm2O3, BaCO3, and CuO. The powder mixture was calcined at 9008C for 24 h in O2 gas with a ¯ow rate of 50 cm3 minÿ1. The same procedure was repeated after crushing the calcined powder mixture. The twice-calcined samples were ground into powder again, and pressed to form pellets. The pelletized samples were sintered in a ¯owing low-oxygen-pressure gas of 0.1% O2+Ar at 9008C for 40 h. Then the pellets were quenched to room temperature while the gas was kept ¯owing. The obtained pellet samples were annealed in ¯owing O2 gas for dierent lengths of period, as shown in Table 1. Each sample was quenched to room temperature in O2 gas. The phases contained in the samples were analyzed by powder X-ray diractometry (XRD). The present sintered samples were found to be single phase of Sm-123. Oxygen contents of these samples were determined by a coulometric titration method [11, 12]. Microstructures of the samples were observed with a high resolution transmission electron microscope (TEM) equipped with an energy dispersive X-ray spectroscope (EDX). The d.c. magnetization of the samples was measured using a SQUID magnetometer. The Jc value was estimated by the following relation based on Bean's model [13]. Jc 20 DM=d;
Fig. 1. Temperature dependence of magnetic susceptibility of samples prepared with dierent lengths of oxygen annealing (at 3508C).
1
SmBa2Cu3Oy
121
Fig. 2. Magnetic-®eld dependence of the critical current density of samples prepared with dierent lengths of oxygen annealing (at 3508C) at 77 K.
where DM is the dierence in magnetization (in emu/cm3), d is the average grain size (in cm) which was measured by optical microscopic observation. The irreversibility ®eld (Hirr) was determined at the magnetic ®eld where Jc fell below 1000 A/cm2: Hirr H Jc 103 A=cm2 :
2
The pinning force, Fp, was estimated by the following relation: Fp Jc H; where H is the external ®eld.
Fig. 3. Oxygen-content dependence of critical current density at the peak point of the curves given in Fig. 2.
3
122
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Fig. 4. Temperature dependence of the magnetic ®eld, Hp, at the peak point of the curves given in Fig. 2. for samples 1 and 3.
Fig. 5. (a) A bright ®eld TEM image and (b) corresponding electron diraction pattern from a grain containing twins in sample 1.
SmBa2Cu3Oy
123
RESULTS AND DISCUSSION
Figure 1 shows temperature dependences of the d.c. magnetic susceptibility of three samples. All the samples have Tcs higher than 94 K (Table 1). Figure 2 shows magnetic-®eld dependences of Jc of the same samples at 77 K. The sample annealed at 3508C for 40 h (sample 1) exhibits a clear peak eect around 1 T, whereas the samples annealed at 3508C for 100 h (sample 2) and at 3508C for 200 h (sample 3) show weak peak eect. Table 1 summarizes the oxygen content and the lattice parameters of the present samples. The oxygen content increases with increasing the duration of sample annealing. This means that the shorter the annealing duration is, the more is contained the oxygen de®ciency in the sample. For the change in oxygen content from 6.84 to 6.95, the lattice parameters shift little, being within experimental error limits. Plackowski et al. reported in Ref. [14] that the dierence in oxygen content in the range higher than 6.8 yielded only slight changes in the three lattice parameters, a, b and c. Figure 3 shows the oxygen content dependence of Jc at the peak. Sample 1 is more de®cient in oxygen than the other two. Thus, the oxygen de®ciency seems to play a role of ¯ux pinning for yielding a peak eect. The value of Jc at the peak increases with increasing oxygen de®ciency. Figure 4 shows the temperature dependence of the magnetic-®eld strength at a peak, Hp, in the Jc- vs -H characteristics of samples 1 and 3. (Note that the Hp- vs -T curve for sample 2 is not shown in Fig. 4 since it was essentially the same as that for sample 3.) Although the Hp- vs -T curve for sample 3 is located at a lower point than that for sample 1, both curves decrease at the same rate with temperature. TEM images and the corresponding electron diraction patterns for dierent grains in sample 1 are shown in Figs 5 and 6. Figure 5 is for an orthorhombic region. Twin boundaries which caused splitting of some diraction spots along h110i were frequently observed in this region. As shown in Fig. 6, it is obvious that sample 1 contains a tetragonal phase as well. The disap-
Fig. 6. (a) A bright ®eld TEM image and (b) corresponding electron diraction pattern from a grain containing tetragonal regions in sample 1.
124
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Fig. 7. Pinning force, Fp [Equation (3)], with respect to a normalized magnetic ®eld H/Hirr.
pearance of both twin boundaries and split spots con®rmed that the grain in Fig. 6(a) was a tetragonal phase. Since a tetragonal structure is considered to contain less oxygen than an orthorhombic structure, the oxygen de®ciency in sample 1 is likely to be due to the presence of the tetragonal regions. Figure 7 shows the dependence of pinning force Fp [as de®ned in Equation (3)] on a normalized magnetic ®eld, H/Hirr for the three samples. All the samples have a peak in the Fp- vs -(H/ Hirr) curve at nearly the same position. This would mean that the inter-pinning-center-distance is nearly the same for all the samples. The height of the peak is the highest for sample 1, indicating that it has the strongest pinning force. This also indicates that the oxygen de®ciency contributes mainly to the appearance of a peak eect. CONCLUSION
Cation-stoichiometric SmBa2Cu3O7 ÿ d samples were prepared by solid-state reaction under a low oxygen partial pressure and subsequent annealing in ¯owing O2 gas at a low temperature (3508C). The oxygen content was controlled by changing the duration of post-annealing. The sample with the largest peak in the Jc- vs -H characteristics among the samples studied in this work had an oxygen content as low as 7 ÿ d = 6.84, while the Tc value remained at 94 K. TEM observations revealed that this sample contained grains with tetragonal regions in them. On the other hand, the sample which showed the lowest peak had the oxygen content, 7 ÿ d, larger than 6.89. Local oxygen de®ciency in cation-stoichiometric Sm-123 sample was thus likely to play a role of ¯ux pinning to yield a Jc peak at high magnetic ®elds. The appearance of the Jc peak eect in cation-stoichiometric Sm-123 superconductors was found to be dependent on the oxygen content. AcknowledgementsÐThis work was supported by a Grant-in-Aid for Scienti®c Research under a contract No. 08455299 from The Ministry of Education, Science and Culture of Japan and also by International and General Collaborative Research Project Grants-1997 of the Materials and Structures Laboratory, Tokyo Institute of Technology.
SmBa2Cu3Oy
125
REFERENCES 1. S. Jin, T. H. Tiefel, R. C. Sherwood, M. E. Davis, R. B. van Dover, G. W. Kammlott, R. A. Fastnacht and H. D. Keith, Appl. Phys. Lett. 52, 2047 (1988). 2. K. Salama, V. Selvamanickam, L. Gao and K. Sun, Appl. Phys. Lett. 54, 2352 (1989). 3. N. Murakami, N. Morita, K. Doi and M. Miyamoto, Jpn. J. Appl. Phys. 28, 1189 (1989). 4. T. Wada, N. Suzuki, T. Maeda, A. Maeda, S. Uchida, K. Uchinokura and S. Tanaka, Appl. Phys. Lett. 52, 1989 (1988). 5. S. I. Yoo, M. Murakami and N. Sakai, Jpn. J. Appl. Phys. 33, 1000 (1994). 6. M. Nakamura, Y. Yamada, T. Hirayama, Y. Ikuhara, Y. Shiohara and S. Tanaka, Physica C 259, 295 (1996). 7. M. Kawano, K. Fujinami, H. Suematsu, T. Onda, M. Hayakawa, M. Karppinen and H. Ogiwara, Materi Sci & Engineering B, (in press). 8. A. Erb, J. Y. Genoud, F. Marti, M. DaÈumling, E. Walker and R. FluÈkiger, J. of Low Temp. Phys. 105, 1033 (1996). 9. H. Suematsu, H. Okamura, S. Nagaya and H. Yamauchi, Advances in Superconductivity X, (in press). 10. H. Kutami, Y. Yamada, S. Koyama, J. G. Wen, T. Egi, Z. Nakagawa and Y. Shiohara, Jpn. J. App. Phys. 36, 674 (1997). 11. K. Kurusu, H. Tanaka and K. Shintomi, Analyst 114, 1341 (1989). 12. M. Karppinen, A. Fukuoka, L. NiinistoÈ and H. Yamauchi, Supercond. Sci. Technol. 8, 1 (1995). 13. E. M. Gyorgy, R. B. van Dover, K. A. Jackson, L. F. Schneemeyer and J. V. Waszczak, Appl. Phys. Lett. 55, 283 (1989). 14. T. Plackowski, C. Sulkowski, Z. Bukowski, D. Wlosewicz and K. Rogacki, Physica C 254, 331 (1995).
Applied Superconductivity Vol. 5, Nos 1±6, pp. 119±125, 1997 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0964-1807(98)00014-3 0964-1807/98 $19.00 + 0.00
PEAK EFFECT IN Jc OF THE CATION-STOICHIOMETRIC SmBa2Cu3Oy SUPERCONDUCTOR M. KAWANO*$, H. SUEMATSU*, T. ONDA%, M. HAYAKAWA%, H. OGIWARA$, M. KARPPINEN*} and H. YAMAUCHI*} *Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan $Department of Electrical Engineering, Shonan Institute of Technology, Tujido, Fujisawa 251-0046, Japan %Department of Mechanical Engineering, Tottori University, Koyama, Tottori 680-0945, Japan }Laboratory of lnorganic and Analytical Chemistry, Helsinki University of Technology, FIN-02150 Espoo, Finland AbstractÐCation-stoichometic SmBa2Cu3O7 ÿ d samples were prepared by a solid-state reaction method and exhibited a peak eect in their Jc vs H characteristics. Oxygen contents of the samples were determined by a coulometric titration method. A sample annealed at 3508C for 40 h showed the largest peak among those studied in this work. The oxygen content of this sample was 7 ÿ d = 6.84, which was lower than those of other samples. Tetragonal regions were revealed in the orthorhombic matrix of this sample by a transmission electron microscope (TEM) observation. On the other hand, a sample annealed at the same temperature (3508C) for a long period (200 h) showed a small peak. The former sample was thought to contain some oxygen de®ciency which would play a role of pinning centers to cause a peak eect in the Jc vs H characteristics. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
It is known that melt-grown Y-Ba-Cu-O superconducting bulks possess high critical current densities (Jc) even at liquid nitrogen temperature [1±3]. REBa2Cu3O7 ÿ d (RE: re-earth, RE-123) are superconductors isomorphous to YBa2Cu3O7 ÿ d (Y-123) with rare earth (RE) ions occupying the Y site. When RE-123 samples were synthesized in a reducing atmosphere, Tc rose to 93 K for La-123 [4] and 96 K in Nd-123 [5]. Moreover such RE-123 superconducting samples showed a peak in the Jc- vs -H (magnetic ®eld applied) characteristics. This characteristic is called a peak eect. The eect has commonly been observed in superconducting bulks containing an RE-123 superconducting phase prepared by a melt-growth process [5], a top-seeded solution-growth method [6], and a solid-state reaction method [7]. Recently, such a peak eect was observed not only in melt-grown samples containing RE-123 but also in Y-123 single crystals [8]. Several cases have been reported for the formation of ¯ux pinning centers. For example, a phase separation due to spinodal decomposition of an unstable solid solution such as Nd1 + xBa2 ÿ xCu3Oy caused spatially periodic variation in the cation ratio [6]. The region where Nd substitutes for the Ba site, showing the lower Tc, alternates with one with zero Nd substitution, having the higher Tc. As the magnitude of applied magnetic ®eld increases, the region with the lower Tc becomes non-superconducting and would then contribute to ¯ux pinning [5]. Tiny Nd4Ba2Cu2O10 crystals were found to form in the Nd-123 phase when prepared by a meltgrowth technique, and were suspected to work as a pinning center [6]. However the eectiveness of such a second phase in ¯ux pinning is usually limited to relatively low magnetic ®elds [9]. Twin boundaries in Y-123 were reported to work as a pinning center [10]. Previously we related the inhomogeneously distributed oxygen vacancies to ¯ux pinning centers [8]. In previous works, however, oxygen contents were not measured quantitatively, but just estimated using a sort of phase diagram. In this work, oxygen contents of the samples with dierent Jc- vs -H }Corresponding author. 119
M. KAWANO et al.
120
Table 1. Tc, oxygen content and lattice parameters of samples annealed at 3508C for dierent lengths of period
Sample 1 2 3
Sample annealing temperature and duration in ¯owing O2 gas 3508C, 40 h 3508C, 100 h 3508C, 200 h
Tc (K) 94 95 94
Oxygen content Lattice Lattice parameter b Lattice 7ÿd parameter a (AÊ) (AÊ) parameter c (AÊ) 6.84 6.89 6.95
3.90620.002 3.90720.001 3.90920.002
3.84620.001 3.84920.001 3.84720.001
11.71920.004 11.72020.002 11.72320.005
characteristics are determined by means of a coulometric titration method. Then, the eect of the oxygen content on the peak eect is discussed. EXPERIMENTAL
Cation-stoichiometric SmBa2Cu3O7 ÿ d (Sm-123) samples were synthesized by a solid-state reaction method from a mixture of powders of Sm2O3, BaCO3, and CuO. The powder mixture was calcined at 9008C for 24 h in O2 gas with a ¯ow rate of 50 cm3 minÿ1. The same procedure was repeated after crushing the calcined powder mixture. The twice-calcined samples were ground into powder again, and pressed to form pellets. The pelletized samples were sintered in a ¯owing low-oxygen-pressure gas of 0.1% O2+Ar at 9008C for 40 h. Then the pellets were quenched to room temperature while the gas was kept ¯owing. The obtained pellet samples were annealed in ¯owing O2 gas for dierent lengths of period, as shown in Table 1. Each sample was quenched to room temperature in O2 gas. The phases contained in the samples were analyzed by powder X-ray diractometry (XRD). The present sintered samples were found to be single phase of Sm-123. Oxygen contents of these samples were determined by a coulometric titration method [11, 12]. Microstructures of the samples were observed with a high resolution transmission electron microscope (TEM) equipped with an energy dispersive X-ray spectroscope (EDX). The d.c. magnetization of the samples was measured using a SQUID magnetometer. The Jc value was estimated by the following relation based on Bean's model [13]. Jc 20 DM=d;
Fig. 1. Temperature dependence of magnetic susceptibility of samples prepared with dierent lengths of oxygen annealing (at 3508C).
1
SmBa2Cu3Oy
121
Fig. 2. Magnetic-®eld dependence of the critical current density of samples prepared with dierent lengths of oxygen annealing (at 3508C) at 77 K.
where DM is the dierence in magnetization (in emu/cm3), d is the average grain size (in cm) which was measured by optical microscopic observation. The irreversibility ®eld (Hirr) was determined at the magnetic ®eld where Jc fell below 1000 A/cm2: Hirr H Jc 103 A=cm2 :
2
The pinning force, Fp, was estimated by the following relation: Fp Jc H; where H is the external ®eld.
Fig. 3. Oxygen-content dependence of critical current density at the peak point of the curves given in Fig. 2.
3
122
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Fig. 4. Temperature dependence of the magnetic ®eld, Hp, at the peak point of the curves given in Fig. 2. for samples 1 and 3.
Fig. 5. (a) A bright ®eld TEM image and (b) corresponding electron diraction pattern from a grain containing twins in sample 1.
SmBa2Cu3Oy
123
RESULTS AND DISCUSSION
Figure 1 shows temperature dependences of the d.c. magnetic susceptibility of three samples. All the samples have Tcs higher than 94 K (Table 1). Figure 2 shows magnetic-®eld dependences of Jc of the same samples at 77 K. The sample annealed at 3508C for 40 h (sample 1) exhibits a clear peak eect around 1 T, whereas the samples annealed at 3508C for 100 h (sample 2) and at 3508C for 200 h (sample 3) show weak peak eect. Table 1 summarizes the oxygen content and the lattice parameters of the present samples. The oxygen content increases with increasing the duration of sample annealing. This means that the shorter the annealing duration is, the more is contained the oxygen de®ciency in the sample. For the change in oxygen content from 6.84 to 6.95, the lattice parameters shift little, being within experimental error limits. Plackowski et al. reported in Ref. [14] that the dierence in oxygen content in the range higher than 6.8 yielded only slight changes in the three lattice parameters, a, b and c. Figure 3 shows the oxygen content dependence of Jc at the peak. Sample 1 is more de®cient in oxygen than the other two. Thus, the oxygen de®ciency seems to play a role of ¯ux pinning for yielding a peak eect. The value of Jc at the peak increases with increasing oxygen de®ciency. Figure 4 shows the temperature dependence of the magnetic-®eld strength at a peak, Hp, in the Jc- vs -H characteristics of samples 1 and 3. (Note that the Hp- vs -T curve for sample 2 is not shown in Fig. 4 since it was essentially the same as that for sample 3.) Although the Hp- vs -T curve for sample 3 is located at a lower point than that for sample 1, both curves decrease at the same rate with temperature. TEM images and the corresponding electron diraction patterns for dierent grains in sample 1 are shown in Figs 5 and 6. Figure 5 is for an orthorhombic region. Twin boundaries which caused splitting of some diraction spots along h110i were frequently observed in this region. As shown in Fig. 6, it is obvious that sample 1 contains a tetragonal phase as well. The disap-
Fig. 6. (a) A bright ®eld TEM image and (b) corresponding electron diraction pattern from a grain containing tetragonal regions in sample 1.
124
M. KAWANO et al.
Fig. 7. Pinning force, Fp [Equation (3)], with respect to a normalized magnetic ®eld H/Hirr.
pearance of both twin boundaries and split spots con®rmed that the grain in Fig. 6(a) was a tetragonal phase. Since a tetragonal structure is considered to contain less oxygen than an orthorhombic structure, the oxygen de®ciency in sample 1 is likely to be due to the presence of the tetragonal regions. Figure 7 shows the dependence of pinning force Fp [as de®ned in Equation (3)] on a normalized magnetic ®eld, H/Hirr for the three samples. All the samples have a peak in the Fp- vs -(H/ Hirr) curve at nearly the same position. This would mean that the inter-pinning-center-distance is nearly the same for all the samples. The height of the peak is the highest for sample 1, indicating that it has the strongest pinning force. This also indicates that the oxygen de®ciency contributes mainly to the appearance of a peak eect. CONCLUSION
Cation-stoichiometric SmBa2Cu3O7 ÿ d samples were prepared by solid-state reaction under a low oxygen partial pressure and subsequent annealing in ¯owing O2 gas at a low temperature (3508C). The oxygen content was controlled by changing the duration of post-annealing. The sample with the largest peak in the Jc- vs -H characteristics among the samples studied in this work had an oxygen content as low as 7 ÿ d = 6.84, while the Tc value remained at 94 K. TEM observations revealed that this sample contained grains with tetragonal regions in them. On the other hand, the sample which showed the lowest peak had the oxygen content, 7 ÿ d, larger than 6.89. Local oxygen de®ciency in cation-stoichiometric Sm-123 sample was thus likely to play a role of ¯ux pinning to yield a Jc peak at high magnetic ®elds. The appearance of the Jc peak eect in cation-stoichiometric Sm-123 superconductors was found to be dependent on the oxygen content. AcknowledgementsÐThis work was supported by a Grant-in-Aid for Scienti®c Research under a contract No. 08455299 from The Ministry of Education, Science and Culture of Japan and also by International and General Collaborative Research Project Grants-1997 of the Materials and Structures Laboratory, Tokyo Institute of Technology.
SmBa2Cu3Oy
125
REFERENCES 1. S. Jin, T. H. Tiefel, R. C. Sherwood, M. E. Davis, R. B. van Dover, G. W. Kammlott, R. A. Fastnacht and H. D. Keith, Appl. Phys. Lett. 52, 2047 (1988). 2. K. Salama, V. Selvamanickam, L. Gao and K. Sun, Appl. Phys. Lett. 54, 2352 (1989). 3. N. Murakami, N. Morita, K. Doi and M. Miyamoto, Jpn. J. Appl. Phys. 28, 1189 (1989). 4. T. Wada, N. Suzuki, T. Maeda, A. Maeda, S. Uchida, K. Uchinokura and S. Tanaka, Appl. Phys. Lett. 52, 1989 (1988). 5. S. I. Yoo, M. Murakami and N. Sakai, Jpn. J. Appl. Phys. 33, 1000 (1994). 6. M. Nakamura, Y. Yamada, T. Hirayama, Y. Ikuhara, Y. Shiohara and S. Tanaka, Physica C 259, 295 (1996). 7. M. Kawano, K. Fujinami, H. Suematsu, T. Onda, M. Hayakawa, M. Karppinen and H. Ogiwara, Materi Sci & Engineering B, (in press). 8. A. Erb, J. Y. Genoud, F. Marti, M. DaÈumling, E. Walker and R. FluÈkiger, J. of Low Temp. Phys. 105, 1033 (1996). 9. H. Suematsu, H. Okamura, S. Nagaya and H. Yamauchi, Advances in Superconductivity X, (in press). 10. H. Kutami, Y. Yamada, S. Koyama, J. G. Wen, T. Egi, Z. Nakagawa and Y. Shiohara, Jpn. J. App. Phys. 36, 674 (1997). 11. K. Kurusu, H. Tanaka and K. Shintomi, Analyst 114, 1341 (1989). 12. M. Karppinen, A. Fukuoka, L. NiinistoÈ and H. Yamauchi, Supercond. Sci. Technol. 8, 1 (1995). 13. E. M. Gyorgy, R. B. van Dover, K. A. Jackson, L. F. Schneemeyer and J. V. Waszczak, Appl. Phys. Lett. 55, 283 (1989). 14. T. Plackowski, C. Sulkowski, Z. Bukowski, D. Wlosewicz and K. Rogacki, Physica C 254, 331 (1995).