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Biaxially aligned YBCO film tapes fabricated by all pulsed laser deposition
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Applied Superconductivity Vol. 4, Nos 10±11, pp. 487±493, 1996 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0964-1807/96 $15.00 + 0.00 S0964-1807(97)00035-5
BIAXIALLY ALIGNED YBCO FILM TAPES FABRICATED BY ALL PULSED LASER DEPOSITION K. HASEGAWA*1, K. FUJINO*, H. MUKAI*, M. KONISHI*, K. HAYASHI*, K. SATO*, S. HONJO$, Y. SATO$, H. ISHII$ and Y. IWATA$ *Osaka Research Laboratories, Sumitomo Electric Industries Ltd, 1-1-3 Shimaya, Konohana-ku, Osaka, 554 Japan $Power Engineering R&D Center, Tokyo Electric Power Company, 4-1 Egasaki-cho, Tsurumi-ku, Yokohama, 230 Japan AbstractÐBiaxially aligned yttria-stabilized zirconia (YSZ) ®lms on Ni-based alloy substrates were realized with high deposition rate of 0.5 mm minÿ1 by the inclined substrate deposition (ISD) technique without ion beam assistance. The microstructure of YSZ was examined to study the growth mechanism of biaxial alignment by ISD. Columnar structures toward the plasma plume suggested a self-shadowing eect in the ISD process. To raise Ic values, YBCO thickness was increased up to 5 mm. Thick YBCO ®lms with high Jc values were realized on the ISD-grown YSZ. Long YBCO tapes with biaxial alignment were successfully fabricated using continuous pulsed laser deposition and a high Ic value of 37.0 A (77.3 K, 0 T) at a 75 cm voltage tap spacing was achieved. # 1998 Elsevier Science Ltd. All rights reserved
1. INTRODUCTION
High-temperature superconducting tapes and wires with high Jc values are necessary for power applications. We have reported on the fabrication of long and ¯exible superconducting tapes using continuous pulsed laser deposition (PLD) which has the advantage of a high deposition rate. One hundred pieces of 1 m long YBCO tape with a Jc value of more than 104 A cmÿ2 were successfully fabricated [1]. Biaxial alignment of YBCO is essential to improve Jc values. Iijima et al. ®rst reported biaxially aligned yttria-stabilized zirconia (YSZ) and YBCO thin ®lms on polycrystalline substrate by ion beam assisted deposition (IBAD) [2]. We found that biaxial alignment is possible by inclining substrate under speci®c condition of PLD without ion beam assistance [3, 4]. We call this new PLD technique ISD (inclined substrate deposition). The best Jc value of 4.3 105 A cmÿ2 was achieved for a short sample [4], and the 1 m long YBCO ®lm tape with Jc=1.5 105 A cmÿ2 was successfully fabricated by the ISD technique [5]. As an application using high Jc YBCO tapes, a magnetic-shielding-type fault current limiter was fabricated and showed good current-limiting properties [6, 7]. Concerning the growth mechanism of the biaxial aligned ®lms in IBAD process, Bradley et al. developed a selective resputtering model based on ion channeling [8], and Sonnenberg et al. proposed a growth controlled model based on etching rate anisotropy [9]. These were caused by an assisted ion beam. On the other hand, in the case of ISD without ion beam assistance, the mechanism of biaxial alignment was not understood. This study reports on the ISD technique, the microstructure related to the mechanism of biaxial alignment, and the high Ic properties of the thick YBCO ®lms.
2. EXPERIMENTAL
The superconducting tape consisted of YBCO ®lm on a ¯exible Ni-based alloy substrate with a YSZ buer layer. Hastelloy tape was employed as the substrate. The substrate, whose size was 10 mm wide, 80±160 mm thick and 1 m long, was continuously sliding on a heater stage during 1
Author to whom correspondence should be addressed. 487
488
K. HASEGAWA et al.
deposition. The substrate temperature ranged from room temperature to 7508C. Both YSZ and YBCO layers were deposited by PLD with KrF (l = 248 nm) excimer laser. The thickness of YSZ ranged from 1 to 2 mm and that of YBCO from 2 to 5 mm. The tape surface was protected with sputtered Ag layer. The structure and orientation of the ®lms were characterized by scanning electron microscopy (SEM), X-ray diraction (XRD) and pole ®gure measurement. Ic was measured by the conventional four-probe method in liquid nitrogen. 3. RESULTS AND DISCUSSION
In order to obtain biaxial alignment of YSZ, we investigated various conditions such as substrate temperature, gas pressure and deposition rate using PLD. Figure 1(a) shows the usual arrangement of PLD in which the substrate surface was parallel to the target face. In this parallel arranged PLD, YSZ ®lms which had preferred orientation normal to the substrate were obtained under control of the deposition conditions. Figure 2 shows XRD patterns of the YSZ ®lms grown by parallel arranged PLD. The (100) oriented ®lm as shown in Fig. 2(a) was deposited at a low substrate temperature and the (111) oriented ®lm as shown in Fig. 2(b) was deposited at a high substrate temperature. These ®lms were randomly oriented in the substrate plane because all conditions were isotropic in a direction parallel to the substrate surface. Figure 1(b) shows ISD technique in which substrate was inclined with respect to the direction of plasma plume in PLD. The preferred orientation of YSZ was aected by the inclined angle [4]. The angle between substrate normal and the plasma plume was 458. YSZ ®lms, (100) oriented normal to the substrate and biaxially aligned, were obtained at high substrate temperatures; (100) oriented YSZ is convenient for YBCO growth because of the lattice match. Figure 3 shows a typical YSZ (111) pole ®gure of a (100) oriented ®lm grown by ISD. The direction of the plasma plume is shown by an arrow and corresponded to f = 1808 and a' = 458 in Fig. 3. The YSZ ®lm was highly aligned in the substrate plane, and the full width at half maximum
Fig. 1. Schematic diagram of PLD: (a) parallel arranged PLD; (b) ISD technique.
Biaxially aligned YBCO ®lm tapes
489
Fig. 2. XRD patterns of YSZ ®lms grown by parallel arranged PLD: (a) (100) oriented ®lm deposited at a low substrate temperature; (b) (111) oriented ®lm deposited at a high substrate temperature.
(FWHM) of YSZ (111) poles evaluated from f scan measurement was 198. The ISD technique utilizes PLD itself, which has the advantage of a high deposition rate. The deposition rate of 0.5 mm minÿ1 was realized to form biaxially aligned YSZ ®lms. Cross-sectional observation was carried out to study the mechanism of biaxial alignment by ISD. Since anisotropic arrangement of PLD is important in the ISD technique, the direction of the cross-section was designed as shown by an arrow in Fig. 3. Figure 4(a) shows a fractured cross-sectional SEM image of biaxially aligned YSZ and YBCO. The columnar structure of YSZ toward the plasma plume was clearly observed. This result is dierent from those by IBAD in which the direction of columnar structure was almost normal to the substrate [9, 10].
Fig. 3. YSZ (111) pole ®gure of (100) oriented ®lm deposited at a high substrate temperature by the ISD technique.
490
K. HASEGAWA et al.
Dirks et al. reported that columnar microstructure was commonly observed in vapor deposited ®lms and more prominent in oblique incident deposition due to self-shadowing [11]. Selfshadowing means geometric shadowing of incident atoms by those within the growing ®lms and makes columnar structure more nearly normal to the substrate than the vapor beam direction. Figure 4(b) shows a schematic drawing of the relationship between the plasma plume, the columnar structure and the substrate normal. The angle between columnar structure and the substrate normal was about 208, which was less than the angle between plasma plume and the substrate normal. These features support the self-shadowing eect in the ISD process. It seems that the biaxial alignment was caused by the growth direction of the self-shadowing combined with the preferred orientation normal to the substrate. YBCO ®lms were deposited on the ISD-grown YSZ. It was not necessary to incline the substrate in the YBCO deposition because YBCO was epitaxially grown on a biaxially aligned YSZ
Fig. 4. (a) Cross-section SEM image of biaxially aligned YSZ and YBCO; (b) schematic drawing of the relationship between the plasma plume, the columnar structure and the substrate normal.
Biaxially aligned YBCO ®lm tapes
491
Fig. 5. Ic values versus YBCO thickness.
buer layer. The cross-sectional microstructure of the YBCO was ¯at in contrast to the YSZ as shown in Fig. 4(a). In view of applying superconducting tapes to power apparatus, not only an improvement of the Jc values of YBCO important, but an increase of the Ic values is equally and practically important. To raise the Ic values, it is necessary to increase the YBCO thickness while maintaining high Jc values. Figure 5 shows the Ic values versus the YBCO thickness. The Ic values became higher according to the increase of YBCO thickness up to about 5 mm. The Ic value of 51.2 A was obtained where the YBCO thickness was 3.4 mm and the Jc value was 1.6 105 A cmÿ2. Figure 6 shows the surface SEM image of this high-Ic YBCO ®lm. Although white particles were observed, a dense and uniform matrix was realized in thick YBCO ®lms and contributed to the high Ic values. Long YBCO tapes were fabricated using the high-Ic process above. Figure 7 shows (103) pole ®gures of YBCO at the front and the end parts of a 50 cm long tape. The YBCO had (001) orientation normal to the substrate and the highly biaxial alignment as well as the YSZ buer layer. The FWHM of the YBCO (103) poles at the front and the end part were 15.0 and 15.28, respectively. The thickness of YBCO were both 2.9 mm. Thus, long and uniform YBCO tapes were successfully fabricated with biaxial alignment. Table 1 shows Ic values with several lengths.
Fig. 6. Surface SEM image of the YBCO thick ®lm.
K. HASEGAWA et al.
492
Fig. 7. YBCO (103) pole ®gures of a 50 cm long tape: (a) front part; (b) end part..
The Ic of 37.0 A at a 75 cm voltage tap spacing was achieved. These results show that the ISD technique is promising in the fabrication of long and high-Jc YBCO tapes, with high Ic values. CONCLUSION
Biaxially aligned YSZ ®lms on Hastelloy substrates were realized with a high deposition rate of 0.5 mm minÿ1 by the ISD technique in which the substrate was inclined with respect to the direction of the plasma plume in PLD. Cross-sectional observation showed that the YSZ had a columnar structure towards the plasma plume, which supports the self-shadowing eect in the ISD process. Thick YBCO ®lms with high Jc values were realized on the ISD-grown YSZ. Long YBCO tapes with high Ic values were successfully fabricated by continuous PLD and the Ic value of 37.0 A at a 75 cm voltage tap spacing was achieved. Table 1. Summary of Ic values and length 77.3 K, 0 T Length (cm)
Ic (A)
75 45 5
37.0 44.4 52.0
Biaxially aligned YBCO ®lm tapes
493
REFERENCES 1. K. Fujino, N. Yoshida, N. Hayashi, S. Okuda, T. Hara and H. Ishii, Advances in Superconductivity VI, p. 763, Springer, Tokyo, (1994). 2. Y. Iijima, N. Tanabe, O. Kono and Y. Ikeno, Appl. Phys. Lett. 60, 759 (1992). 3. K. Fujino, N. Yoshida, S. Okuda, T. Hara, T. Ohkuma and H. Ishii, Advances in Superconductivity VII, p. 629, Springer, Tokyo, (1995). 4. K. Hasegawa, N. Yoshida, K. Fujino, H. Mukai, K. Hayashi, K. Sato, T. Ohkuma, S. Honjo, H. Ishii and T. Hara, Proceedings of the 16th International Cryogenic Engineering Conference/International Cryogenic Materials Conference, p. 1413, Elsevier, Amsterdam, (1997). 5. K. Fujino, K. Hasegawa, H. Mukai, K. Sato, T. Hara, T. Ohkuma, H. Ishii and S. Honjo, Advances in Superconductivity VIII, p. 675, Springer, Tokyo, (1996). 6. S. Honjo, S. Harada, T. Ohkuma and T. Hara, National Convention Record I.E.E. Japan 5, 323 (1995). 7. K. Hasegawa, N. Yoshida, K. Fujino, H. Mukai, K. Hayashi, K. Sato, S. Honjo, Y. Sato, T. Ohkuma, H. Ishii, Y. Iwata and T. Hara., Advances in Superconductivity IX. p. 745, Springer, Tokyo, (1997). 8. R. M. Bradley, J. M. E. Harper and D. A. Smith, J. Appl. Phys. 60, 4160 (1986). 9. N. Sonnenberg, A. S. Longo, M. J. Cima, B. P. Chang, K. G. Ressler, P. C. McIntyre and Y. P. Liu, J. Appl. Phys. 74, 1027 (1993). 10. Y. Iijima, M. Hosaka, N. Tanabe, N. Sadakata, T. Saito, O. Kono and J. Yoshitomi, Advances in Superconductivity VIII, p. 659, Springer, Tokyo, (1996). 11. A. G. Dirks and H. J. Leamy, Thin Solid Films 47, 219 (1977).
Applied Superconductivity Vol. 4, Nos 10±11, pp. 487±493, 1996 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0964-1807/96 $15.00 + 0.00 S0964-1807(97)00035-5
BIAXIALLY ALIGNED YBCO FILM TAPES FABRICATED BY ALL PULSED LASER DEPOSITION K. HASEGAWA*1, K. FUJINO*, H. MUKAI*, M. KONISHI*, K. HAYASHI*, K. SATO*, S. HONJO$, Y. SATO$, H. ISHII$ and Y. IWATA$ *Osaka Research Laboratories, Sumitomo Electric Industries Ltd, 1-1-3 Shimaya, Konohana-ku, Osaka, 554 Japan $Power Engineering R&D Center, Tokyo Electric Power Company, 4-1 Egasaki-cho, Tsurumi-ku, Yokohama, 230 Japan AbstractÐBiaxially aligned yttria-stabilized zirconia (YSZ) ®lms on Ni-based alloy substrates were realized with high deposition rate of 0.5 mm minÿ1 by the inclined substrate deposition (ISD) technique without ion beam assistance. The microstructure of YSZ was examined to study the growth mechanism of biaxial alignment by ISD. Columnar structures toward the plasma plume suggested a self-shadowing eect in the ISD process. To raise Ic values, YBCO thickness was increased up to 5 mm. Thick YBCO ®lms with high Jc values were realized on the ISD-grown YSZ. Long YBCO tapes with biaxial alignment were successfully fabricated using continuous pulsed laser deposition and a high Ic value of 37.0 A (77.3 K, 0 T) at a 75 cm voltage tap spacing was achieved. # 1998 Elsevier Science Ltd. All rights reserved
1. INTRODUCTION
High-temperature superconducting tapes and wires with high Jc values are necessary for power applications. We have reported on the fabrication of long and ¯exible superconducting tapes using continuous pulsed laser deposition (PLD) which has the advantage of a high deposition rate. One hundred pieces of 1 m long YBCO tape with a Jc value of more than 104 A cmÿ2 were successfully fabricated [1]. Biaxial alignment of YBCO is essential to improve Jc values. Iijima et al. ®rst reported biaxially aligned yttria-stabilized zirconia (YSZ) and YBCO thin ®lms on polycrystalline substrate by ion beam assisted deposition (IBAD) [2]. We found that biaxial alignment is possible by inclining substrate under speci®c condition of PLD without ion beam assistance [3, 4]. We call this new PLD technique ISD (inclined substrate deposition). The best Jc value of 4.3 105 A cmÿ2 was achieved for a short sample [4], and the 1 m long YBCO ®lm tape with Jc=1.5 105 A cmÿ2 was successfully fabricated by the ISD technique [5]. As an application using high Jc YBCO tapes, a magnetic-shielding-type fault current limiter was fabricated and showed good current-limiting properties [6, 7]. Concerning the growth mechanism of the biaxial aligned ®lms in IBAD process, Bradley et al. developed a selective resputtering model based on ion channeling [8], and Sonnenberg et al. proposed a growth controlled model based on etching rate anisotropy [9]. These were caused by an assisted ion beam. On the other hand, in the case of ISD without ion beam assistance, the mechanism of biaxial alignment was not understood. This study reports on the ISD technique, the microstructure related to the mechanism of biaxial alignment, and the high Ic properties of the thick YBCO ®lms.
2. EXPERIMENTAL
The superconducting tape consisted of YBCO ®lm on a ¯exible Ni-based alloy substrate with a YSZ buer layer. Hastelloy tape was employed as the substrate. The substrate, whose size was 10 mm wide, 80±160 mm thick and 1 m long, was continuously sliding on a heater stage during 1
Author to whom correspondence should be addressed. 487
488
K. HASEGAWA et al.
deposition. The substrate temperature ranged from room temperature to 7508C. Both YSZ and YBCO layers were deposited by PLD with KrF (l = 248 nm) excimer laser. The thickness of YSZ ranged from 1 to 2 mm and that of YBCO from 2 to 5 mm. The tape surface was protected with sputtered Ag layer. The structure and orientation of the ®lms were characterized by scanning electron microscopy (SEM), X-ray diraction (XRD) and pole ®gure measurement. Ic was measured by the conventional four-probe method in liquid nitrogen. 3. RESULTS AND DISCUSSION
In order to obtain biaxial alignment of YSZ, we investigated various conditions such as substrate temperature, gas pressure and deposition rate using PLD. Figure 1(a) shows the usual arrangement of PLD in which the substrate surface was parallel to the target face. In this parallel arranged PLD, YSZ ®lms which had preferred orientation normal to the substrate were obtained under control of the deposition conditions. Figure 2 shows XRD patterns of the YSZ ®lms grown by parallel arranged PLD. The (100) oriented ®lm as shown in Fig. 2(a) was deposited at a low substrate temperature and the (111) oriented ®lm as shown in Fig. 2(b) was deposited at a high substrate temperature. These ®lms were randomly oriented in the substrate plane because all conditions were isotropic in a direction parallel to the substrate surface. Figure 1(b) shows ISD technique in which substrate was inclined with respect to the direction of plasma plume in PLD. The preferred orientation of YSZ was aected by the inclined angle [4]. The angle between substrate normal and the plasma plume was 458. YSZ ®lms, (100) oriented normal to the substrate and biaxially aligned, were obtained at high substrate temperatures; (100) oriented YSZ is convenient for YBCO growth because of the lattice match. Figure 3 shows a typical YSZ (111) pole ®gure of a (100) oriented ®lm grown by ISD. The direction of the plasma plume is shown by an arrow and corresponded to f = 1808 and a' = 458 in Fig. 3. The YSZ ®lm was highly aligned in the substrate plane, and the full width at half maximum
Fig. 1. Schematic diagram of PLD: (a) parallel arranged PLD; (b) ISD technique.
Biaxially aligned YBCO ®lm tapes
489
Fig. 2. XRD patterns of YSZ ®lms grown by parallel arranged PLD: (a) (100) oriented ®lm deposited at a low substrate temperature; (b) (111) oriented ®lm deposited at a high substrate temperature.
(FWHM) of YSZ (111) poles evaluated from f scan measurement was 198. The ISD technique utilizes PLD itself, which has the advantage of a high deposition rate. The deposition rate of 0.5 mm minÿ1 was realized to form biaxially aligned YSZ ®lms. Cross-sectional observation was carried out to study the mechanism of biaxial alignment by ISD. Since anisotropic arrangement of PLD is important in the ISD technique, the direction of the cross-section was designed as shown by an arrow in Fig. 3. Figure 4(a) shows a fractured cross-sectional SEM image of biaxially aligned YSZ and YBCO. The columnar structure of YSZ toward the plasma plume was clearly observed. This result is dierent from those by IBAD in which the direction of columnar structure was almost normal to the substrate [9, 10].
Fig. 3. YSZ (111) pole ®gure of (100) oriented ®lm deposited at a high substrate temperature by the ISD technique.
490
K. HASEGAWA et al.
Dirks et al. reported that columnar microstructure was commonly observed in vapor deposited ®lms and more prominent in oblique incident deposition due to self-shadowing [11]. Selfshadowing means geometric shadowing of incident atoms by those within the growing ®lms and makes columnar structure more nearly normal to the substrate than the vapor beam direction. Figure 4(b) shows a schematic drawing of the relationship between the plasma plume, the columnar structure and the substrate normal. The angle between columnar structure and the substrate normal was about 208, which was less than the angle between plasma plume and the substrate normal. These features support the self-shadowing eect in the ISD process. It seems that the biaxial alignment was caused by the growth direction of the self-shadowing combined with the preferred orientation normal to the substrate. YBCO ®lms were deposited on the ISD-grown YSZ. It was not necessary to incline the substrate in the YBCO deposition because YBCO was epitaxially grown on a biaxially aligned YSZ
Fig. 4. (a) Cross-section SEM image of biaxially aligned YSZ and YBCO; (b) schematic drawing of the relationship between the plasma plume, the columnar structure and the substrate normal.
Biaxially aligned YBCO ®lm tapes
491
Fig. 5. Ic values versus YBCO thickness.
buer layer. The cross-sectional microstructure of the YBCO was ¯at in contrast to the YSZ as shown in Fig. 4(a). In view of applying superconducting tapes to power apparatus, not only an improvement of the Jc values of YBCO important, but an increase of the Ic values is equally and practically important. To raise the Ic values, it is necessary to increase the YBCO thickness while maintaining high Jc values. Figure 5 shows the Ic values versus the YBCO thickness. The Ic values became higher according to the increase of YBCO thickness up to about 5 mm. The Ic value of 51.2 A was obtained where the YBCO thickness was 3.4 mm and the Jc value was 1.6 105 A cmÿ2. Figure 6 shows the surface SEM image of this high-Ic YBCO ®lm. Although white particles were observed, a dense and uniform matrix was realized in thick YBCO ®lms and contributed to the high Ic values. Long YBCO tapes were fabricated using the high-Ic process above. Figure 7 shows (103) pole ®gures of YBCO at the front and the end parts of a 50 cm long tape. The YBCO had (001) orientation normal to the substrate and the highly biaxial alignment as well as the YSZ buer layer. The FWHM of the YBCO (103) poles at the front and the end part were 15.0 and 15.28, respectively. The thickness of YBCO were both 2.9 mm. Thus, long and uniform YBCO tapes were successfully fabricated with biaxial alignment. Table 1 shows Ic values with several lengths.
Fig. 6. Surface SEM image of the YBCO thick ®lm.
K. HASEGAWA et al.
492
Fig. 7. YBCO (103) pole ®gures of a 50 cm long tape: (a) front part; (b) end part..
The Ic of 37.0 A at a 75 cm voltage tap spacing was achieved. These results show that the ISD technique is promising in the fabrication of long and high-Jc YBCO tapes, with high Ic values. CONCLUSION
Biaxially aligned YSZ ®lms on Hastelloy substrates were realized with a high deposition rate of 0.5 mm minÿ1 by the ISD technique in which the substrate was inclined with respect to the direction of the plasma plume in PLD. Cross-sectional observation showed that the YSZ had a columnar structure towards the plasma plume, which supports the self-shadowing eect in the ISD process. Thick YBCO ®lms with high Jc values were realized on the ISD-grown YSZ. Long YBCO tapes with high Ic values were successfully fabricated by continuous PLD and the Ic value of 37.0 A at a 75 cm voltage tap spacing was achieved. Table 1. Summary of Ic values and length 77.3 K, 0 T Length (cm)
Ic (A)
75 45 5
37.0 44.4 52.0
Biaxially aligned YBCO ®lm tapes
493
REFERENCES 1. K. Fujino, N. Yoshida, N. Hayashi, S. Okuda, T. Hara and H. Ishii, Advances in Superconductivity VI, p. 763, Springer, Tokyo, (1994). 2. Y. Iijima, N. Tanabe, O. Kono and Y. Ikeno, Appl. Phys. Lett. 60, 759 (1992). 3. K. Fujino, N. Yoshida, S. Okuda, T. Hara, T. Ohkuma and H. Ishii, Advances in Superconductivity VII, p. 629, Springer, Tokyo, (1995). 4. K. Hasegawa, N. Yoshida, K. Fujino, H. Mukai, K. Hayashi, K. Sato, T. Ohkuma, S. Honjo, H. Ishii and T. Hara, Proceedings of the 16th International Cryogenic Engineering Conference/International Cryogenic Materials Conference, p. 1413, Elsevier, Amsterdam, (1997). 5. K. Fujino, K. Hasegawa, H. Mukai, K. Sato, T. Hara, T. Ohkuma, H. Ishii and S. Honjo, Advances in Superconductivity VIII, p. 675, Springer, Tokyo, (1996). 6. S. Honjo, S. Harada, T. Ohkuma and T. Hara, National Convention Record I.E.E. Japan 5, 323 (1995). 7. K. Hasegawa, N. Yoshida, K. Fujino, H. Mukai, K. Hayashi, K. Sato, S. Honjo, Y. Sato, T. Ohkuma, H. Ishii, Y. Iwata and T. Hara., Advances in Superconductivity IX. p. 745, Springer, Tokyo, (1997). 8. R. M. Bradley, J. M. E. Harper and D. A. Smith, J. Appl. Phys. 60, 4160 (1986). 9. N. Sonnenberg, A. S. Longo, M. J. Cima, B. P. Chang, K. G. Ressler, P. C. McIntyre and Y. P. Liu, J. Appl. Phys. 74, 1027 (1993). 10. Y. Iijima, M. Hosaka, N. Tanabe, N. Sadakata, T. Saito, O. Kono and J. Yoshitomi, Advances in Superconductivity VIII, p. 659, Springer, Tokyo, (1996). 11. A. G. Dirks and H. J. Leamy, Thin Solid Films 47, 219 (1977).