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Effects of heat-treatment conditions on microstructure of Y123 films deposited by TFA-MOD method
Physica C 392–396 (2003) 922–926 www.elsevier.com/locate/physc
Effects of heat-treatment conditions on microstructure of Y123 films deposited by TFA-MOD method J. Shibata *, Y. Tokunaga, R. Teranishi, H. Fuji, T. Honjo, T. Izumi, Y. Shiohara Superconductivity Research Laboratory, ISTEC, Shinonome 1-Chome 10-13, Koto-ku, Tokyo 135-0062, Japan Received 13 November 2002; accepted 31 January 2003
Abstract Y123 precursor and final films were deposited at various heating rates in the calcination process of metal organic deposition method using trifluoroacetates. The superconducting characteristics were evaluated in the final films. The microstructures and the distributions of chemical compositions of these films were also investigated using transmission electron microscopy and energy dispersive X-ray spectroscopy. In the Y123 precursor film prepared by heating at the rate of 0.2 C/min in the temperature range from 200 to 250 C, Cu was distributed uniformly. On the other hand, in the precursor film prepared by heat-treating at the rate of 5 C/min from 200 to 250 C, Cu was found to be segregated in the vicinity of the surface of the film. This Cu segregation in the precursor film might cause the differences of crystallinity and superconducting properties of the final film. The former precursor film was grown into a c-axis oriented Y123 film with good crystallinity, and exhibited the high JC value of 2 · 106 A/cm2 . However, the final film prepared by heat-treating the latter precursor film had many stacking faults of CuO planes, and showed the lower JC value of 0.3 · 106 A/cm2 . 2003 Elsevier B.V. All rights reserved. PACS: 74.72.B; 61.16.B Keywords: Metal organic deposition; Trifluoroacetate; Calcination; Transmission electron microscopy; Energy dispersive X-ray spectroscopy
1. Introduction Metal organic deposition using trifluoroacetates (TFA-MOD) is a promising method for producing superconducting electric wires and tapes inexpensively. In this method, the superconducting films are formed by coating substrates with the solution
*
Corresponding author. Tel.: +81-3-3536-5711; fax: +81-33536-5717. E-mail address: [email protected] (J. Shibata).
containing TFA salts, and then heat-treating the samples at the two steps of calcination and crystallization. McIntyre et al. have reported that Y123 films were formed on LaAlO3 (LAO) substrates with the high JC value over 5 · 106 A/cm2 [1]. Recently, Fuji et al. and Araki et al. have succeeded in producing Y123 films on CeO2 /IBADYSZ/Hastelloy with high JC [2,3]. However, it is necessary to increase the thickness of the films in order to apply them to electric wires which transport a large current. In the multi-coating TFA-MOD method, the coating and calcination
0921-4534/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-4534(03)01151-1
J. Shibata et al. / Physica C 392–396 (2003) 922–926
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processes are repeated several times alternately to increase the film thickness. At that time, uniformity of the microstructure and the distribution of chemical composition in the precursor film is important for producing the thick film with high JC . In the previous paper, we have reported that the Y123 precursor film contains a Y–Ba–Cu–O–F amorphous matrix and CuO crystals [4]. In the present study, we deposited the Y123 precursor and final films with changing a heating rate from 200 to 250 C during the calcination, in order to investigate the effects of the heat-treatment condition of the calcination on the microstructure of the final films. In addition, we have observed the quenched precursor films prepared by cooling rapidly during the calcination process, using transmission electron microscopy (TEM). 2. Experimental Y123 precursor and final superconducting films were deposited by TFA-MOD process. First, (0 0 1)LAO substrates were spin-coated with solution containing Y-, Ba- and Cu-TFA. Then, these samples were heat-treated to 400 C with different heating rates in the temperature range from 200 to 250 C in an O2 gas flow with P H2 O of 2.1%. Finally, these films were heated at 775 C for 1 h in an Ar/O2 gas flow with P H2 O of 4.2%. Furthermore, in order to investigate the calcination process, the quenched films were prepared by cooling rapidly from 200, 220, 250, 270 and 300 C during the heat-treatment for calcination. Microstructures and chemical compositions of these films were investigated by means of TEM. Cross-sectional specimens were prepared by the standard processes for TEM specimen preparation. The transmission electron microscope used in this work was JEM-2010F (JEOL) operating at 200 kV. 3. Results and discussion 3.1. Effects of heating rate during calcination process Fig. 1(a) shows a cross-sectional TEM photograph of the Y123 final film prepared by heating at
Fig. 1. Cross-sectional HRTEM images of the Y123 final films deposited by heat-treating at (a) 0.2 C/min; (b) 5 C/min, from 200 to 250 C during the calcination process, respectively. These photographs were taken from the direction of [0 1 0]Y123 k [0 1 0]LAO . White arrows in (a) indicate typical examples of the stacking faults of CuO planes.
the rate of 0.2 C/min from 200 to 250 C during the calcination process, observed from the direction of [0 1 0]Y123 k[0 1 0]LAO . As shown in this figure, the c-axis oriented Y123 film is grown with good crystallinity in this case. a-axis oriented particles of Y123 were scarcely observed in this film. In the X-ray diffraction pattern (XRD), FWHM of (0 0 5)Y123 peak of this film was 0.25. Furthermore, this film has exhibited the high JC value (at 77 K, 0 T) of 2 · 106 A/cm2 . Fig. 1(b) shows the cross-sectional image of the final film formed by heating at 5 C/min from 200 to 250 C during the calcination. In this film, many stacking faults of CuO planes are observed; cplanes of Y123 seem to be in a wave shape. In the selected-area electron diffraction pattern (SAEDP) taken at the film, (0 0 3)Y123 spots have had streaks. If extra CuO planes are inserted regularly, Y123
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structure becomes to have the same structure as Y124 and Y248, which have the lower TC values; Y124 has a double CuO chain and Y248 has a double and a single CuO chain alternately. FWHM of (0 0 5)Y123 peak of this final film is 0.6, and the JC (at 77 K, 0 T) is 0.3 · 106 A/cm2 . The differences of the crystallinity and the superconducting property between these Y123 final films are considered to be affected by the microstructure and the distribution of chemical composition of each precursor film. Figs. 2(a) and (b) show cross-sectional images and SAEDPs of the Y123 films deposited by heattreating at the different rates of 0.2 and 5 C/min from 200 to 250 C, respectively. We have reported that the Y123 precursor film contains an amorphous matrix and CuO crystals in the previous
paper [4]. There were also seen CuO particles in these precursor films. Furthermore, SAEDP using about a 0.4 lm aperture in diameter obtained from the Y123 precursor film deposited and heated at 5 C/min has many rings, which correspond to the lattice spacings of CuO crystal. This indicates that CuO particles are grown in this film. Figs. 3(a) and (b) show results of energy dispersive X-ray spectroscopy (EDS) measurements taken at the points in Figs. 2(a) and (b), respectively. Electron beam used in this work was about 100 nm in diameter at the points marked as 1, 2 and 3; 1 nm at the point of 4. As shown in these figures, in the case of heating at 0.2 C/min, Cu is distributed uniformly in the film. At this time, relative quantities of Cu were determined by using the Cu-K peak with the energy of about 8.0 keV, because the peaks with low energy such as the Cu-L peak of 0.9 keV are more scattered than the peaks with the high energy. However, in the case of heat-treatment at 5 C/min, atm% ratios of Ba to Cu which were estimated from the intensity of Ba and Cu peaks are 3:1, 1:1 and 1:3 at point 1, point 2 and point 3, respectively. From a result of this, quite a bit of Cu-element is found to exist near the film surface. It is suggested that this segregation of Cu in the precursor film could enhance the nucleation and growth of CuO in this area. In addition, a Y-rich layer is observed at the surface of both films, as shown at the point of 4 in Figs. 3(a) and (b). However, this layer is not significant for the epitaxial growth of the c-axis oriented Y123 film, since the layer was not observed in the final film after high temperature annealing. 3.2. Observation of calcination process
Fig. 2. Low-magnification TEM images and SAEDPs of the Y123 precursor films prepared by changing the heating rate in the temperature range from 200 to 250 C: (a) 0.2 C/min; (b) 5 C/min.
Figs. 4(a) and (b) show cross-sectional images of the precursor film, which was quenched from 220 C during the calcination. This film was heated at 0.2 C/min in the temperature range from 200 to 220 C. In the precursor film quenched from 200 C, no crystal such as CuO was able to be observed. The thickness of the film was about 250 nm. However, in the quenched film cooled rapidly from 220 C, the crystallization of CuO has occurred, and the thickness of the film becomes to about 400–500 nm, as shown in Figs. 4(a) and
J. Shibata et al. / Physica C 392–396 (2003) 922–926
Y
Y
Point 4
O
Point 4
O Y Ba Cu
Cu Cu
Point 3
Cu
Point 3
Cu Ba
Intensity (a.u)
Y Y Cu
Cu
Y
Ba
F
Ba
Intensity (a.u)
925
Point 2
Ba
Y Y Cu
Ba
Point 2
Cu
Y
Y Y
Ba
Y Ba
Point 1
Cu
Point 1
O Cu
Y
5
(a)
Cu
Y
Y
10
15
Y
20
Energy (keV)
5
(b)
10
15
20
Energy (keV)
Fig. 3. EDS spectra taken at the points in (a) Fig. 2(a); (b) Fig. 2(b). In the Y123 precursor film heat-treated at 5 C/min from 200 to 250 C, Cu is segregated in the vicinity of the film surface.
(b). From high-resolutional electron microscopic (HRTEM) observation, CuO crystals are less than 10 nm in diameter. Fig. 4(c) shows EDS spectra taken at the points in Fig. 4(a). This figure indicates that the Cu-element is distributed uniformly in this film. From the TEM observation of the quenched film cooled from 250 C, the number of CuO particles increased and the size of them became large. From the results of these TEM observations, thermal decomposition of TFA salts firstly occurs in the entire precursor film, and at the same time the thickness of the film increases remarkably. In FT-IR measurements of the quenched films over 270 C, the peak which corresponds absorption of CF3 disappeared. Therefore, not only Cu-TFA but
also Y-, Ba-TFA are suggested to be decomposed and changed to be oxides or oxifluorides. As described above, it was found that the thermal decomposition of Cu-TFA occurs in the entire film in the case of heating gradually from 200 to 250 C. In contrast, in the case of heating the film rapidly, the crystallization of CuO tends to occur firstly near the surface of the film. At that time, F-element decreases in the vicinity of the film surface due to HF removal. This is suggested to cause the migration of Cu-TFA to the film surface, if there are some interactions between F and any cationic elements. We should investigate the mechanism of the precursor formation in more details, and put the migration of the elements into formulas in the near future.
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4. Conclusion We have investigated the effects of heat-treatment conditions in the calcination of the TFAMOD processes on the microstructure of the Y123 final films. It was found that the heating rate during thermal decomposition of TFA salts from 200 to 250 C have an influence on the crystallinity of the final films. The difference in the crystallinity of the final films was found to be resulted from the distributions of chemical compositions in the precursor films. In the Y123 precursor film prepared at the heating rate of 0.2 C/min from 200 to 250 C, Cu is distributed uniformly in the entire film. The final film prepared by heat-treating this precursor film has a good crystallinity and exhibits the high JC value of 2 · 106 A/cm2 . On the other hand, in the Y123 precursor film prepared at the heating rate of 5 C/min, Cu is segregated in the vicinity of the film surface. The final film formed by heating this precursor film has many stacking faults of CuO planes with JC of 0.3 · 106 A/cm2 .
Acknowledgements 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.
References
Fig. 4. Cross-sectional TEM images and EDS spectra taken at the quenched Y123 precursor film, which was prepared by cooling rapidly from 220 C during the calcination process: (a) low-magnification TEM image; (b) HRTEM image taken in this film; (c) EDS spectra obtained at the points of 1, 2 and 3. CuO crystals can be observed in the amorphous matrix, and Cu is distributed uniformly in this film as shown in (c).
[1] P.C. McIntyre, M.J. Cima, J.A. Smith Jr., R.B. Hallock, M.P. Siegal, J.M. Phillips, J. Appl. Phys. 71 (1992) 1868. [2] H. Fuji, T. Honjo, Y. Nakamura, T. Izumi, T. Araki, I. Hirabayashi, Y. Shiohara, Y. Iijima, K. Takeda, Physica C 357–360 (2001) 1011. [3] T. Araki, Y. Takahashi, K. Yamagiwa, Y. Iijima, K. Takeda, Y. Yamada, J. Shibata, T. Hirayama, I. Hirabayashi, Physica C 357–360 (2001) 991. [4] J. Shibata, T. Honjo, H. Fuji, T. Araki, I. Hirabayashi, T. Hirayama, T. Izumi, Y. Shiohara, T. Yamamoto, Y. Ikuhara, J. Mater. Res. 17 (2002) 1266.
Effects of heat-treatment conditions on microstructure of Y123 films deposited by TFA-MOD method J. Shibata *, Y. Tokunaga, R. Teranishi, H. Fuji, T. Honjo, T. Izumi, Y. Shiohara Superconductivity Research Laboratory, ISTEC, Shinonome 1-Chome 10-13, Koto-ku, Tokyo 135-0062, Japan Received 13 November 2002; accepted 31 January 2003
Abstract Y123 precursor and final films were deposited at various heating rates in the calcination process of metal organic deposition method using trifluoroacetates. The superconducting characteristics were evaluated in the final films. The microstructures and the distributions of chemical compositions of these films were also investigated using transmission electron microscopy and energy dispersive X-ray spectroscopy. In the Y123 precursor film prepared by heating at the rate of 0.2 C/min in the temperature range from 200 to 250 C, Cu was distributed uniformly. On the other hand, in the precursor film prepared by heat-treating at the rate of 5 C/min from 200 to 250 C, Cu was found to be segregated in the vicinity of the surface of the film. This Cu segregation in the precursor film might cause the differences of crystallinity and superconducting properties of the final film. The former precursor film was grown into a c-axis oriented Y123 film with good crystallinity, and exhibited the high JC value of 2 · 106 A/cm2 . However, the final film prepared by heat-treating the latter precursor film had many stacking faults of CuO planes, and showed the lower JC value of 0.3 · 106 A/cm2 . 2003 Elsevier B.V. All rights reserved. PACS: 74.72.B; 61.16.B Keywords: Metal organic deposition; Trifluoroacetate; Calcination; Transmission electron microscopy; Energy dispersive X-ray spectroscopy
1. Introduction Metal organic deposition using trifluoroacetates (TFA-MOD) is a promising method for producing superconducting electric wires and tapes inexpensively. In this method, the superconducting films are formed by coating substrates with the solution
*
Corresponding author. Tel.: +81-3-3536-5711; fax: +81-33536-5717. E-mail address: [email protected] (J. Shibata).
containing TFA salts, and then heat-treating the samples at the two steps of calcination and crystallization. McIntyre et al. have reported that Y123 films were formed on LaAlO3 (LAO) substrates with the high JC value over 5 · 106 A/cm2 [1]. Recently, Fuji et al. and Araki et al. have succeeded in producing Y123 films on CeO2 /IBADYSZ/Hastelloy with high JC [2,3]. However, it is necessary to increase the thickness of the films in order to apply them to electric wires which transport a large current. In the multi-coating TFA-MOD method, the coating and calcination
0921-4534/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-4534(03)01151-1
J. Shibata et al. / Physica C 392–396 (2003) 922–926
923
processes are repeated several times alternately to increase the film thickness. At that time, uniformity of the microstructure and the distribution of chemical composition in the precursor film is important for producing the thick film with high JC . In the previous paper, we have reported that the Y123 precursor film contains a Y–Ba–Cu–O–F amorphous matrix and CuO crystals [4]. In the present study, we deposited the Y123 precursor and final films with changing a heating rate from 200 to 250 C during the calcination, in order to investigate the effects of the heat-treatment condition of the calcination on the microstructure of the final films. In addition, we have observed the quenched precursor films prepared by cooling rapidly during the calcination process, using transmission electron microscopy (TEM). 2. Experimental Y123 precursor and final superconducting films were deposited by TFA-MOD process. First, (0 0 1)LAO substrates were spin-coated with solution containing Y-, Ba- and Cu-TFA. Then, these samples were heat-treated to 400 C with different heating rates in the temperature range from 200 to 250 C in an O2 gas flow with P H2 O of 2.1%. Finally, these films were heated at 775 C for 1 h in an Ar/O2 gas flow with P H2 O of 4.2%. Furthermore, in order to investigate the calcination process, the quenched films were prepared by cooling rapidly from 200, 220, 250, 270 and 300 C during the heat-treatment for calcination. Microstructures and chemical compositions of these films were investigated by means of TEM. Cross-sectional specimens were prepared by the standard processes for TEM specimen preparation. The transmission electron microscope used in this work was JEM-2010F (JEOL) operating at 200 kV. 3. Results and discussion 3.1. Effects of heating rate during calcination process Fig. 1(a) shows a cross-sectional TEM photograph of the Y123 final film prepared by heating at
Fig. 1. Cross-sectional HRTEM images of the Y123 final films deposited by heat-treating at (a) 0.2 C/min; (b) 5 C/min, from 200 to 250 C during the calcination process, respectively. These photographs were taken from the direction of [0 1 0]Y123 k [0 1 0]LAO . White arrows in (a) indicate typical examples of the stacking faults of CuO planes.
the rate of 0.2 C/min from 200 to 250 C during the calcination process, observed from the direction of [0 1 0]Y123 k[0 1 0]LAO . As shown in this figure, the c-axis oriented Y123 film is grown with good crystallinity in this case. a-axis oriented particles of Y123 were scarcely observed in this film. In the X-ray diffraction pattern (XRD), FWHM of (0 0 5)Y123 peak of this film was 0.25. Furthermore, this film has exhibited the high JC value (at 77 K, 0 T) of 2 · 106 A/cm2 . Fig. 1(b) shows the cross-sectional image of the final film formed by heating at 5 C/min from 200 to 250 C during the calcination. In this film, many stacking faults of CuO planes are observed; cplanes of Y123 seem to be in a wave shape. In the selected-area electron diffraction pattern (SAEDP) taken at the film, (0 0 3)Y123 spots have had streaks. If extra CuO planes are inserted regularly, Y123
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structure becomes to have the same structure as Y124 and Y248, which have the lower TC values; Y124 has a double CuO chain and Y248 has a double and a single CuO chain alternately. FWHM of (0 0 5)Y123 peak of this final film is 0.6, and the JC (at 77 K, 0 T) is 0.3 · 106 A/cm2 . The differences of the crystallinity and the superconducting property between these Y123 final films are considered to be affected by the microstructure and the distribution of chemical composition of each precursor film. Figs. 2(a) and (b) show cross-sectional images and SAEDPs of the Y123 films deposited by heattreating at the different rates of 0.2 and 5 C/min from 200 to 250 C, respectively. We have reported that the Y123 precursor film contains an amorphous matrix and CuO crystals in the previous
paper [4]. There were also seen CuO particles in these precursor films. Furthermore, SAEDP using about a 0.4 lm aperture in diameter obtained from the Y123 precursor film deposited and heated at 5 C/min has many rings, which correspond to the lattice spacings of CuO crystal. This indicates that CuO particles are grown in this film. Figs. 3(a) and (b) show results of energy dispersive X-ray spectroscopy (EDS) measurements taken at the points in Figs. 2(a) and (b), respectively. Electron beam used in this work was about 100 nm in diameter at the points marked as 1, 2 and 3; 1 nm at the point of 4. As shown in these figures, in the case of heating at 0.2 C/min, Cu is distributed uniformly in the film. At this time, relative quantities of Cu were determined by using the Cu-K peak with the energy of about 8.0 keV, because the peaks with low energy such as the Cu-L peak of 0.9 keV are more scattered than the peaks with the high energy. However, in the case of heat-treatment at 5 C/min, atm% ratios of Ba to Cu which were estimated from the intensity of Ba and Cu peaks are 3:1, 1:1 and 1:3 at point 1, point 2 and point 3, respectively. From a result of this, quite a bit of Cu-element is found to exist near the film surface. It is suggested that this segregation of Cu in the precursor film could enhance the nucleation and growth of CuO in this area. In addition, a Y-rich layer is observed at the surface of both films, as shown at the point of 4 in Figs. 3(a) and (b). However, this layer is not significant for the epitaxial growth of the c-axis oriented Y123 film, since the layer was not observed in the final film after high temperature annealing. 3.2. Observation of calcination process
Fig. 2. Low-magnification TEM images and SAEDPs of the Y123 precursor films prepared by changing the heating rate in the temperature range from 200 to 250 C: (a) 0.2 C/min; (b) 5 C/min.
Figs. 4(a) and (b) show cross-sectional images of the precursor film, which was quenched from 220 C during the calcination. This film was heated at 0.2 C/min in the temperature range from 200 to 220 C. In the precursor film quenched from 200 C, no crystal such as CuO was able to be observed. The thickness of the film was about 250 nm. However, in the quenched film cooled rapidly from 220 C, the crystallization of CuO has occurred, and the thickness of the film becomes to about 400–500 nm, as shown in Figs. 4(a) and
J. Shibata et al. / Physica C 392–396 (2003) 922–926
Y
Y
Point 4
O
Point 4
O Y Ba Cu
Cu Cu
Point 3
Cu
Point 3
Cu Ba
Intensity (a.u)
Y Y Cu
Cu
Y
Ba
F
Ba
Intensity (a.u)
925
Point 2
Ba
Y Y Cu
Ba
Point 2
Cu
Y
Y Y
Ba
Y Ba
Point 1
Cu
Point 1
O Cu
Y
5
(a)
Cu
Y
Y
10
15
Y
20
Energy (keV)
5
(b)
10
15
20
Energy (keV)
Fig. 3. EDS spectra taken at the points in (a) Fig. 2(a); (b) Fig. 2(b). In the Y123 precursor film heat-treated at 5 C/min from 200 to 250 C, Cu is segregated in the vicinity of the film surface.
(b). From high-resolutional electron microscopic (HRTEM) observation, CuO crystals are less than 10 nm in diameter. Fig. 4(c) shows EDS spectra taken at the points in Fig. 4(a). This figure indicates that the Cu-element is distributed uniformly in this film. From the TEM observation of the quenched film cooled from 250 C, the number of CuO particles increased and the size of them became large. From the results of these TEM observations, thermal decomposition of TFA salts firstly occurs in the entire precursor film, and at the same time the thickness of the film increases remarkably. In FT-IR measurements of the quenched films over 270 C, the peak which corresponds absorption of CF3 disappeared. Therefore, not only Cu-TFA but
also Y-, Ba-TFA are suggested to be decomposed and changed to be oxides or oxifluorides. As described above, it was found that the thermal decomposition of Cu-TFA occurs in the entire film in the case of heating gradually from 200 to 250 C. In contrast, in the case of heating the film rapidly, the crystallization of CuO tends to occur firstly near the surface of the film. At that time, F-element decreases in the vicinity of the film surface due to HF removal. This is suggested to cause the migration of Cu-TFA to the film surface, if there are some interactions between F and any cationic elements. We should investigate the mechanism of the precursor formation in more details, and put the migration of the elements into formulas in the near future.
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4. Conclusion We have investigated the effects of heat-treatment conditions in the calcination of the TFAMOD processes on the microstructure of the Y123 final films. It was found that the heating rate during thermal decomposition of TFA salts from 200 to 250 C have an influence on the crystallinity of the final films. The difference in the crystallinity of the final films was found to be resulted from the distributions of chemical compositions in the precursor films. In the Y123 precursor film prepared at the heating rate of 0.2 C/min from 200 to 250 C, Cu is distributed uniformly in the entire film. The final film prepared by heat-treating this precursor film has a good crystallinity and exhibits the high JC value of 2 · 106 A/cm2 . On the other hand, in the Y123 precursor film prepared at the heating rate of 5 C/min, Cu is segregated in the vicinity of the film surface. The final film formed by heating this precursor film has many stacking faults of CuO planes with JC of 0.3 · 106 A/cm2 .
Acknowledgements 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.
References
Fig. 4. Cross-sectional TEM images and EDS spectra taken at the quenched Y123 precursor film, which was prepared by cooling rapidly from 220 C during the calcination process: (a) low-magnification TEM image; (b) HRTEM image taken in this film; (c) EDS spectra obtained at the points of 1, 2 and 3. CuO crystals can be observed in the amorphous matrix, and Cu is distributed uniformly in this film as shown in (c).
[1] P.C. McIntyre, M.J. Cima, J.A. Smith Jr., R.B. Hallock, M.P. Siegal, J.M. Phillips, J. Appl. Phys. 71 (1992) 1868. [2] H. Fuji, T. Honjo, Y. Nakamura, T. Izumi, T. Araki, I. Hirabayashi, Y. Shiohara, Y. Iijima, K. Takeda, Physica C 357–360 (2001) 1011. [3] T. Araki, Y. Takahashi, K. Yamagiwa, Y. Iijima, K. Takeda, Y. Yamada, J. Shibata, T. Hirayama, I. Hirabayashi, Physica C 357–360 (2001) 991. [4] J. Shibata, T. Honjo, H. Fuji, T. Araki, I. Hirabayashi, T. Hirayama, T. Izumi, Y. Shiohara, T. Yamamoto, Y. Ikuhara, J. Mater. Res. 17 (2002) 1266.