- Email: [email protected]
Measurement of local critical currents in TFA-MOD processed coated conductors by use of scanning Hall-probe microscopy
Physica C 471 (2011) 1041–1044
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Measurement of local critical currents in TFA-MOD processed coated conductors by use of scanning Hall-probe microscopy K. Shiohara a,⇑, K. Higashikawa a, T. Kawaguchi a, M. Inoue a, T. Kiss a, M. Yoshizumi b, T. Izumi b a b
Kyushu University, 744 Motooka, Nishi, Fukuoka City, Fukuoka 819-0395, Japan SRL-ISTEC, 10-13 Shinonome 1-chome, Koto-ku, Tokyo 135-0062, Japan
a r t i c l e
i n f o
Article history: Available online 13 May 2011 Keywords: Characterization of superconducting materials Visualization of critical current distribution Scanning Hall-probe microscopy TFA-MOD processed superconducting coated conductor
a b s t r a c t We have carried out 2-dimensional (2D) measurement of local critical current in a TrifluoroacetatesMetal Organic Deposition (TFA-MOD) processed YBCO coated conductor using scanning Hall-probe microscopy. Recently, remarkable R&D accomplishments on the fabrication processes of coated conductors have been conducted extensively and reported. The TFA-MOD process has been expected as an attractive process to produce coated conductors with high performance at a low production cost due to a simple process using non-vacuum equipments. On the other hand, enhancement of critical currents and homogenization of the critical current distribution in the coated conductors are definitely very important for practical applications. According to our measurements, we can detect positions and spatial distribution of defects in the conductor. This kind of information will be very helpful for the improvement of the TFA-MOD process and for the design of the conductor intended for practical electric power device applications. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction A large number of fabrication processes for high critical temperature, Tc, RE-123 (RE means Rare Earth) coated conductors (CCs) have been reported. For example, even for the deposition process for a superconducting layer, at least the following four processes have been conducted mainly in the world: Pulse Laser Deposition (PLD) process, Metal Organic Deposition (MOD) process, Metal Organic Chemical Vapor Deposition (MOCVD) process, and Reactive Co-Evaporation (RCE) [1–3]. In particular, a trifluoroacetates (TFA)-MOD (TFA-MOD) process has been expected as an attractive process to fabricate high-performance CCs with low production cost due to a simple process using non-vacuum equipments and high material yields [1–3]. On the other hand, enhancement of critical currents and homogenization of the critical current distribution in the CCs has been still remained as very important subjects to be investigated for practical applications. Until now, we have several characterization techniques including spatially resolved measurements of flux flow dissipation and current distributions [4–8]. It is extremely important and useful for CC fabrication to obtain such information about the spatial distributions of the superconductive properties. These distributions (non-uniformity) are, in general, caused by kinds and dispersions of defects and ⇑ Corresponding author. Address: Faculty of Information Science and Electrical Engineering, Kyushu University, #546, West-2 Bldg, Motooka, Nishi, Fukuoka City, Fukuoka 819-0395, Japan. Tel.: +81 92 802 3678; fax: +81 92 802 3677. E-mail address: [email protected] (K. Shiohara). 0921-4534/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2011.05.119
the inhomogeneous qualities of the superconducting phase crystals. This means that it is indispensable to understand the locations of the defects and their influence on the critical current for the performance improvement of the CCs. For that, it will be very helpful to characterize in-plane distribution of critical current density in a CC in a non-destructive manner. In this study, using a non-contact and non-destructive evaluation method by means of scanning Hall-probe microscopy (SHPM), we investigated in-plane critical current density, Jc, distribution of TFA-MOD processed CCs. Furthermore, we showed that the relationship of applied external magnetic fields and local critical current density could also be characterized by this method.
2. Experimental Fig. 1 shows the SHPM system. This system was cooled by thermal conduction from the LN2 tank. A sample was located in the bore of the coil, and was cooled together with the coil. A typical temperature was controlled to be 79 K. A Hall-probe was fixed on a triaxial stage head and scanned by a stepping motor. The active area of the Hall sensor is 50 lm 50 lm and the spatial resolutions of the stage are 1 lm in the xy-plane and 0.25 lm in the z-axis. External magnetic fields were applied to the sample, and then magnetic field distribution around the sample was measured by scanning the Hall probe sensor. In this study, the measurement was carried out (1) when external magnetic fields were applied to the sample and (2) when the external fields were
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K. Shiohara et al. / Physica C 471 (2011) 1041–1044
removed after that, i.e., remanent magnetic field. When we assumed in-plane distributions of current densities, i.e., sheet current densities, we could estimate the corresponding distributions of sheet current densities from that of measured magnetic fields by considering an inverse problem of Biot-Savart law. The thickness of the superconducting layer is very thin (1 lm), and the width/ thickness aspect ratio is high, therefore 2D sheet current can be a reasonable assumption [6]. In our previous study, we obtained a good agreement between the theoretical and the experimental results that the relationships of distance from the CC’s surface to the Hall-probe sensor and the magnetic fields on the plane (B\tape) [6]. This distance between the CC’s surface and Hall-probe sensor is the ‘‘lift-off’’ and shown in the images of Fig. 2. When sufficiently high external magnetic fields were applied to the sample, the shielding currents flow at their critical current densities in almost all the regions of the sample. Similarly, when we remove the external magnetic fields after applying a sufficient magnetic field, the loop currents trapping the remanent magnetic fields flow at their critical current densities. This is the principle of the visualization of the critical current density distribution in the sample.
3. Results and discussion
Measured magnetic field Bz (mT)
Fig. 1. Photograph of the scanning Hall-probe microscope.
30
Bz
Lift-off: 400 µm
Jx
Cut-off: 380 µm
Jy
Cut-off: 380 µm
J
Cut-off: 380 µm
Sheet current density Jx (A/mm) Sheet current density Jy (A/mm)
Lift-off: 340 µm
10 -10
Sample A
Sheet current density J (A/mm)
By 2D scanning of the Hall sensor, we could obtain the 2D distribution of ‘‘Bz’’ that is the perpendicular component of the field at
32
Sample B Cut-off: 380 µm
0 -32 32
Sample A
Sample B Cut-off: 380 µm
0 -32 32
Sample A
Sample B Cut-off: 380 µm
16 0
Fig. 2. Perpendicular component of the remanent field, Bz, measured by SHPM, longitudinal component of sheet current, Jx, transverse component of sheet current, Jy, and amplitude of sheet current, J (sample A: high Ic sample, sample B: low Ic sample).
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K. Shiohara et al. / Physica C 471 (2011) 1041–1044
Cut-off:380 µm
Fig. 3. Measured magnetic field, Bz, and magnetic field on the plane (B\tape), Bz0, for different applied magnetic fields ( : 5 mT, : remanent after 105 mT).
Cut-off:380 µm
Fig. 4. Longitudinal sheet current density, Jx for different applied magnetic fields ( : 5 mT, : 10 mT, : 30 mT, : 60 mT, : 90 mT and : remanent after 105 mT).
the position of lift-off distance from the CC’s surface. The measurement was carried out when external magnetic fields were applied to the sample and when the external fields were removed after
: 30 mT,
(b)
Insufficient penetration
ex
: 60 mT,
: 90 mT and
that, i.e., remanent magnetic field. The critical current distribution in the CC could be estimated by conversion from the 2D magnetic field distribution based on the Biot-Savart law [5–9]. We investigated current distributions as well as local Jc distributions for two different 1 cm-wide YBCO CCs fabricated by TFAMOD processes. Fig. 2 shows a set of visualized results from SHPM: 2D distributions of the measured magnetic fields and the corresponding sheet current densities at the remanent magnetic fields after the application of 105 mT which was much larger than the penetration field. The lift-off distances from the samples A and B are 400 lm and 340 lm, respectively. The cut-off wavelengths in the inversion analysis of both samples are 380 lm. Improvement of the spatial resolution, i.e., decrease of the wavelength, amplifies a noise at spatial harmonics when we calculated sheet current density from the measured magnetic field ‘‘Bz’’. It should be noted that a half of the cut-off wavelength corresponds to the spatial resolution in this analysis. As a result, we could confirm that the sample A exhibit a typical rooftop-like distribution of magnetic field and the corresponding uniform distribution of sheet current densities. On the other hand, sample B is unclear in the visualized magnetic field. This result indicates that homogeneity as well as Jc properties of sample B are much inferior than those of sample A. Fig. 3 shows the measured magnetic fields ‘‘Bz’’ by the Hall sensor scanned along the width direction across the CC (the arrow ‘‘"’’ in Fig. 2). When we inversely calculated the sheet current density by
(a)
Remanent
: 10 mT,
Magnetic field at the point indicated in Fig.2 Bz0 (mT)
Fig. 5. Magnetic field dependence of (a) Ic and (b) Jc at 79 K.
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K. Shiohara et al. / Physica C 471 (2011) 1041–1044
the Biot-Savart law, we could also obtain the magnetic fields on the plane ‘‘Bz0’’. The plotted ‘‘d’’ line in Fig. 3 is the remnant magnetic fields after removal of the applied external magnetic fields of 105 mT. The other lines are the results of the shielding magnetic field distributions with changing the applied external magnetic fields (5, 10, 30, 60 and 90 mT) from zero-field sequentially. Furthermore, the width distribution of longitudinal component of the sheet current density, Jx, were also shown in Fig. 4. It can be seen that the amplitude of Jx, which should be equal to the local Jc, decreased as the external field increased. By integrating shielding current across the tape, we could estimate critical current, Ic, of the tape as far as the external field is larger than the full penetration field as shown in Fig. 5a. Two points indicated as ‘‘insufficient penetration’’ in Fig. 5a means that the external magnetic field does not fully penetrate into the center of the coated conductor as shown in the results of square ‘‘h’’ and diamond shape ‘‘}’’ plotted lines in Fig. 4. However, we could obtain local Jc-B property from both results shown in Figs. 3 and 4. Fig. 5b shows Jc-B properties at 79 K at a position indicated by a circle on sample A in Fig. 2. If we compare the field dependences of these two results, we can see that the Ic is influenced by its self-field at low external field. Namely, intrinsic Jc at low field has higher value than the nominal value estimated from Ic. From the results of Fig. 5, it could be clearly demonstrated that we could also succeed in evaluation of the critical current vs. magnetic field properties including the self magnetic fields by SHPM. 4. Conclusions In this study, we investigated the local critical currents in the TFA-MOD processed coated conductors by scanning Hall-probe microscopy that is a non-contact and non-destructive evaluation method. It enabled to visualize 2D distribution of local critical current densities. As a result, it was found that the homogeneities and the critical current properties were quite different for the two short
samples. Furthermore, the current density profile along its width direction was also obtained as a function of external magnetic field. From this analysis, we could also estimate magnetic-field dependences of critical currents and of local critical current density. These findings will be helpful for the homogenization of the properties of CCs and for the precise estimation of the AC losses which will be influence by the homogeneity and by the local Jc-B characteristics. Acknowledgment This work was supported by ‘‘New Energy and Industrial Technology Development Organization (NEDO) as a part of the Rare Metal Substitute Materials Development Project’’, and ‘‘JSPS: KAKENHI (20360143)’’. References [1] T. Izumi, H. Fuji, Y. Aoki, R. Teranishi, J. Matsuda, K. Nakaoka, Y. Kitoh, S. Nomoto, Y. Yamada, A. Yajima, T. Saitoh, Y. Shiohara, Physica C 445–448 (2006) 533. [2] T. Izumi, M. Yoshizumi, J. Matsuda, K. Nakaoka, Y. Kitoh, T. Nakanishi, A. Nakai, K. Suzuki, Y. Yamada, A. Yajima, T. Saitoh, Y. Shiohara, Physica C 463–465 (2007) 510. [3] Y. Shiohara, M. Yoshizumi, T. Izumi, Y. Yamada, Supercond. Sci. Technol. 21 (2008) 034002. [4] K. Koyanagi, T. Kiss, M. Inoue, T. Nakamura, K. Imamura, M. Takeo, Y. Shiohara, Physica C 445–448 (2006) 677. [5] T. Kiss, M. Inoue, T. Shoyama, S. Koyanagi, D. Mitsui, T. Nakamura, K. Imamura, A. Ibi, Y. Yamada, T. Kato, T. Hirayama, Y. Shiohara, IEEE Trans. Appl. Supercond. 17 (2007) 3211. [6] M. Inoue, K. Abiru, Y. Honda, T. Kiss, Y. Iijima, K. Kakimoto, T. Saitoh, K. Nakao, Y. Shiohara, IEEE Trans. Appl. Supercond. 19 (2009) 2847. [7] R.B. Dinner, M.R. Beasley, K.A. Moler, Rev. Sci. Instrum. 76 (2005) 103702. [8] R.B. Dinner, K.A. Moler, D.M. Feldmann, M.R. Beasley, Phys. Rev. B 75 (2007) 144503. [9] B.J. Roth, N.G. Sepulveda, J.P. Wikswo Jr., J. Appl. Phys. 65 (2001) 361.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Measurement of local critical currents in TFA-MOD processed coated conductors by use of scanning Hall-probe microscopy K. Shiohara a,⇑, K. Higashikawa a, T. Kawaguchi a, M. Inoue a, T. Kiss a, M. Yoshizumi b, T. Izumi b a b
Kyushu University, 744 Motooka, Nishi, Fukuoka City, Fukuoka 819-0395, Japan SRL-ISTEC, 10-13 Shinonome 1-chome, Koto-ku, Tokyo 135-0062, Japan
a r t i c l e
i n f o
Article history: Available online 13 May 2011 Keywords: Characterization of superconducting materials Visualization of critical current distribution Scanning Hall-probe microscopy TFA-MOD processed superconducting coated conductor
a b s t r a c t We have carried out 2-dimensional (2D) measurement of local critical current in a TrifluoroacetatesMetal Organic Deposition (TFA-MOD) processed YBCO coated conductor using scanning Hall-probe microscopy. Recently, remarkable R&D accomplishments on the fabrication processes of coated conductors have been conducted extensively and reported. The TFA-MOD process has been expected as an attractive process to produce coated conductors with high performance at a low production cost due to a simple process using non-vacuum equipments. On the other hand, enhancement of critical currents and homogenization of the critical current distribution in the coated conductors are definitely very important for practical applications. According to our measurements, we can detect positions and spatial distribution of defects in the conductor. This kind of information will be very helpful for the improvement of the TFA-MOD process and for the design of the conductor intended for practical electric power device applications. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction A large number of fabrication processes for high critical temperature, Tc, RE-123 (RE means Rare Earth) coated conductors (CCs) have been reported. For example, even for the deposition process for a superconducting layer, at least the following four processes have been conducted mainly in the world: Pulse Laser Deposition (PLD) process, Metal Organic Deposition (MOD) process, Metal Organic Chemical Vapor Deposition (MOCVD) process, and Reactive Co-Evaporation (RCE) [1–3]. In particular, a trifluoroacetates (TFA)-MOD (TFA-MOD) process has been expected as an attractive process to fabricate high-performance CCs with low production cost due to a simple process using non-vacuum equipments and high material yields [1–3]. On the other hand, enhancement of critical currents and homogenization of the critical current distribution in the CCs has been still remained as very important subjects to be investigated for practical applications. Until now, we have several characterization techniques including spatially resolved measurements of flux flow dissipation and current distributions [4–8]. It is extremely important and useful for CC fabrication to obtain such information about the spatial distributions of the superconductive properties. These distributions (non-uniformity) are, in general, caused by kinds and dispersions of defects and ⇑ Corresponding author. Address: Faculty of Information Science and Electrical Engineering, Kyushu University, #546, West-2 Bldg, Motooka, Nishi, Fukuoka City, Fukuoka 819-0395, Japan. Tel.: +81 92 802 3678; fax: +81 92 802 3677. E-mail address: [email protected] (K. Shiohara). 0921-4534/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2011.05.119
the inhomogeneous qualities of the superconducting phase crystals. This means that it is indispensable to understand the locations of the defects and their influence on the critical current for the performance improvement of the CCs. For that, it will be very helpful to characterize in-plane distribution of critical current density in a CC in a non-destructive manner. In this study, using a non-contact and non-destructive evaluation method by means of scanning Hall-probe microscopy (SHPM), we investigated in-plane critical current density, Jc, distribution of TFA-MOD processed CCs. Furthermore, we showed that the relationship of applied external magnetic fields and local critical current density could also be characterized by this method.
2. Experimental Fig. 1 shows the SHPM system. This system was cooled by thermal conduction from the LN2 tank. A sample was located in the bore of the coil, and was cooled together with the coil. A typical temperature was controlled to be 79 K. A Hall-probe was fixed on a triaxial stage head and scanned by a stepping motor. The active area of the Hall sensor is 50 lm 50 lm and the spatial resolutions of the stage are 1 lm in the xy-plane and 0.25 lm in the z-axis. External magnetic fields were applied to the sample, and then magnetic field distribution around the sample was measured by scanning the Hall probe sensor. In this study, the measurement was carried out (1) when external magnetic fields were applied to the sample and (2) when the external fields were
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K. Shiohara et al. / Physica C 471 (2011) 1041–1044
removed after that, i.e., remanent magnetic field. When we assumed in-plane distributions of current densities, i.e., sheet current densities, we could estimate the corresponding distributions of sheet current densities from that of measured magnetic fields by considering an inverse problem of Biot-Savart law. The thickness of the superconducting layer is very thin (1 lm), and the width/ thickness aspect ratio is high, therefore 2D sheet current can be a reasonable assumption [6]. In our previous study, we obtained a good agreement between the theoretical and the experimental results that the relationships of distance from the CC’s surface to the Hall-probe sensor and the magnetic fields on the plane (B\tape) [6]. This distance between the CC’s surface and Hall-probe sensor is the ‘‘lift-off’’ and shown in the images of Fig. 2. When sufficiently high external magnetic fields were applied to the sample, the shielding currents flow at their critical current densities in almost all the regions of the sample. Similarly, when we remove the external magnetic fields after applying a sufficient magnetic field, the loop currents trapping the remanent magnetic fields flow at their critical current densities. This is the principle of the visualization of the critical current density distribution in the sample.
3. Results and discussion
Measured magnetic field Bz (mT)
Fig. 1. Photograph of the scanning Hall-probe microscope.
30
Bz
Lift-off: 400 µm
Jx
Cut-off: 380 µm
Jy
Cut-off: 380 µm
J
Cut-off: 380 µm
Sheet current density Jx (A/mm) Sheet current density Jy (A/mm)
Lift-off: 340 µm
10 -10
Sample A
Sheet current density J (A/mm)
By 2D scanning of the Hall sensor, we could obtain the 2D distribution of ‘‘Bz’’ that is the perpendicular component of the field at
32
Sample B Cut-off: 380 µm
0 -32 32
Sample A
Sample B Cut-off: 380 µm
0 -32 32
Sample A
Sample B Cut-off: 380 µm
16 0
Fig. 2. Perpendicular component of the remanent field, Bz, measured by SHPM, longitudinal component of sheet current, Jx, transverse component of sheet current, Jy, and amplitude of sheet current, J (sample A: high Ic sample, sample B: low Ic sample).
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K. Shiohara et al. / Physica C 471 (2011) 1041–1044
Cut-off:380 µm
Fig. 3. Measured magnetic field, Bz, and magnetic field on the plane (B\tape), Bz0, for different applied magnetic fields ( : 5 mT, : remanent after 105 mT).
Cut-off:380 µm
Fig. 4. Longitudinal sheet current density, Jx for different applied magnetic fields ( : 5 mT, : 10 mT, : 30 mT, : 60 mT, : 90 mT and : remanent after 105 mT).
the position of lift-off distance from the CC’s surface. The measurement was carried out when external magnetic fields were applied to the sample and when the external fields were removed after
: 30 mT,
(b)
Insufficient penetration
ex
: 60 mT,
: 90 mT and
that, i.e., remanent magnetic field. The critical current distribution in the CC could be estimated by conversion from the 2D magnetic field distribution based on the Biot-Savart law [5–9]. We investigated current distributions as well as local Jc distributions for two different 1 cm-wide YBCO CCs fabricated by TFAMOD processes. Fig. 2 shows a set of visualized results from SHPM: 2D distributions of the measured magnetic fields and the corresponding sheet current densities at the remanent magnetic fields after the application of 105 mT which was much larger than the penetration field. The lift-off distances from the samples A and B are 400 lm and 340 lm, respectively. The cut-off wavelengths in the inversion analysis of both samples are 380 lm. Improvement of the spatial resolution, i.e., decrease of the wavelength, amplifies a noise at spatial harmonics when we calculated sheet current density from the measured magnetic field ‘‘Bz’’. It should be noted that a half of the cut-off wavelength corresponds to the spatial resolution in this analysis. As a result, we could confirm that the sample A exhibit a typical rooftop-like distribution of magnetic field and the corresponding uniform distribution of sheet current densities. On the other hand, sample B is unclear in the visualized magnetic field. This result indicates that homogeneity as well as Jc properties of sample B are much inferior than those of sample A. Fig. 3 shows the measured magnetic fields ‘‘Bz’’ by the Hall sensor scanned along the width direction across the CC (the arrow ‘‘"’’ in Fig. 2). When we inversely calculated the sheet current density by
(a)
Remanent
: 10 mT,
Magnetic field at the point indicated in Fig.2 Bz0 (mT)
Fig. 5. Magnetic field dependence of (a) Ic and (b) Jc at 79 K.
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K. Shiohara et al. / Physica C 471 (2011) 1041–1044
the Biot-Savart law, we could also obtain the magnetic fields on the plane ‘‘Bz0’’. The plotted ‘‘d’’ line in Fig. 3 is the remnant magnetic fields after removal of the applied external magnetic fields of 105 mT. The other lines are the results of the shielding magnetic field distributions with changing the applied external magnetic fields (5, 10, 30, 60 and 90 mT) from zero-field sequentially. Furthermore, the width distribution of longitudinal component of the sheet current density, Jx, were also shown in Fig. 4. It can be seen that the amplitude of Jx, which should be equal to the local Jc, decreased as the external field increased. By integrating shielding current across the tape, we could estimate critical current, Ic, of the tape as far as the external field is larger than the full penetration field as shown in Fig. 5a. Two points indicated as ‘‘insufficient penetration’’ in Fig. 5a means that the external magnetic field does not fully penetrate into the center of the coated conductor as shown in the results of square ‘‘h’’ and diamond shape ‘‘}’’ plotted lines in Fig. 4. However, we could obtain local Jc-B property from both results shown in Figs. 3 and 4. Fig. 5b shows Jc-B properties at 79 K at a position indicated by a circle on sample A in Fig. 2. If we compare the field dependences of these two results, we can see that the Ic is influenced by its self-field at low external field. Namely, intrinsic Jc at low field has higher value than the nominal value estimated from Ic. From the results of Fig. 5, it could be clearly demonstrated that we could also succeed in evaluation of the critical current vs. magnetic field properties including the self magnetic fields by SHPM. 4. Conclusions In this study, we investigated the local critical currents in the TFA-MOD processed coated conductors by scanning Hall-probe microscopy that is a non-contact and non-destructive evaluation method. It enabled to visualize 2D distribution of local critical current densities. As a result, it was found that the homogeneities and the critical current properties were quite different for the two short
samples. Furthermore, the current density profile along its width direction was also obtained as a function of external magnetic field. From this analysis, we could also estimate magnetic-field dependences of critical currents and of local critical current density. These findings will be helpful for the homogenization of the properties of CCs and for the precise estimation of the AC losses which will be influence by the homogeneity and by the local Jc-B characteristics. Acknowledgment This work was supported by ‘‘New Energy and Industrial Technology Development Organization (NEDO) as a part of the Rare Metal Substitute Materials Development Project’’, and ‘‘JSPS: KAKENHI (20360143)’’. References [1] T. Izumi, H. Fuji, Y. Aoki, R. Teranishi, J. Matsuda, K. Nakaoka, Y. Kitoh, S. Nomoto, Y. Yamada, A. Yajima, T. Saitoh, Y. Shiohara, Physica C 445–448 (2006) 533. [2] T. Izumi, M. Yoshizumi, J. Matsuda, K. Nakaoka, Y. Kitoh, T. Nakanishi, A. Nakai, K. Suzuki, Y. Yamada, A. Yajima, T. Saitoh, Y. Shiohara, Physica C 463–465 (2007) 510. [3] Y. Shiohara, M. Yoshizumi, T. Izumi, Y. Yamada, Supercond. Sci. Technol. 21 (2008) 034002. [4] K. Koyanagi, T. Kiss, M. Inoue, T. Nakamura, K. Imamura, M. Takeo, Y. Shiohara, Physica C 445–448 (2006) 677. [5] T. Kiss, M. Inoue, T. Shoyama, S. Koyanagi, D. Mitsui, T. Nakamura, K. Imamura, A. Ibi, Y. Yamada, T. Kato, T. Hirayama, Y. Shiohara, IEEE Trans. Appl. Supercond. 17 (2007) 3211. [6] M. Inoue, K. Abiru, Y. Honda, T. Kiss, Y. Iijima, K. Kakimoto, T. Saitoh, K. Nakao, Y. Shiohara, IEEE Trans. Appl. Supercond. 19 (2009) 2847. [7] R.B. Dinner, M.R. Beasley, K.A. Moler, Rev. Sci. Instrum. 76 (2005) 103702. [8] R.B. Dinner, K.A. Moler, D.M. Feldmann, M.R. Beasley, Phys. Rev. B 75 (2007) 144503. [9] B.J. Roth, N.G. Sepulveda, J.P. Wikswo Jr., J. Appl. Phys. 65 (2001) 361.