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]Influence of misorientation angle on third harmonic voltages V3 induced by YBCO thin films on bi-crystal substrates
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Physics Procedia 27 (2012) 292 – 295
ISS2011
Influence of misorientation angle on third harmonic voltages V3 induced by YBCO thin films on bi-crystal substrates Kensuke Oktia, Masashi Tanigawa, Shinpei Kasahara, Yuta Hatanaka, Tetsuro Sueyoshi, Takanori Fujiyoshi Department of Computer Science and Electrical Engineering, Kumamoto University, 2-39-1, Kurokami, Kumamoto 860-8555, Japan
Abstract
YBa2Cu3Oy thin films have been deposited on SrTiO3 bi-crystal substrates with various misorientation angles and a STO single-crystal substrate by a pulsed laser deposition method. The magnetic field dependence of third harmonic voltage V3 vs. coil current I0 was measured by the third harmonic voltage method when the coils were mounted on the crystal grain boundary. The V3 increases monotonically with increasing I0 in the single-crystal substrate. However, the V3 increases strangely with increasing I0 in the bi-crystal substrate with low-angle grain boundary in low-magnetic fields. On the other hand, the V3 increases monotonically in the bi-crystal substrates with high-angle grain boundary. © 2012 2011 Published Published by by Elsevier ElsevierB.V. Ltd. Selection Selection and/or and/orpeer-review peer-reviewunder underresponsibility responsibilityofofISS ISSProgram ProgramCommittee Committee. Keywords: Third harmonic voltage method; Crystal grain boundary; Bi-crystal substrate
1. Introduction High critical current density Jc and its high uniformity are required for the development of long-length YBa2Cu3Oy (YBCO) coated conductors [1]. Then, it is important to investigate the local distribution of Jc in YBCO coated conductors to detect crystal grain boundary, crystal defects, etc. As the typical measurement technique for nondestructive measurement of Jc, there are the third harmonic voltage method [2] and the permanent magnet method [3]. The third harmonic voltage method is very useful to nondestructively estimate the Jc distribution of a superconducting film without contact. In this method, an ac magnetic field is applied by a small coil placed just above the superconducting film. The harmonic voltages are generated due to the nonlinear magnetic response of the superconducting film and Jc is determined by detecting the largest harmonic voltage: the third harmonic voltage V3. The validity of this method was clarified by some research groups theoretically and experimentally [4-9]. Recently, we have presented an interesting measurement result of a YBCO thin film on a SrTiO3 (STO) bi-crystal substrate using the third harmonic voltage method [10]. In this study, we measured the third harmonic voltage V3 as a function of the coil current I0 in YBCO thin films deposited on STO bi-crystal substrates with various misorientation angles and a STO single-crystal substrate. The influence of the misorientation angle on V3 and the magnetic field dependence of V3 were investigated.
Corresponding author. Tel.: +81-96-342-3640 ; fax: +81-96-342-3630 . E-mail address: [email protected] .
1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of ISS Program Committee doi:10.1016/j.phpro.2012.03.468
Kensuke Oktia et al. / Physics Procedia 27 (2012) 292 – 295
2. Experimental The YBCO thin films were deposited on STO bi-crystal substrates with nominally symmetric [100] tilt boundaries and a STO single-crystal substrate by a pulsed laser deposition method. The detail of the sample preparation has been published elsewhere [11]. The size and thickness of the YBCO thin films are 10 mm u 10mm and about 280nm, respectively. Table 1 shows the misorientation angle for STO bi-crystal substrates. We prepared the sample E which has an artificial defect in surface of YBCO thin film for comparison. The artificial defect was made by hand-scratched using cut-knife, and the width of ditch is about 20 Pm. Fig. 1 shows the X-ray I scans of the (102) peak of the YBCO thin films on STO bi-crystal substrates and STO single-crystal substrate. We confirmed that the peak appears whenever I shifts by the misorientation angle. In the third harmonic voltage method, we used the two kinds of small coils, the drive and pick-up coils. The internal pick-up coil (1.0 mm in inner diameter, 2.4 mm in outer diameter, 1.0 mm in height and 200 turns winding) and the external drive coil (2.4 mm in inner diameter, 5.0 mm in outer diameter, 1.0 mm in height and 400 turns winding) were wound and the coils were made of the enamel wire 50 Pm in diameter. Fig. 2 shows the block diagram of an electronic system for measuring a V3-I0 curve by the third harmonic voltage method. The frequency f of the coil current was changed in the range of 0.5 ~ 10 kHz. The ac current I0 cos 2Sft was supplied from a signal generator to the drive coil, and the third harmonic voltage induced in the pick-up coil was measured with a lock-in amplifier. The voltage of a shunt resistance was measured with a digital multimeter to monitor the current through the drive coil. The coils were placed so that these axes were parallel to the c-axis of the YBCO thin film and were mounted about 0.2 mm above the YBCO thin film. The YBCO thin film protected by a kapton sheet was fixed on the sample holder of the measurement system and cooled by liquid N2. A magnetic field was applied by a neodymium magnet and was measured by two Hall devices put on the corners of the sample holder.
Sample
Material
Table 1 Specification of the samples Substrate Misorientation angle
A
YBCO
STO single-crystal
-
B
YBCO
STO bi-crystal
2T = 10.0º
C
YBCO
STO bi-crystal
2T = 22.6º
D
YBCO
STO bi-crystal
2T = 36.8º
E
YBCO
STO single-crystal
scratch defect
Sample A
Intensity [a.u.]
Sample B
Sample C
Sample D
0
100
200
300
I [deg.]
Fig. 1. The I scans of (102) peak of YBCO thin films.
Fig. 2. Schematic illustration of electronic system.
3. Results and discussion Fig. 3 (a) and (b) show the dependence of V3 and V3/fI0 on I0 for Sample A and Sample B in the self-magnetic field, respectively. The coils were mounted at the center of the YBCO thin films. This area is just above the crystal grain
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boundary as shown in Fig. 3 (b). In Fig. 3 (a), the typical third harmonic voltage curves induced by the YBCO thin film on STO single-crystal substrate for various frequency (0.5 ~ 10kHz). The V3 increases monotonically with increasing I0 over a certain threshold value. Fig. 3 (b) shows the V3-I0 curves of Sample B in which the misorientation angle of the substrate is 2T = 10.0º. Unlike the behavior of V3 in Sample A, the V3 of Sample B decreases once after having increased, and then increases again with increasing I0. In Sample B, after the shielding current flowing across the crystal grain boundary (inter-grain current) reaches the critical value, the main part of the shielding current flows along the crystal grain boundary [12], because the inter-grain critical current is much smaller than the intra-grain critical current [13]. This complicated behavior of the shielding current affects the generation of V3. We must also consider the edge effect [14, 15] by the crystal grain boundary. Fig. 4 (a) and (b) show the V3-I0 curves of Sample D in which the misorientation angle of the substrate is 2T = 36.8º and Sample E with the artificial defect at the center, respectively. Unlike the behavior of V3 in Sample B, the V3 of Sample D increase monotonically with increasing I0. The monotonic increase of V3 with increasing I0 is also observed in Sample E. However, the amplitude of V3 in Sample D is smaller than that of Sample E in high frequency region. The shielding current cannot flow across the artificial defect in Sample E, the schematic illustration of the shielding current flowing in Sample E is shown in the Fig. 4 (b) [15]. On the other hand, a few shielding current flows across the crystal grain boundary in Sample D. Therefore, the behavior of V3 depends on the amount of the inter-grain current strongly. (b)
2 Sample A B = 0T 500Hz 1kHz 2kHz 5kHz 10kHz
1
Coil
2
YBCO thin film V3 / л2 [mV]
V3 / л2 [mV]
(a)
0
1
Sample B B = 0T 500Hz 1kHz 2kHz 5kHz 10kHz
Crystal grain-boundary
0
8 20 V3 / f I0 [PH]
V3 / f I0 [PH]
6 4
10
2 0 0
20
40 60 I0 / л2 [mA]
0 0
80
20
40 60 I0 / л2 [mA]
80
Fig. 3. The third harmonic voltage V3/2 and the normalized V3/fI0 as function of the coil current I0/2. 2
1
0 0
(b) Sample D B = 0T 500Hz 1kHz 2kHz 5kHz 10kHz
20
2
Crystal grain-boundary
V3 / л2 [mV]
V3 / л2 [mV]
(a)
40 60 I0 / л2 [mA]
80
1
0 0
Shielding current
Sample E B = 0T 500Hz 1kHz 2kHz 5kHz 10kHz
20
defect
40 60 I0 / л2 [mA]
Fig. 4. The third harmonic voltage V3/2 vs. the coil current I0/2.
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Fig. 5 (a) and (b) show the magnetic field dependence of V3-I0 curves in 10 kHz for Sample B and Sample C, respectively. In Sample B, the V3 in low-magnetic field shows the two-step increase behavior. When the magnetic field increased, the behavior of V3 has changed from a two-step increase into a monotonic one. The shielding current in high-magnetic field is composed mainly of the intra-grain current flowing along the grain boundary, because the inter-grain critical current reduces drastically with increasing magnetic field [10]. In Sample C, the V3 increases from zero point in all the magnetic field, because the magnetic flux which generated from the drive coil was not completely shielded by the YBCO thin film. The V3 shows a little two-step increase behavior. It is considered that only a few shielding current flows across the crystal grain boundary, because the misorientation angle of Sample C is larger than that of Sample B. (b)
1.5
Sample B f = 10kHz 0T 0.01T 0.03T 0.05T 0.07T 0.1T 0.15T 0.2T 0.25T
1
0.5
0 0
20 40 60 Coil current I0 / л2 [mA]
V3 / л2 [mV]
V3 / л2 [mV]
(a)
1.5
Sample C f = 10kHz 0T 0.01T 0.03T 1 0.05T 0.07T 0.1T 0.15T 0.2T 0.5 0.25T
80
0 0
20 40 60 Coil current I0 / л2 [mA]
80
Fig. 5. The magnetic field dependence of V3–I0 curves in 10 kHz.
4. Conclusion In this study, we measured the V3-I0 curves in YBCO thin films on STO bi-crystal substrates with various misorientation angles using the third harmonic voltage method. In the YBCO thin films on STO bi-crystal substrates with 2T = 10.0º and 22.6º, the V3 exhibits the two-step increase. In the case of 2T = 36.8º, however, the V3 increases monotonically. From measuring result of the YBCO thin film with the artificial defect, we can confirm that the intergrain current hardly flows across the crystal grain boundary in the case of high-angle grain boundary.
References [1] T. Izumi, Y. Shiohara, Physica C 470 (2010) 967. [2] J. H. Claasen, M. E. Reeves, R. J. Soulen Jr., Rev. Sci. Instrum. 62 (1991) 996. [3] A. Saito, K. Takeishi, Y. Takano, T. Nakamura, M. Yokoo, M. Mukaida, et al, Physica C 426–431 (2005) 1122. [4] H. Yamasaki, Y. Mawatari, Y. Nakagawa, Appl. Phys. Lett. 82 (2003) 3275. [5] H. Yamasaki, Y. Mawatari, Y. Nakagawa, IEEE Trans. Appl. Supercond. 15 (2005) 3636. [6] H. Yamada, A. Bitoh, Y. Mitsumo, I. Imai, K. Nomura, K. Kanayama et al, Physica C 433 (2005) 59. [7] H. Yamada, T. Minakuchi, T. Furuta, K. Takegami, S. Nakagawa, K. Kanayama et al, J. Phys. : Conf. Ser. 97 (2008) 012005. [8] K. Ohki, H. Yamasaki, K. Develos-Bagarinao, Y. Nakagawa, Supercond. Sci. Technol. 21 (2008) 045004. [9] S. Ota, S. Ono, J. H. Lee, A. Saito, H. Yamasaki, S. Ohshima, Physica C 469 (2009) 1454. [10] K. Okita, Y. Hatanaka, A. Adachi, T. Sueyoshi, T. Fujiyoshi, Supercond. Sci. Technol. 24 (2011) 125004. [11] T. Sueyoshi, M. Watanabe, M. Haruta, T. Fujiyoshi, K. Miyahara, T. Ikegami et al, Physica C 468 (2008) 1266. [12] N. F. Heinig, R. D. Redwing, J. E. Nordman, D. C. Larbalestier, Phys. Rev. B 60 (1999) 1409. [13] D. Dimos, P. Chaudhari, J. Mannhart, F. K. LeGoues, Phys. Rev. Lett. 61 (1988) 219. [14] T. Nadami, E. S. Otabe, M. Kiuchi, T. Matsushita, Physica C 412-414 (2004) 1011. [15] A. Kamitani, T. Takayama, A. Tanaka, S. Ikuno, Physica C 470 (2010) 1189.
Physics Procedia 27 (2012) 292 – 295
ISS2011
Influence of misorientation angle on third harmonic voltages V3 induced by YBCO thin films on bi-crystal substrates Kensuke Oktia, Masashi Tanigawa, Shinpei Kasahara, Yuta Hatanaka, Tetsuro Sueyoshi, Takanori Fujiyoshi Department of Computer Science and Electrical Engineering, Kumamoto University, 2-39-1, Kurokami, Kumamoto 860-8555, Japan
Abstract
YBa2Cu3Oy thin films have been deposited on SrTiO3 bi-crystal substrates with various misorientation angles and a STO single-crystal substrate by a pulsed laser deposition method. The magnetic field dependence of third harmonic voltage V3 vs. coil current I0 was measured by the third harmonic voltage method when the coils were mounted on the crystal grain boundary. The V3 increases monotonically with increasing I0 in the single-crystal substrate. However, the V3 increases strangely with increasing I0 in the bi-crystal substrate with low-angle grain boundary in low-magnetic fields. On the other hand, the V3 increases monotonically in the bi-crystal substrates with high-angle grain boundary. © 2012 2011 Published Published by by Elsevier ElsevierB.V. Ltd. Selection Selection and/or and/orpeer-review peer-reviewunder underresponsibility responsibilityofofISS ISSProgram ProgramCommittee Committee. Keywords: Third harmonic voltage method; Crystal grain boundary; Bi-crystal substrate
1. Introduction High critical current density Jc and its high uniformity are required for the development of long-length YBa2Cu3Oy (YBCO) coated conductors [1]. Then, it is important to investigate the local distribution of Jc in YBCO coated conductors to detect crystal grain boundary, crystal defects, etc. As the typical measurement technique for nondestructive measurement of Jc, there are the third harmonic voltage method [2] and the permanent magnet method [3]. The third harmonic voltage method is very useful to nondestructively estimate the Jc distribution of a superconducting film without contact. In this method, an ac magnetic field is applied by a small coil placed just above the superconducting film. The harmonic voltages are generated due to the nonlinear magnetic response of the superconducting film and Jc is determined by detecting the largest harmonic voltage: the third harmonic voltage V3. The validity of this method was clarified by some research groups theoretically and experimentally [4-9]. Recently, we have presented an interesting measurement result of a YBCO thin film on a SrTiO3 (STO) bi-crystal substrate using the third harmonic voltage method [10]. In this study, we measured the third harmonic voltage V3 as a function of the coil current I0 in YBCO thin films deposited on STO bi-crystal substrates with various misorientation angles and a STO single-crystal substrate. The influence of the misorientation angle on V3 and the magnetic field dependence of V3 were investigated.
Corresponding author. Tel.: +81-96-342-3640 ; fax: +81-96-342-3630 . E-mail address: [email protected] .
1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of ISS Program Committee doi:10.1016/j.phpro.2012.03.468
Kensuke Oktia et al. / Physics Procedia 27 (2012) 292 – 295
2. Experimental The YBCO thin films were deposited on STO bi-crystal substrates with nominally symmetric [100] tilt boundaries and a STO single-crystal substrate by a pulsed laser deposition method. The detail of the sample preparation has been published elsewhere [11]. The size and thickness of the YBCO thin films are 10 mm u 10mm and about 280nm, respectively. Table 1 shows the misorientation angle for STO bi-crystal substrates. We prepared the sample E which has an artificial defect in surface of YBCO thin film for comparison. The artificial defect was made by hand-scratched using cut-knife, and the width of ditch is about 20 Pm. Fig. 1 shows the X-ray I scans of the (102) peak of the YBCO thin films on STO bi-crystal substrates and STO single-crystal substrate. We confirmed that the peak appears whenever I shifts by the misorientation angle. In the third harmonic voltage method, we used the two kinds of small coils, the drive and pick-up coils. The internal pick-up coil (1.0 mm in inner diameter, 2.4 mm in outer diameter, 1.0 mm in height and 200 turns winding) and the external drive coil (2.4 mm in inner diameter, 5.0 mm in outer diameter, 1.0 mm in height and 400 turns winding) were wound and the coils were made of the enamel wire 50 Pm in diameter. Fig. 2 shows the block diagram of an electronic system for measuring a V3-I0 curve by the third harmonic voltage method. The frequency f of the coil current was changed in the range of 0.5 ~ 10 kHz. The ac current I0 cos 2Sft was supplied from a signal generator to the drive coil, and the third harmonic voltage induced in the pick-up coil was measured with a lock-in amplifier. The voltage of a shunt resistance was measured with a digital multimeter to monitor the current through the drive coil. The coils were placed so that these axes were parallel to the c-axis of the YBCO thin film and were mounted about 0.2 mm above the YBCO thin film. The YBCO thin film protected by a kapton sheet was fixed on the sample holder of the measurement system and cooled by liquid N2. A magnetic field was applied by a neodymium magnet and was measured by two Hall devices put on the corners of the sample holder.
Sample
Material
Table 1 Specification of the samples Substrate Misorientation angle
A
YBCO
STO single-crystal
-
B
YBCO
STO bi-crystal
2T = 10.0º
C
YBCO
STO bi-crystal
2T = 22.6º
D
YBCO
STO bi-crystal
2T = 36.8º
E
YBCO
STO single-crystal
scratch defect
Sample A
Intensity [a.u.]
Sample B
Sample C
Sample D
0
100
200
300
I [deg.]
Fig. 1. The I scans of (102) peak of YBCO thin films.
Fig. 2. Schematic illustration of electronic system.
3. Results and discussion Fig. 3 (a) and (b) show the dependence of V3 and V3/fI0 on I0 for Sample A and Sample B in the self-magnetic field, respectively. The coils were mounted at the center of the YBCO thin films. This area is just above the crystal grain
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Kensuke Oktia et al. / Physics Procedia 27 (2012) 292 – 295
boundary as shown in Fig. 3 (b). In Fig. 3 (a), the typical third harmonic voltage curves induced by the YBCO thin film on STO single-crystal substrate for various frequency (0.5 ~ 10kHz). The V3 increases monotonically with increasing I0 over a certain threshold value. Fig. 3 (b) shows the V3-I0 curves of Sample B in which the misorientation angle of the substrate is 2T = 10.0º. Unlike the behavior of V3 in Sample A, the V3 of Sample B decreases once after having increased, and then increases again with increasing I0. In Sample B, after the shielding current flowing across the crystal grain boundary (inter-grain current) reaches the critical value, the main part of the shielding current flows along the crystal grain boundary [12], because the inter-grain critical current is much smaller than the intra-grain critical current [13]. This complicated behavior of the shielding current affects the generation of V3. We must also consider the edge effect [14, 15] by the crystal grain boundary. Fig. 4 (a) and (b) show the V3-I0 curves of Sample D in which the misorientation angle of the substrate is 2T = 36.8º and Sample E with the artificial defect at the center, respectively. Unlike the behavior of V3 in Sample B, the V3 of Sample D increase monotonically with increasing I0. The monotonic increase of V3 with increasing I0 is also observed in Sample E. However, the amplitude of V3 in Sample D is smaller than that of Sample E in high frequency region. The shielding current cannot flow across the artificial defect in Sample E, the schematic illustration of the shielding current flowing in Sample E is shown in the Fig. 4 (b) [15]. On the other hand, a few shielding current flows across the crystal grain boundary in Sample D. Therefore, the behavior of V3 depends on the amount of the inter-grain current strongly. (b)
2 Sample A B = 0T 500Hz 1kHz 2kHz 5kHz 10kHz
1
Coil
2
YBCO thin film V3 / л2 [mV]
V3 / л2 [mV]
(a)
0
1
Sample B B = 0T 500Hz 1kHz 2kHz 5kHz 10kHz
Crystal grain-boundary
0
8 20 V3 / f I0 [PH]
V3 / f I0 [PH]
6 4
10
2 0 0
20
40 60 I0 / л2 [mA]
0 0
80
20
40 60 I0 / л2 [mA]
80
Fig. 3. The third harmonic voltage V3/2 and the normalized V3/fI0 as function of the coil current I0/2. 2
1
0 0
(b) Sample D B = 0T 500Hz 1kHz 2kHz 5kHz 10kHz
20
2
Crystal grain-boundary
V3 / л2 [mV]
V3 / л2 [mV]
(a)
40 60 I0 / л2 [mA]
80
1
0 0
Shielding current
Sample E B = 0T 500Hz 1kHz 2kHz 5kHz 10kHz
20
defect
40 60 I0 / л2 [mA]
Fig. 4. The third harmonic voltage V3/2 vs. the coil current I0/2.
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Fig. 5 (a) and (b) show the magnetic field dependence of V3-I0 curves in 10 kHz for Sample B and Sample C, respectively. In Sample B, the V3 in low-magnetic field shows the two-step increase behavior. When the magnetic field increased, the behavior of V3 has changed from a two-step increase into a monotonic one. The shielding current in high-magnetic field is composed mainly of the intra-grain current flowing along the grain boundary, because the inter-grain critical current reduces drastically with increasing magnetic field [10]. In Sample C, the V3 increases from zero point in all the magnetic field, because the magnetic flux which generated from the drive coil was not completely shielded by the YBCO thin film. The V3 shows a little two-step increase behavior. It is considered that only a few shielding current flows across the crystal grain boundary, because the misorientation angle of Sample C is larger than that of Sample B. (b)
1.5
Sample B f = 10kHz 0T 0.01T 0.03T 0.05T 0.07T 0.1T 0.15T 0.2T 0.25T
1
0.5
0 0
20 40 60 Coil current I0 / л2 [mA]
V3 / л2 [mV]
V3 / л2 [mV]
(a)
1.5
Sample C f = 10kHz 0T 0.01T 0.03T 1 0.05T 0.07T 0.1T 0.15T 0.2T 0.5 0.25T
80
0 0
20 40 60 Coil current I0 / л2 [mA]
80
Fig. 5. The magnetic field dependence of V3–I0 curves in 10 kHz.
4. Conclusion In this study, we measured the V3-I0 curves in YBCO thin films on STO bi-crystal substrates with various misorientation angles using the third harmonic voltage method. In the YBCO thin films on STO bi-crystal substrates with 2T = 10.0º and 22.6º, the V3 exhibits the two-step increase. In the case of 2T = 36.8º, however, the V3 increases monotonically. From measuring result of the YBCO thin film with the artificial defect, we can confirm that the intergrain current hardly flows across the crystal grain boundary in the case of high-angle grain boundary.
References [1] T. Izumi, Y. Shiohara, Physica C 470 (2010) 967. [2] J. H. Claasen, M. E. Reeves, R. J. Soulen Jr., Rev. Sci. Instrum. 62 (1991) 996. [3] A. Saito, K. Takeishi, Y. Takano, T. Nakamura, M. Yokoo, M. Mukaida, et al, Physica C 426–431 (2005) 1122. [4] H. Yamasaki, Y. Mawatari, Y. Nakagawa, Appl. Phys. Lett. 82 (2003) 3275. [5] H. Yamasaki, Y. Mawatari, Y. Nakagawa, IEEE Trans. Appl. Supercond. 15 (2005) 3636. [6] H. Yamada, A. Bitoh, Y. Mitsumo, I. Imai, K. Nomura, K. Kanayama et al, Physica C 433 (2005) 59. [7] H. Yamada, T. Minakuchi, T. Furuta, K. Takegami, S. Nakagawa, K. Kanayama et al, J. Phys. : Conf. Ser. 97 (2008) 012005. [8] K. Ohki, H. Yamasaki, K. Develos-Bagarinao, Y. Nakagawa, Supercond. Sci. Technol. 21 (2008) 045004. [9] S. Ota, S. Ono, J. H. Lee, A. Saito, H. Yamasaki, S. Ohshima, Physica C 469 (2009) 1454. [10] K. Okita, Y. Hatanaka, A. Adachi, T. Sueyoshi, T. Fujiyoshi, Supercond. Sci. Technol. 24 (2011) 125004. [11] T. Sueyoshi, M. Watanabe, M. Haruta, T. Fujiyoshi, K. Miyahara, T. Ikegami et al, Physica C 468 (2008) 1266. [12] N. F. Heinig, R. D. Redwing, J. E. Nordman, D. C. Larbalestier, Phys. Rev. B 60 (1999) 1409. [13] D. Dimos, P. Chaudhari, J. Mannhart, F. K. LeGoues, Phys. Rev. Lett. 61 (1988) 219. [14] T. Nadami, E. S. Otabe, M. Kiuchi, T. Matsushita, Physica C 412-414 (2004) 1011. [15] A. Kamitani, T. Takayama, A. Tanaka, S. Ikuno, Physica C 470 (2010) 1189.