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Growth and characterization of a-axis oriented ErBa2Cu3O7−δ films using double buffer layers
Physica C 426–431 (2005) 1424–1428 www.elsevier.com/locate/physc
Growth and characterization of a-axis oriented ErBa2Cu3O7d films using double buffer layers Y. Shingai a,f,*, M. Mukaida a,f, K. Matsumoto b,f, Y. Yoshida c,f, A. Ichinose S. Horii e,f, K. Koike a, F. Hirose a, A. Saito a, S. Ohshima a a
d,f
,
Yamagata University, Jonan 4-3-16, Yonezawa, Yamagata 992-8510, Japan b Kyoto University, Sakyou-ku, Kyoto, Kyoto 606-8510, Japan c Nagoya University, Chikusa-ku, Nagoya, Aichi 464-8603, Japan d CREPI, Nagasaka, 2-6-1, Yokosuka, Kanagawa 240-0196, Japan e University of Tokyo, Hongo 7-3-1,Bunkyo-ku, Tokyo 113-8685, Japan f CREST JST, Honmachi 4-1-8, Kawaguchi, Saitama 332-0012, Japan Received 23 November 2004; accepted 11 March 2005
Abstract We have grown a-axis oriented ErBa2Cu3O7d (ErBCO) films. Crystallinity of ErBCO films grown on Gd2CuO4 (GCO) buffer layers are investigated at various growth temperatures, in order to find a condition to fabricate a-axis oriented ErBCO films. a-Axis oriented ErBCO films are also grown at a high growth substrate temperature by using YBa2Cu3O7d (YBCO)/GCO double buffer layers. We have successfully fabricated a-axis oriented ErBCO films with the GCO buffer layer by optimizing the substrate temperature during growth. Furthermore, we optimized growth temperatures for a-axis oriented ErBCO films are increased by using the YBCO/GCO buffer layer. Ó 2005 Elsevier B.V. All rights reserved. PACS: 74.72.Jt; 74.76.Bz Keywords: a-Axis oriented ErBa2Cu3O7d films; Selective preferred orientation control; REBa2Cu3O7d films
1. Introduction * Corresponding author. Address: Mukaida Laboratory, Department of Electronic Engineering, Faculty of Industry, Yamagata University, Jonan 4-3-16, Yonezawa, Yamagata 992-8510, Japan. Tel./fax: +81 238 26 3316. E-mail address: [email protected] (Y. Shingai).
Recently, YBa2Cu3O7d (YBCO) films are attracting attentions to be applied for power applications. However, the YBCO transport properties (at 77 K) are inferior to that of metallic superconducting material in a magnetic field. Then, the
0921-4534/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.03.056
Y. Shingai et al. / Physica C 426–431 (2005) 1424–1428
researches are making efforts to fabricate the HTS with high transport properties in magnetic field. A superconducting material with pinning centers has high transport properties compared with a superconducting material without pinning centers. Now, we are trying to grow YBCO films with a boundary between an a-axis oriented YBCO grains and a c-axis oriented YBCO grains (a/c boundary) [1], in order to study how the a/c boundary in the YBCO film acts as a pinning center. Recently, REBa2Cu3O7d (RE: rare earth elements) superconducting materials attract attentions as a substitution of YBCO [2–4]. Here, we focused on ErBa2Cu3O7d (ErBCO). The ErBCO have some advantages. For example, the ErBCO have a higher superconducting transition temperature (TC) than that of YBCO, no substitution problems between RE and Ba atoms and easy controlling of oxygen contents in a superconducting film by annealing [5]. However, a-axis oriented ErBCO films have not been reported yet. Then, the growth of a-axis oriented ErBCO films is necessary to fabricate ErBCO films with a/c boundaries. In this study, we investigate preferred orientation of ErBCO films on a Gd2CuO4 (GCO) buffer layer at varied growth temperatures, in order to find a condition to fabricate a-axis oriented ErBCO films.
2. Experimental Substrates used in this study were (1 0 0)-oriented SrLaGaO4 (SLGO). ErBCO and YBCO films and Gd2CuO4 (GCO) buffer layers were grown by pulsed ArF excimer laser deposition. The laser targets were sintered ErBCO ceramic, a sintered YBCO ceramic and a sintered GCO ceramic. The substrate was attached with silver paste to a rotating metal-substrate holder, which was heated by a ramp heater. The substrate temperature was monitored by a thermocouple and calibrated by an optical pyrometer. Table 1 shows a list of lattice constants and crystal structure of materials used in this study. During the ErBCO and YBCO film growth, the oxygen pressures were 400 mTorr. The oxygen pressure for the growth of a GCO buffer layer
1425
Table 1 Lattice constants and crystalline structures of materials used in this study Material
Crystalline structure
Lattice constant (nm)
SrLaGaO4
K2NiF4
Gd2CuO4
K2NiF4
YBa2Cu3O7d
REBa2Cu3O7d
ErBa2Cu3O7d
REBa2Cu3O7d
a = b = 0.384, c = 1.268 a = b = 0.389, c = 1.189 a = 0.382, b = 0.388, c = 1.168 a = 0.381, b = 0.388, c = 1.166
was fixed at 40 mTorr with an oxygen flow rate of 5 sccm. The substrate temperatures at the growth of the GCO buffer layer and an YBCO buffer layer were 730 and 660 °C, respectively. The typical growth time of GCO buffer layers were 5 min, YBCO buffer layers were 10 min, and ErBCO films were 60 min. After the growth, the ErBCO films were cooled down rapidly to room temperature in O2 gas with increased atmospheric pressure. The other hands, YBCO and GCO buffer layers were cooled down slowly in the same pressure as the growth of the films. The preferred orientations of ErBCO films grown in this study were determined by X-ray h/2h diffraction with Cu-Ka radiation. The inplane orientations of the ErBCO films were evaluated by X-ray /-scan by using a (1 0 2) plane of ErBCO. Lattice images and grain azimuth were observed by a TEM. The q–T curves of grown ErBCO films were observed by a 4-probe method in magnetic fields (direction of current k [0 1 0] SLGO, direction of magnetic field k [1 0 0] SLGO).
3. Results and discussion 3.1. a-Axis oriented ErBCO films using GCO buffer layers An a-axis oriented YBCO film was obtained on a SLGO substrate at optimized growth conditions [6]. When using a GCO buffer layer, growth window for the grown of a-axis oriented YBCO film
1426
Y. Shingai et al. / Physica C 426–431 (2005) 1424–1428
Here, an I2 0 0 means the intensity of the ErBCO(2 0 0) peak characterized by X-ray h/2h diffraction and an I0 0 6 means the ErBCO(0 0 6) peak too. (The a-axis oriented ratio defined in this study is a simple indicator, because the I2 0 0 dose not equal to I0 0 6 at the same quantity of a-axis oriented grains and c-axis oriented grains.) By increasing the substrate temperature, the a-axis oriented ratio was decreased similar to the a-axis oriented ratio of YBCO films [1]. In Fig. 1, it was confirmed that a-axis oriented films were grown on the GCO buffer at a substrate temperature lower than 680 °C. The ErBCO film grown at 680 °C with the GCO buffer layer was also confirmed to have a c-axis in plane alignment by using X-ray /-scan.
a-axis oriented ratio (%)
100
50
0 650
700 Substrate temperature (OC)
750
Fig. 1. Preferred orientations of ErBCO films with respect to the substrate temperature during the growth.
3.2. Fabrication of a-axis oriented ErBCO films at a high growth temperature
is wider than that of without the buffer layer. Then, ErBCO films were grown on SLGO substrates with GCO buffer layers at several substrate temperatures. Fig. 1 shows a-axis oriented ratios of ErBCO films with several substrate temperatures during the growth of ErBCO films. Here, the a-axis oriented ratio was defined by using I2 0 0 . I2 0 0 þ I0 0 6
Growth temperature higher than 700 °C is necessary for a-axis oriented ErBCO films with good electrical properties [2,5]. A bc-plane of YBCO has a size between a bc-plane of ErBCO and a bc-plane of GCO. In addition, an a-axis oriented YBCO film grown on a GCO buffer layer has good crystallinity [7]. For these reasons, high crystalline
ð1Þ
sub.
ErBCO
006
a.u.
a.u.
200
200
sub.
ErBCO
46 (a)
47 48 2θ (degree)
49
46 (b)
47 48 2θ (degree)
49
Fig. 2. The typical X-ray diffraction patterns of ErBCO films grown at 700 °C. An ErBCO film without (a) and with (b) YBCO/GCO double buffer layers.
Y. Shingai et al. / Physica C 426–431 (2005) 1424–1428
quality a-axis oriented ErBCO films were expected to be grown at above 700 °C by using an YBCO film as a buffer layer. Then, an ErBCO film was grown on YBCO/GCO double buffer layers at 700 °C. 3.3. Characterized crystallinity of the ErBCO film with YBCO/GCO double buffer layers grown at 700 °C The ErBCO films were confirmed to have c-axis oriented ErBCO grains by an X-ray h/2h diffrac-
ErBCO (102) 400
Intensity (cps)
300
tion pattern as shown in Fig. 2. In Fig. 2(a), an ErBCO film without the YBCO buffer layer has the ErBCO(0 0 5) peak, on the other hand an ErBCO films with the YBCO buffer layer has no ErBCO(0 0 5) peaks. Then, using the buffer layer with the lattice between ErBCO and GCO, we could obtain a-axis oriented ErBCO films at a high growth temperature. These ErBCO films had a c-axis in plane alignment as shown in Fig. 3 as characterized by X-ray /-scan using the ErBCO(1 0 2). The observed surface morphologies of the grown ErBCO films were shown in Fig. 4. In Fig. 4(b), grains of ErBCO films with YBCO buffer layers can be confirmed to have a regular rectangular shape from the SEM image, then we believe ErBCO films with YBCO buffer layers have better in-plane orientation than ErBCO films without YBCO buffer layers (from Figs. 3 and 4(a)). 3.4. Evaluated transport properties of the ErBCO film with YBCO/GCO double buffer layers grown at 700 °C
200
100
0
1427
0
90
180 φ (degree)
270
360
Fig. 3. The X-ray /-scan patterns of an ErBCO film with YBCO/GCO double buffer layers using a (1 0 2) ErBCO.
The q–T curves of the ErBCO film with YBCO/ GCO by using a 4-probe method in magnetic fields were shown in Fig. 5. This film had a TC = 85 K without applied magnetic field (in Fig. 5). This TC is lower than that of an ErBCO bulk. We guessed that the ErBCO film has not enough carriers. Because the q–T curves of this film had a semiconducting behavior at the high temperature. It
Fig. 4. The surface morphologies of the ErBCO films by using SEM. An ErBCO film without (a) and with (b) YBCO/GCO double buffer layers. These ErBCO films were grown at the same substrate temperature of 700 °C.
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Y. Shingai et al. / Physica C 426–431 (2005) 1424–1428
the transport properties of the ErBCO film with YBCO/GCO double buffer layers grown at 700 °C. The TC is 85 K without any applied magnetic fields. One of the reason for low TC is low carriers in the ErBCO films.
1 B = 0,1,3,5,7,9 (T)
ρ(T)/ρ(at 95K)
0.8
0.6
0.4
Acknowledgments
0.2
The authors would like to thank Prof. M. Kusunoki of Kinki University, and Mr. K. Aizawa for valuable discussions.
0
70
75
80 85 Temperature (K)
90
95
Fig. 5. q–T curves of the a-axis oriented ErBCO film with YBCO/GCO double buffer layer grown at 700 °C evaluated in some magnetic fields.
was expected to improve these properties by annealing.
4. Conclusions We have successfully fabricated a-axis oriented ErBCO films by optimizing the substrate temperature during growth. Furthermore, we also exhibited that the optimized growth temperature for a-axis oriented ErBCO films is increased by using the YBCO/GCO buffer layer. We have evaluated
References [1] Y. Shingai, M. Mukaida, K. Matsumoto, Y. Yoshida, A. Ichinose, S. Horii, A. Saito, S. Shingai, Physica C 412–414 (2004) 1296. [2] T. Ohhazama, M. Mukaida, A. Ichinose, K. Matsumoto, Y. Yoshida, S. Horii, A. Saito, S. Shingai, Physica C 412–414 (2004) 1301. [3] Q.X. Jia, S.R. Foltyn, P.N. Arendt, H. Wrang, J.L. MacManus-Driscoll, Y. Coulter, Y. Li, M.P. Maley, M. Hawley, Appl. Phys. Lett. 83 (2003) 1388. [4] M. Itoh, Y. Yoshida, Y. Ichino, M. Miura, Y. Takai, K. Matsumoto, A. Ichinose, M. Mukaida, S. Horii, Physica C 412–414 (2004) 833. [5] S. Horii, A. Ichinose, M. Mukaida, K. Matsumoto, T. Ohazama, Y. Yoshida, J. Shimoyama, K. Kishio, Jpn. J. Appl. Phys. 43 (2004) L1223. [6] M. Mukaida, S. Miyazawa, Appl. Phys. Lett. 63 (1993) 999. [7] M. Mukaida, Jpn. J. Appl. Phys. 36 (1997) L767.
Growth and characterization of a-axis oriented ErBa2Cu3O7d films using double buffer layers Y. Shingai a,f,*, M. Mukaida a,f, K. Matsumoto b,f, Y. Yoshida c,f, A. Ichinose S. Horii e,f, K. Koike a, F. Hirose a, A. Saito a, S. Ohshima a a
d,f
,
Yamagata University, Jonan 4-3-16, Yonezawa, Yamagata 992-8510, Japan b Kyoto University, Sakyou-ku, Kyoto, Kyoto 606-8510, Japan c Nagoya University, Chikusa-ku, Nagoya, Aichi 464-8603, Japan d CREPI, Nagasaka, 2-6-1, Yokosuka, Kanagawa 240-0196, Japan e University of Tokyo, Hongo 7-3-1,Bunkyo-ku, Tokyo 113-8685, Japan f CREST JST, Honmachi 4-1-8, Kawaguchi, Saitama 332-0012, Japan Received 23 November 2004; accepted 11 March 2005
Abstract We have grown a-axis oriented ErBa2Cu3O7d (ErBCO) films. Crystallinity of ErBCO films grown on Gd2CuO4 (GCO) buffer layers are investigated at various growth temperatures, in order to find a condition to fabricate a-axis oriented ErBCO films. a-Axis oriented ErBCO films are also grown at a high growth substrate temperature by using YBa2Cu3O7d (YBCO)/GCO double buffer layers. We have successfully fabricated a-axis oriented ErBCO films with the GCO buffer layer by optimizing the substrate temperature during growth. Furthermore, we optimized growth temperatures for a-axis oriented ErBCO films are increased by using the YBCO/GCO buffer layer. Ó 2005 Elsevier B.V. All rights reserved. PACS: 74.72.Jt; 74.76.Bz Keywords: a-Axis oriented ErBa2Cu3O7d films; Selective preferred orientation control; REBa2Cu3O7d films
1. Introduction * Corresponding author. Address: Mukaida Laboratory, Department of Electronic Engineering, Faculty of Industry, Yamagata University, Jonan 4-3-16, Yonezawa, Yamagata 992-8510, Japan. Tel./fax: +81 238 26 3316. E-mail address: [email protected] (Y. Shingai).
Recently, YBa2Cu3O7d (YBCO) films are attracting attentions to be applied for power applications. However, the YBCO transport properties (at 77 K) are inferior to that of metallic superconducting material in a magnetic field. Then, the
0921-4534/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.03.056
Y. Shingai et al. / Physica C 426–431 (2005) 1424–1428
researches are making efforts to fabricate the HTS with high transport properties in magnetic field. A superconducting material with pinning centers has high transport properties compared with a superconducting material without pinning centers. Now, we are trying to grow YBCO films with a boundary between an a-axis oriented YBCO grains and a c-axis oriented YBCO grains (a/c boundary) [1], in order to study how the a/c boundary in the YBCO film acts as a pinning center. Recently, REBa2Cu3O7d (RE: rare earth elements) superconducting materials attract attentions as a substitution of YBCO [2–4]. Here, we focused on ErBa2Cu3O7d (ErBCO). The ErBCO have some advantages. For example, the ErBCO have a higher superconducting transition temperature (TC) than that of YBCO, no substitution problems between RE and Ba atoms and easy controlling of oxygen contents in a superconducting film by annealing [5]. However, a-axis oriented ErBCO films have not been reported yet. Then, the growth of a-axis oriented ErBCO films is necessary to fabricate ErBCO films with a/c boundaries. In this study, we investigate preferred orientation of ErBCO films on a Gd2CuO4 (GCO) buffer layer at varied growth temperatures, in order to find a condition to fabricate a-axis oriented ErBCO films.
2. Experimental Substrates used in this study were (1 0 0)-oriented SrLaGaO4 (SLGO). ErBCO and YBCO films and Gd2CuO4 (GCO) buffer layers were grown by pulsed ArF excimer laser deposition. The laser targets were sintered ErBCO ceramic, a sintered YBCO ceramic and a sintered GCO ceramic. The substrate was attached with silver paste to a rotating metal-substrate holder, which was heated by a ramp heater. The substrate temperature was monitored by a thermocouple and calibrated by an optical pyrometer. Table 1 shows a list of lattice constants and crystal structure of materials used in this study. During the ErBCO and YBCO film growth, the oxygen pressures were 400 mTorr. The oxygen pressure for the growth of a GCO buffer layer
1425
Table 1 Lattice constants and crystalline structures of materials used in this study Material
Crystalline structure
Lattice constant (nm)
SrLaGaO4
K2NiF4
Gd2CuO4
K2NiF4
YBa2Cu3O7d
REBa2Cu3O7d
ErBa2Cu3O7d
REBa2Cu3O7d
a = b = 0.384, c = 1.268 a = b = 0.389, c = 1.189 a = 0.382, b = 0.388, c = 1.168 a = 0.381, b = 0.388, c = 1.166
was fixed at 40 mTorr with an oxygen flow rate of 5 sccm. The substrate temperatures at the growth of the GCO buffer layer and an YBCO buffer layer were 730 and 660 °C, respectively. The typical growth time of GCO buffer layers were 5 min, YBCO buffer layers were 10 min, and ErBCO films were 60 min. After the growth, the ErBCO films were cooled down rapidly to room temperature in O2 gas with increased atmospheric pressure. The other hands, YBCO and GCO buffer layers were cooled down slowly in the same pressure as the growth of the films. The preferred orientations of ErBCO films grown in this study were determined by X-ray h/2h diffraction with Cu-Ka radiation. The inplane orientations of the ErBCO films were evaluated by X-ray /-scan by using a (1 0 2) plane of ErBCO. Lattice images and grain azimuth were observed by a TEM. The q–T curves of grown ErBCO films were observed by a 4-probe method in magnetic fields (direction of current k [0 1 0] SLGO, direction of magnetic field k [1 0 0] SLGO).
3. Results and discussion 3.1. a-Axis oriented ErBCO films using GCO buffer layers An a-axis oriented YBCO film was obtained on a SLGO substrate at optimized growth conditions [6]. When using a GCO buffer layer, growth window for the grown of a-axis oriented YBCO film
1426
Y. Shingai et al. / Physica C 426–431 (2005) 1424–1428
Here, an I2 0 0 means the intensity of the ErBCO(2 0 0) peak characterized by X-ray h/2h diffraction and an I0 0 6 means the ErBCO(0 0 6) peak too. (The a-axis oriented ratio defined in this study is a simple indicator, because the I2 0 0 dose not equal to I0 0 6 at the same quantity of a-axis oriented grains and c-axis oriented grains.) By increasing the substrate temperature, the a-axis oriented ratio was decreased similar to the a-axis oriented ratio of YBCO films [1]. In Fig. 1, it was confirmed that a-axis oriented films were grown on the GCO buffer at a substrate temperature lower than 680 °C. The ErBCO film grown at 680 °C with the GCO buffer layer was also confirmed to have a c-axis in plane alignment by using X-ray /-scan.
a-axis oriented ratio (%)
100
50
0 650
700 Substrate temperature (OC)
750
Fig. 1. Preferred orientations of ErBCO films with respect to the substrate temperature during the growth.
3.2. Fabrication of a-axis oriented ErBCO films at a high growth temperature
is wider than that of without the buffer layer. Then, ErBCO films were grown on SLGO substrates with GCO buffer layers at several substrate temperatures. Fig. 1 shows a-axis oriented ratios of ErBCO films with several substrate temperatures during the growth of ErBCO films. Here, the a-axis oriented ratio was defined by using I2 0 0 . I2 0 0 þ I0 0 6
Growth temperature higher than 700 °C is necessary for a-axis oriented ErBCO films with good electrical properties [2,5]. A bc-plane of YBCO has a size between a bc-plane of ErBCO and a bc-plane of GCO. In addition, an a-axis oriented YBCO film grown on a GCO buffer layer has good crystallinity [7]. For these reasons, high crystalline
ð1Þ
sub.
ErBCO
006
a.u.
a.u.
200
200
sub.
ErBCO
46 (a)
47 48 2θ (degree)
49
46 (b)
47 48 2θ (degree)
49
Fig. 2. The typical X-ray diffraction patterns of ErBCO films grown at 700 °C. An ErBCO film without (a) and with (b) YBCO/GCO double buffer layers.
Y. Shingai et al. / Physica C 426–431 (2005) 1424–1428
quality a-axis oriented ErBCO films were expected to be grown at above 700 °C by using an YBCO film as a buffer layer. Then, an ErBCO film was grown on YBCO/GCO double buffer layers at 700 °C. 3.3. Characterized crystallinity of the ErBCO film with YBCO/GCO double buffer layers grown at 700 °C The ErBCO films were confirmed to have c-axis oriented ErBCO grains by an X-ray h/2h diffrac-
ErBCO (102) 400
Intensity (cps)
300
tion pattern as shown in Fig. 2. In Fig. 2(a), an ErBCO film without the YBCO buffer layer has the ErBCO(0 0 5) peak, on the other hand an ErBCO films with the YBCO buffer layer has no ErBCO(0 0 5) peaks. Then, using the buffer layer with the lattice between ErBCO and GCO, we could obtain a-axis oriented ErBCO films at a high growth temperature. These ErBCO films had a c-axis in plane alignment as shown in Fig. 3 as characterized by X-ray /-scan using the ErBCO(1 0 2). The observed surface morphologies of the grown ErBCO films were shown in Fig. 4. In Fig. 4(b), grains of ErBCO films with YBCO buffer layers can be confirmed to have a regular rectangular shape from the SEM image, then we believe ErBCO films with YBCO buffer layers have better in-plane orientation than ErBCO films without YBCO buffer layers (from Figs. 3 and 4(a)). 3.4. Evaluated transport properties of the ErBCO film with YBCO/GCO double buffer layers grown at 700 °C
200
100
0
1427
0
90
180 φ (degree)
270
360
Fig. 3. The X-ray /-scan patterns of an ErBCO film with YBCO/GCO double buffer layers using a (1 0 2) ErBCO.
The q–T curves of the ErBCO film with YBCO/ GCO by using a 4-probe method in magnetic fields were shown in Fig. 5. This film had a TC = 85 K without applied magnetic field (in Fig. 5). This TC is lower than that of an ErBCO bulk. We guessed that the ErBCO film has not enough carriers. Because the q–T curves of this film had a semiconducting behavior at the high temperature. It
Fig. 4. The surface morphologies of the ErBCO films by using SEM. An ErBCO film without (a) and with (b) YBCO/GCO double buffer layers. These ErBCO films were grown at the same substrate temperature of 700 °C.
1428
Y. Shingai et al. / Physica C 426–431 (2005) 1424–1428
the transport properties of the ErBCO film with YBCO/GCO double buffer layers grown at 700 °C. The TC is 85 K without any applied magnetic fields. One of the reason for low TC is low carriers in the ErBCO films.
1 B = 0,1,3,5,7,9 (T)
ρ(T)/ρ(at 95K)
0.8
0.6
0.4
Acknowledgments
0.2
The authors would like to thank Prof. M. Kusunoki of Kinki University, and Mr. K. Aizawa for valuable discussions.
0
70
75
80 85 Temperature (K)
90
95
Fig. 5. q–T curves of the a-axis oriented ErBCO film with YBCO/GCO double buffer layer grown at 700 °C evaluated in some magnetic fields.
was expected to improve these properties by annealing.
4. Conclusions We have successfully fabricated a-axis oriented ErBCO films by optimizing the substrate temperature during growth. Furthermore, we also exhibited that the optimized growth temperature for a-axis oriented ErBCO films is increased by using the YBCO/GCO buffer layer. We have evaluated
References [1] Y. Shingai, M. Mukaida, K. Matsumoto, Y. Yoshida, A. Ichinose, S. Horii, A. Saito, S. Shingai, Physica C 412–414 (2004) 1296. [2] T. Ohhazama, M. Mukaida, A. Ichinose, K. Matsumoto, Y. Yoshida, S. Horii, A. Saito, S. Shingai, Physica C 412–414 (2004) 1301. [3] Q.X. Jia, S.R. Foltyn, P.N. Arendt, H. Wrang, J.L. MacManus-Driscoll, Y. Coulter, Y. Li, M.P. Maley, M. Hawley, Appl. Phys. Lett. 83 (2003) 1388. [4] M. Itoh, Y. Yoshida, Y. Ichino, M. Miura, Y. Takai, K. Matsumoto, A. Ichinose, M. Mukaida, S. Horii, Physica C 412–414 (2004) 833. [5] S. Horii, A. Ichinose, M. Mukaida, K. Matsumoto, T. Ohazama, Y. Yoshida, J. Shimoyama, K. Kishio, Jpn. J. Appl. Phys. 43 (2004) L1223. [6] M. Mukaida, S. Miyazawa, Appl. Phys. Lett. 63 (1993) 999. [7] M. Mukaida, Jpn. J. Appl. Phys. 36 (1997) L767.