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Superconducting properties in magnetic fields of LTG-Sm1+xBa2−xCu3Oy films on Ni–W textured substrate
Physica C 468 (2008) 1619–1622
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Physica C journal homepage: www.elsevier.com/locate/physc
Superconducting properties in magnetic fields of LTG-Sm1+xBa2 xCu3Oy films on Ni–W textured substrate S. Yamaguchi a, Y. Yoshida a,*, Y. Ichino a, Y. Takai a, Y. Takahashi b, Y. Aoki b, T. Izumi c, Y. Shiohara c a
Department of Electrical Engineering and Computer Science, Nagoya University, Furo-cho, Chikusaku, Nagoya 464-8603, Japan SWCC Showa Cable Systems Co. Ltd., 1-1-18 Tranomon, Minatoku, Tokyo 105-0001, Japan c Superconductivity Research Laboratory Division 4, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan b
a r t i c l e
i n f o
Article history: Available online 27 May 2008 PACS: 74.78.Bz 74.72.Jt Keywords: Sm1+xBa2 xCu3Oy Thin film Low temperature growth Critical current Ni–W textured substrate
a b s t r a c t We have reported a fabrication and the superconducting properties of Sm1+xBa2 xCu3Oy (SmBCO) films prepared by pulsed laser deposition (PLD). In this report, we fabricated the SmBCO coated conductor on CeO2/Ce–Zr–O/Ni–W textured substrate using low temperature growth (LTG) technique and evaluated the superconducting properties. The LTG-SmBCO film on Ni–W showed Tc = 92 K, Jc = 0.7 MA/cm2 in selffield at 77 K and Birr = 11 T. From the measurement of the Jc–B–h, the film showed a peak of Jc(h) at B//c similar to that on CeO2/IBAD-YSZ and MgO substrates. We concluded that the Jc peak at B//c of the LTGSmBCO on Ni–W would be affected by dislocation at grain boundaries. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Coated conductor based on REBa2Cu3Oy (REBCO) films grown on textured metallic substrates with oxide buffer layers have been considered for a number of electric power applications such as magnets, transmission cables, motors and so on. However, further improvement of critical current (Ic) and critical current density (Jc) in high applied magnetic fields are still required for many largescale practical applications of high-temperature superconductors. Improvement of flux-pinning in REBCO films by increasing the density of crystalline defects are a positive route for achieving this goal. We have reported a fabrication and superconducting properties of SmBCO films prepared by low temperature growth (LTG) technique [1,2]. LTG-SmBCO films on single crystal MgO (1 0 0) substrate showed excellent Jc in a high magnetic field at 77 K, which are comparable to that of Nb–Ti wire at 4.2 K [3]. From previous work, it’s reported that the dislocations in SmBCO film on MgO act as c-axis correlated pinning centers (PC) in magnetic fields [4]. The LTG-SmBCO film on PLD-CeO2/ion beam assisted deposition (IBAD)-yttrium stabilized zirconium (YSZ)/Hastelloy (hereaf-
* Corresponding author. Tel.: +81 52 789 5417; fax: +81 52 789 5418. E-mail address: [email protected] (Y. Yoshida). 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2008.05.084
ter referred to as IBAD) showed high crystal quality compared with the PLD-SmBCO film [5]. In this study, we fabricated the LTG-SmBCO coated conductor on Ni-3 at%W (Ni–W) textured substrate and evaluated the superconducting properties in the magnetic field. Furthermore, we carried out a comparative study with the results of the LTG-SmBCO films on Ni–W tape, IBAD tape and MgO substrate. 2. Experimental The LTG-SmBCO films with a thickness of 600 nm were prepared by usual pulsed laser deposition (PLD) technique using KrF excimer laser on Ni–W textured substrate. The PLD-SmBCO films were deposited at substrate temperature (Ts) of 880 °C on sputtered–CeO2/Ce–Zr–O/Ni-3 at%W textured substrate (Ni–W) in which the Ce–Zr–O (CZO) layer is prepared by metal organic deposition method. The LTG-SmBCO films are consisted of two layers [1], one is a seed layer of Sm1.08Ba1.92Cu3Oy with a thickness of 100 nm at 880 °C on substrates and another is an upper layer of Sm1.04Ba1.96Cu3Oy with a thickness of 500 nm at 850 °C on the seed layer. Other deposition conditions are listed in Table 1. The crystallinity and orientation of the LTG-SmBCO films on Ni– W were checked by X-ray h–2h diffraction and u-scan using the (1 0 2) plane of SmBCO. The surface morphologies of LTG-SmBCO films were observed by resonance-mode atomic force microscopy
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S. Yamaguchi et al. / Physica C 468 (2008) 1619–1622
Table 1 Depositional condition of the LTG-SmBCO films
106
Condition
Target
Seed layer: Sm1.08Ba1.92Cu3Oy Upper layer: Sm1.04Ba1.96Cu3Oy CeO2/CZO/Ni-3 at %W CeO2/IBAD-YSZ/Hastelloy C276 MgO (1 0 0) Seed layer: 880 °C (Ni–W), 860 °C (IBAD), 830 °C (MgO) Upper layer: 850 °C (Ni–W), 740–780 °C (IBAD, MgO) 60 mm 400 mTorr KrF (k = 248 nm) 1.6 J/cm2 10 Hz
Substrate
Substrate temperature (Ts) Target-substrate distance Oxygen pressure (pO2) Laser source Laser energy Laser repetition rate
Jc [A/cm2]
PLD-YBCO / CeO2 / YSZ / Y2O3 / RABiTS[9]
Parameters
5
LTG-SmBCO / CeO2 / CZO/Ni-W
10
4 10 PLD-SmBCO / CeO2 / CZO/Ni-W
B//c @ 77 K 3
10
0
2
4
6
8
10
Magnetic Field [T] (AFM). The critical temperature (Tc) and the Jc at a magnetic field were measured by standard four-probe method using a physical property measurement system (PPMS). Zero dissipative critical current and resistance were determined for an electrical field criterion of 1 lV/cm. 3. Results and discussion 3.1. Superconducting properties and surface morphologies of the LTGSmBCO on Ni–W There are many uncertainties in the properties of the superconducting layer on a metallic substrate such as pinning mechanism. Then, we investigated the properties of the LTG-SmBCO on Ni–W against that on IBAD and MgO substrate. Table 2 shows the crystallinity and superconducting properties of SmBCO films on various substrates fabricated by LTG technique. The LTG-SmBCO film on Ni–W grew to biaxial orientation as well as LTG-SmBCO films on IBAD and MgO substrate. However, the critical current density of the LTG-SmBCO film on Ni–W is lower than that on IBAD and MgO. The lowering factors of Jc are reported as follows the crystallinity, decline at grain boundaries, the a-axis orientated grains, 45° rotated grains and so on. Though du gives a degree for in-plane orientation, du of the LTG-SmBCO on Ni–W was not so different from that on IBAD. Furthermore neither a-axis orientation grains nor 45° orientation grains were confirmed from the XRD results. From the surface morphologies, we recognized that grooves, which caused from large tilt grain boundaries between coarse grains of Ni–W substrate, existed on the surface of the SmBCO film. The lowering Jc would be affected by these grooves and the large tilt grain boundaries. Fig. 1 shows the Jc–B for B//c at 77 K in the LTG-SmBCO film on Ni–W compared with PLD-SmBCO film on Ni–W and PLD-YBCO film on RABiTS [6]. Although the LTG-SmBCO film on Ni–W showed higher Jc than PLD-SmBCO film on Ni–W in the self field and the magnetic fields. While Jc = 0.7 MA/cm2 (77 K, self field) in the LTG-SmBCO film was lower than Jc = 1.7 MA/cm2 (77 K, self field) in the PLD-YBCO film, however above B = 5 T, Jc of LTG-SmBCO film on Ni–W was superior to the PLD-YBCO film on RABiTS. Table 2 Sample reference date for the LTG-SmBCO films on various substrates Substrate
du (deg)
nisland (lm2)
Tc (K)
Jcs.f. (MA/cm2)
CeO2/CZO/Ni–W CeO2/IBAD-YSZ/Hastelloy MgO
8.3 7.1 0.9
19 21 20
92.3 93.0 93.0
0.7 4.7 8.0
Here, du is the FWHM of the u-scan corresponding to in-plane texture, nisland is crystal grain density, Tc is critical temperature, and Jcs.f. is critical current density in self-field.
Fig. 1. Magnetic field dependence of Jc for B//c at 77 K of LTG-SmBCO film on Ni–W. For comparison, Jc of PLD-SmBCO film on Ni–W and PLD-YBCO films on RABiTS [9] were also plotted.
Here, we discuss the higher Jc of LTG-SmBCO film at a magnetic field. The surface morphologies of SmBCO films fabricated by PLD and LTG on Ni–W observed by AFM are shown in Fig. 2. We found that the LTG-SmBCO film on Ni–W was grown up by 2D island growth mode as well as that on IBAD and MgO as shown in Ref. [5,7]. From the AFM images, the average grain size (dgrain) of the LTG-SmBCO film was about 230 nm and the value was one-third of PLD-SmBCO film. The REBCO film on Ni–W using by metal-organic deposition (MOD) process was reported that the dgrain was several dozen micro meters [7]. As compared with dgrain of MOD, the dgrain of LTG-SmBCO was very small. LTG-SmBCO films are fabricated at low-Ts and homoepitaxial growth on the seed layer so that the surface diffusion length and the surface resident time of the adatoms shorten. As a result, because the frequency of two dimensional island nucleation increases with Ts, it is considered that the grain size of the LTG-SmBCO film on Ni–W became small. We have reported that the dislocation density (ndisl) in LTG-SmBCO films on MgO was almost same with crystal grain density (nisland) 2 [8], where nisland is determined by 1/dgrain . From these discussions, since the nisland increased by LTG technique, we suggest that the dislocations increased with lowering Ts and the flux pinning force in LTG-SmBCO was enhanced by the dislocations. We considered that the reason of different Jc between LTG-SmBCO and PLDSmBCO on Ni–W was the different number density of the grains. 3.2. Comparative study of pinning centers in LTG-SmBCO films on various substrates Fig. 3a shows the Jc–B for B//c at 77 K of the PLD-SmBCO and the LTG-SmBCO films on various substrates. In spite of the different substrate, the Jc–B curves of the LTG-SmBCO films were higher than those of the PLD-SmBCO films as shown in Fig. 3a. The XRD results showed that the crystallinity of the LTG-SmBCO films were superior to the PLD-SmBCO films, therefore, higher Jc of LTG-SmBCO was achieved in magnetic field. To discuss the pinning effect, Fig. 3b represents normalized Jc by the Jc in the self field (Jc/Jcs.f.–B). The Jc/Jcs.f.–B curves of PLD-SmBCO films were different for each substrate, however, those of LTG-SmBCO films were similar in low magnet field (B < 1 T), while the Jc/Jcs.f.–B above B = 1 T were somewhat different as seen in Fig. 3b. The nisland values of the LTG-SmBCO films shown in Table 2 were a similar value on Ni– W, IBAD and MgO substrates. As stated in previous section, nisland value is proportional to ndisl value so that these films would include almost same ndisl value. We have argued that the dislocations of
S. Yamaguchi et al. / Physica C 468 (2008) 1619–1622
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Fig. 2. Surface morphologies of SmBCO film on Ni–W (scan size 2 2 lm2). (a) Top-view of PLD-SmBCO and (b) LTG-SmBCO film.
a
10
7
Jc [A/cm2]
MgO IBAD Ni-WO PLD-SmBCO LTG-SmBCO
10
6
10
5
104
103
B// c @ 77 K
0
2
4
8
6
10
Magnetic Field [T]
b
10 0
LTG-SmBCO/MgO LTG-SmBCO/IBAD LTG-SmBCO/Ni-W
Jc / Jc
s.f.
10 -1
10 -2
10 -3
B//c @ 77 K
0
2
4
6
8
10
Magnetic Field [T] Fig. 3. Magnetic field dependence of Jc (Jc–B) measured at B//c and 77 K in SmBCO films on various substrate. (a) The Jc–B measured of PLD-SmBCO and LTG-SmBCO films on Ni–W compared with SmBCO films on IBAD [7] and MgO [2] prepared by PLD and LTG technique and (b) Jc of LTG-SmBCO is normalized by the Jc in the self field.
LTG-SmBCO on MgO confirmed by Br-ethanol etching act as c-axis correlated dominant pinning centers at low magnetic fields [8]. Therefore, it is considered that the Jc/Jcs.f.–B curve of the LTG-
SmBCO film on Ni–W showed the similar tendency with LTGSmBCO films on IBAD and MgO at low magnetic fields. Based on these results, we found that the substrate independent Jc–B at low magnetic fields could be obtained by using the LTG technique. On the other hand, under a high magnetic field (B > 1 T), the Smrich low-Tc phases would become a normal state and then contribute to pinning centers [5], so that the Jc/Jcs.f.–B should be affected by the low-Tc phases and the dislocations. In order to investigate the configuration of the pinning centers, we measured the magnetic field applied angular dependence of Jc Jc–B–h at 77 K and 3 T for the LTG-SmBCO film on Ni–W. In this report, we evaluated the pinning centers at B = 3 T at which the difference of the Jc/Jcs.f.–B curves clearly appeared. From the Jc–B–h curves, the LTG-SmBCO film on Ni–W showed peaks of Jc at h = 0° (B//c) and h = 90° (B//ab) as well as at LTG-SmBCO films on MgO and IBAD. The peaks of Jc at B//c and B//ab are reported that the dislocations of act as c-axis correlated pinning centers in LTG-SmBCO thin film [4], and stacking faults are effective as a/b-axis correlated pinning centers in magnetic fields [9]. Fig. 4 shows the Jc–B–h curves at B = 3 T normalized by the Jc in the self field on Ni–W, compared with those on IBAD and MgO. Firstly we focus at B//c, Jc/Jcs.f. in film on Ni–W and IBAD reached 0.1, which value is higher than that on MgO. From Table 2, the nisland of LTG-SmBCO films are almost similar values with on other substrates, while LTG-SmBCO on Ni–W and IBAD had near du value compared with that of MgO. Even under a high magnetic field, the dislocation of the grain boundaries act as c-axis correlated pinning centers. Furthermore, we speculate that pinning centers with a large size play the dominant pinning centers. Namely, Sm-rich low-Tc phases would be the pinning centers. Therefore we suggest that the difference of Jc/Jcs.f. at B//c results from effects of Sm-rich low-Tc phases. Secondly we focus the Jc/Jcs.f. at B//ab. The Jc/Jcs.f. of the film on Ni–W showed the higher value at B//ab, which is compared with that on IBAD and MgO. We speculate that the stacking faults may be affected by the growth conditions, because the surface roughness of interfaced with superconducting layer and buffer layer grown on the substrate were difference between the single crystalline substrate and metal substrate. It is reported that the peak of Jc at B//ab was lowered by the decrease of stacking faults in MOD-YBCO film [10]. We suggest that the difference of Jc/Jcs.f. at B//ab are attributed by the stacking faults acting as a/b-axis correlated pinning centers. In addition, we consider 3D-PC for a wide angle range. Based on above discussion, we concluded that the Jc– B–h curves of the LTG-SmBCO on various substrates would be affected by pinning centers such as dislocations of grain boundary, stacking faults, and Sm-rich low-Tc phases. These pinning centers
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S. Yamaguchi et al. / Physica C 468 (2008) 1619–1622
ducting properties of LTG-SmBCO films on various substrates, we found that the Jc–B of the PLD-SmBCO films depended on the substrate, while the substrate independent Jc–B could be obtained by using the LTG technique. The Jc–B–h curves of the LTG-SmBCO on various substrates were not the same, because they would be affected by dislocations of grain boundary, Sm-rich low-Tc phases, and stacking faults. These pinning centers in superconducting layer are affected and introduced by the surface morphology of buffer layer such as misfit, surface roughness and so on.
B//ab
B//c LTG-SmBCO/MgO LTG-SmBCO/IBAD LTG-SmBCO/Ni-W
JC (θ ) / JC
s.f.
0.2
Acknowledgments
0.1 3 T @ 77 K
0
20
40
60
80
100
120
θ [deg.] Fig. 4. Field angular dependence of Jc at 77 K for the LTG-SmBCO films on various substrate. As indicated in the figure, h = 0° corresponds to B//c, and h = 90° to B//ab.
in superconducting layer are affected and introduced by the surface morphology in buffer layer such as misfit, surface roughness and so on. 4. Conclusion The LTG-SmBCO films with a thickness of 600 nm were fabricated on CeO2/CZO/Ni–W substrate. We confirmed that the films grew to biaxial orientation on Ni–W textured substrate as well as on IBAD tape and MgO substrate. The Jc of LTG-SmBCO film on Ni–W was higher than PLD-SmBCO film on Ni–W in magnetic field in this study. The high Jc of the LTG-SmBCO film on Ni–W was improved in magnetic field at B//c as compared with PLD-SmBCO according that the pinning force enhanced with increasing dislocations by using LTG technique. From the comparison of supercon-
This work was partly supported by Grant-in-Aid for Scientific Research (19676005) and the New Energy and Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications. References [1] M. Itoh, Y. Yoshida, Y. Ichino, M. Miura, Y. Takai, K. Matsumoto, A. Ichinose, S. Horii, M. Mukaida, Physica C 412–414 (2004) 833. [2] Y. Yoshida, K. Matsumoto, Y. Ichino, M. Itoh, A. Ichinose, S. Horii, M. Mukaida, Y. Takai, Jpn. J. Appl. Phys. 44 (2005) L129. [3] M. Miura, Y. Yoshida, Y. Ichino, Y. Takai, K. Matsumoto, A. Ichinose, S. Horii, M. Mukaida, Jpn. J. Appl. Phys. 45 (2006) L11. [4] S.AwajiM.S. Awaji, M. Namba, K. Watanabe, M. Miura, Y. Yoshida, Y. Ichino, Y. Takai, K. Mastumoto, Appl. Phys. Lett. 90 (2007) 122501. [5] M. Miura, Y. Yoshida, T. Ozaki, M. Mutoh, S. Funaki, Y. Takai, K. Matsumoto, A. Ichinose, S. Horii, M. Mukaida, S. Awaji, K. Watanabe, IEEE Trans. Appl. Supercond. 17 (2007) 3247. [6] A. Goyal, S. Kang, K.J. Leonard, P.M. Martin, A.A. Gapud, M. Varela, M. Paranthaman, A.O. Ijaduola, E.D. Specht, J.R. Thompson, D.K. Christen, S.J. Pennycook, F.A. List, Supercond. Sci. Technol. 18 (2005) 1533. [7] S.I. Kim, D.M. Feldmann, D.T. Verebelyi, C. Thieme, X. Li, A.A. Polyanskii, D.C. Larbalestier, Phys. Rev. B 71 (2005) 104501. [8] M. Miura, Y. Yoshida, Y. Ichino, T. Ozaki, Y. Takai, K. Matsumoto, A. Ichinose, S. Horii, M. Mukaida, Jpn. J. Appl. Phys. 45 (2006) L701. [9] E.D. Specht, A. Goyal, J. Li, P.M. Martin, X. Li, M.W. Rupich, Appl. Phys. Lett. 89 (2006) 162510. [10] Z.J. Chen, D.M. Feldmann, D.C. Larbalestier, T.G. Holesinger, X. Li, W. Zhang, M.W. Rupich, Appl. Phys. Lett. 91 (2007) 052508.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Superconducting properties in magnetic fields of LTG-Sm1+xBa2 xCu3Oy films on Ni–W textured substrate S. Yamaguchi a, Y. Yoshida a,*, Y. Ichino a, Y. Takai a, Y. Takahashi b, Y. Aoki b, T. Izumi c, Y. Shiohara c a
Department of Electrical Engineering and Computer Science, Nagoya University, Furo-cho, Chikusaku, Nagoya 464-8603, Japan SWCC Showa Cable Systems Co. Ltd., 1-1-18 Tranomon, Minatoku, Tokyo 105-0001, Japan c Superconductivity Research Laboratory Division 4, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan b
a r t i c l e
i n f o
Article history: Available online 27 May 2008 PACS: 74.78.Bz 74.72.Jt Keywords: Sm1+xBa2 xCu3Oy Thin film Low temperature growth Critical current Ni–W textured substrate
a b s t r a c t We have reported a fabrication and the superconducting properties of Sm1+xBa2 xCu3Oy (SmBCO) films prepared by pulsed laser deposition (PLD). In this report, we fabricated the SmBCO coated conductor on CeO2/Ce–Zr–O/Ni–W textured substrate using low temperature growth (LTG) technique and evaluated the superconducting properties. The LTG-SmBCO film on Ni–W showed Tc = 92 K, Jc = 0.7 MA/cm2 in selffield at 77 K and Birr = 11 T. From the measurement of the Jc–B–h, the film showed a peak of Jc(h) at B//c similar to that on CeO2/IBAD-YSZ and MgO substrates. We concluded that the Jc peak at B//c of the LTGSmBCO on Ni–W would be affected by dislocation at grain boundaries. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Coated conductor based on REBa2Cu3Oy (REBCO) films grown on textured metallic substrates with oxide buffer layers have been considered for a number of electric power applications such as magnets, transmission cables, motors and so on. However, further improvement of critical current (Ic) and critical current density (Jc) in high applied magnetic fields are still required for many largescale practical applications of high-temperature superconductors. Improvement of flux-pinning in REBCO films by increasing the density of crystalline defects are a positive route for achieving this goal. We have reported a fabrication and superconducting properties of SmBCO films prepared by low temperature growth (LTG) technique [1,2]. LTG-SmBCO films on single crystal MgO (1 0 0) substrate showed excellent Jc in a high magnetic field at 77 K, which are comparable to that of Nb–Ti wire at 4.2 K [3]. From previous work, it’s reported that the dislocations in SmBCO film on MgO act as c-axis correlated pinning centers (PC) in magnetic fields [4]. The LTG-SmBCO film on PLD-CeO2/ion beam assisted deposition (IBAD)-yttrium stabilized zirconium (YSZ)/Hastelloy (hereaf-
* Corresponding author. Tel.: +81 52 789 5417; fax: +81 52 789 5418. E-mail address: [email protected] (Y. Yoshida). 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2008.05.084
ter referred to as IBAD) showed high crystal quality compared with the PLD-SmBCO film [5]. In this study, we fabricated the LTG-SmBCO coated conductor on Ni-3 at%W (Ni–W) textured substrate and evaluated the superconducting properties in the magnetic field. Furthermore, we carried out a comparative study with the results of the LTG-SmBCO films on Ni–W tape, IBAD tape and MgO substrate. 2. Experimental The LTG-SmBCO films with a thickness of 600 nm were prepared by usual pulsed laser deposition (PLD) technique using KrF excimer laser on Ni–W textured substrate. The PLD-SmBCO films were deposited at substrate temperature (Ts) of 880 °C on sputtered–CeO2/Ce–Zr–O/Ni-3 at%W textured substrate (Ni–W) in which the Ce–Zr–O (CZO) layer is prepared by metal organic deposition method. The LTG-SmBCO films are consisted of two layers [1], one is a seed layer of Sm1.08Ba1.92Cu3Oy with a thickness of 100 nm at 880 °C on substrates and another is an upper layer of Sm1.04Ba1.96Cu3Oy with a thickness of 500 nm at 850 °C on the seed layer. Other deposition conditions are listed in Table 1. The crystallinity and orientation of the LTG-SmBCO films on Ni– W were checked by X-ray h–2h diffraction and u-scan using the (1 0 2) plane of SmBCO. The surface morphologies of LTG-SmBCO films were observed by resonance-mode atomic force microscopy
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S. Yamaguchi et al. / Physica C 468 (2008) 1619–1622
Table 1 Depositional condition of the LTG-SmBCO films
106
Condition
Target
Seed layer: Sm1.08Ba1.92Cu3Oy Upper layer: Sm1.04Ba1.96Cu3Oy CeO2/CZO/Ni-3 at %W CeO2/IBAD-YSZ/Hastelloy C276 MgO (1 0 0) Seed layer: 880 °C (Ni–W), 860 °C (IBAD), 830 °C (MgO) Upper layer: 850 °C (Ni–W), 740–780 °C (IBAD, MgO) 60 mm 400 mTorr KrF (k = 248 nm) 1.6 J/cm2 10 Hz
Substrate
Substrate temperature (Ts) Target-substrate distance Oxygen pressure (pO2) Laser source Laser energy Laser repetition rate
Jc [A/cm2]
PLD-YBCO / CeO2 / YSZ / Y2O3 / RABiTS[9]
Parameters
5
LTG-SmBCO / CeO2 / CZO/Ni-W
10
4 10 PLD-SmBCO / CeO2 / CZO/Ni-W
B//c @ 77 K 3
10
0
2
4
6
8
10
Magnetic Field [T] (AFM). The critical temperature (Tc) and the Jc at a magnetic field were measured by standard four-probe method using a physical property measurement system (PPMS). Zero dissipative critical current and resistance were determined for an electrical field criterion of 1 lV/cm. 3. Results and discussion 3.1. Superconducting properties and surface morphologies of the LTGSmBCO on Ni–W There are many uncertainties in the properties of the superconducting layer on a metallic substrate such as pinning mechanism. Then, we investigated the properties of the LTG-SmBCO on Ni–W against that on IBAD and MgO substrate. Table 2 shows the crystallinity and superconducting properties of SmBCO films on various substrates fabricated by LTG technique. The LTG-SmBCO film on Ni–W grew to biaxial orientation as well as LTG-SmBCO films on IBAD and MgO substrate. However, the critical current density of the LTG-SmBCO film on Ni–W is lower than that on IBAD and MgO. The lowering factors of Jc are reported as follows the crystallinity, decline at grain boundaries, the a-axis orientated grains, 45° rotated grains and so on. Though du gives a degree for in-plane orientation, du of the LTG-SmBCO on Ni–W was not so different from that on IBAD. Furthermore neither a-axis orientation grains nor 45° orientation grains were confirmed from the XRD results. From the surface morphologies, we recognized that grooves, which caused from large tilt grain boundaries between coarse grains of Ni–W substrate, existed on the surface of the SmBCO film. The lowering Jc would be affected by these grooves and the large tilt grain boundaries. Fig. 1 shows the Jc–B for B//c at 77 K in the LTG-SmBCO film on Ni–W compared with PLD-SmBCO film on Ni–W and PLD-YBCO film on RABiTS [6]. Although the LTG-SmBCO film on Ni–W showed higher Jc than PLD-SmBCO film on Ni–W in the self field and the magnetic fields. While Jc = 0.7 MA/cm2 (77 K, self field) in the LTG-SmBCO film was lower than Jc = 1.7 MA/cm2 (77 K, self field) in the PLD-YBCO film, however above B = 5 T, Jc of LTG-SmBCO film on Ni–W was superior to the PLD-YBCO film on RABiTS. Table 2 Sample reference date for the LTG-SmBCO films on various substrates Substrate
du (deg)
nisland (lm2)
Tc (K)
Jcs.f. (MA/cm2)
CeO2/CZO/Ni–W CeO2/IBAD-YSZ/Hastelloy MgO
8.3 7.1 0.9
19 21 20
92.3 93.0 93.0
0.7 4.7 8.0
Here, du is the FWHM of the u-scan corresponding to in-plane texture, nisland is crystal grain density, Tc is critical temperature, and Jcs.f. is critical current density in self-field.
Fig. 1. Magnetic field dependence of Jc for B//c at 77 K of LTG-SmBCO film on Ni–W. For comparison, Jc of PLD-SmBCO film on Ni–W and PLD-YBCO films on RABiTS [9] were also plotted.
Here, we discuss the higher Jc of LTG-SmBCO film at a magnetic field. The surface morphologies of SmBCO films fabricated by PLD and LTG on Ni–W observed by AFM are shown in Fig. 2. We found that the LTG-SmBCO film on Ni–W was grown up by 2D island growth mode as well as that on IBAD and MgO as shown in Ref. [5,7]. From the AFM images, the average grain size (dgrain) of the LTG-SmBCO film was about 230 nm and the value was one-third of PLD-SmBCO film. The REBCO film on Ni–W using by metal-organic deposition (MOD) process was reported that the dgrain was several dozen micro meters [7]. As compared with dgrain of MOD, the dgrain of LTG-SmBCO was very small. LTG-SmBCO films are fabricated at low-Ts and homoepitaxial growth on the seed layer so that the surface diffusion length and the surface resident time of the adatoms shorten. As a result, because the frequency of two dimensional island nucleation increases with Ts, it is considered that the grain size of the LTG-SmBCO film on Ni–W became small. We have reported that the dislocation density (ndisl) in LTG-SmBCO films on MgO was almost same with crystal grain density (nisland) 2 [8], where nisland is determined by 1/dgrain . From these discussions, since the nisland increased by LTG technique, we suggest that the dislocations increased with lowering Ts and the flux pinning force in LTG-SmBCO was enhanced by the dislocations. We considered that the reason of different Jc between LTG-SmBCO and PLDSmBCO on Ni–W was the different number density of the grains. 3.2. Comparative study of pinning centers in LTG-SmBCO films on various substrates Fig. 3a shows the Jc–B for B//c at 77 K of the PLD-SmBCO and the LTG-SmBCO films on various substrates. In spite of the different substrate, the Jc–B curves of the LTG-SmBCO films were higher than those of the PLD-SmBCO films as shown in Fig. 3a. The XRD results showed that the crystallinity of the LTG-SmBCO films were superior to the PLD-SmBCO films, therefore, higher Jc of LTG-SmBCO was achieved in magnetic field. To discuss the pinning effect, Fig. 3b represents normalized Jc by the Jc in the self field (Jc/Jcs.f.–B). The Jc/Jcs.f.–B curves of PLD-SmBCO films were different for each substrate, however, those of LTG-SmBCO films were similar in low magnet field (B < 1 T), while the Jc/Jcs.f.–B above B = 1 T were somewhat different as seen in Fig. 3b. The nisland values of the LTG-SmBCO films shown in Table 2 were a similar value on Ni– W, IBAD and MgO substrates. As stated in previous section, nisland value is proportional to ndisl value so that these films would include almost same ndisl value. We have argued that the dislocations of
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Fig. 2. Surface morphologies of SmBCO film on Ni–W (scan size 2 2 lm2). (a) Top-view of PLD-SmBCO and (b) LTG-SmBCO film.
a
10
7
Jc [A/cm2]
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6
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103
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b
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Jc / Jc
s.f.
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10 -2
10 -3
B//c @ 77 K
0
2
4
6
8
10
Magnetic Field [T] Fig. 3. Magnetic field dependence of Jc (Jc–B) measured at B//c and 77 K in SmBCO films on various substrate. (a) The Jc–B measured of PLD-SmBCO and LTG-SmBCO films on Ni–W compared with SmBCO films on IBAD [7] and MgO [2] prepared by PLD and LTG technique and (b) Jc of LTG-SmBCO is normalized by the Jc in the self field.
LTG-SmBCO on MgO confirmed by Br-ethanol etching act as c-axis correlated dominant pinning centers at low magnetic fields [8]. Therefore, it is considered that the Jc/Jcs.f.–B curve of the LTG-
SmBCO film on Ni–W showed the similar tendency with LTGSmBCO films on IBAD and MgO at low magnetic fields. Based on these results, we found that the substrate independent Jc–B at low magnetic fields could be obtained by using the LTG technique. On the other hand, under a high magnetic field (B > 1 T), the Smrich low-Tc phases would become a normal state and then contribute to pinning centers [5], so that the Jc/Jcs.f.–B should be affected by the low-Tc phases and the dislocations. In order to investigate the configuration of the pinning centers, we measured the magnetic field applied angular dependence of Jc Jc–B–h at 77 K and 3 T for the LTG-SmBCO film on Ni–W. In this report, we evaluated the pinning centers at B = 3 T at which the difference of the Jc/Jcs.f.–B curves clearly appeared. From the Jc–B–h curves, the LTG-SmBCO film on Ni–W showed peaks of Jc at h = 0° (B//c) and h = 90° (B//ab) as well as at LTG-SmBCO films on MgO and IBAD. The peaks of Jc at B//c and B//ab are reported that the dislocations of act as c-axis correlated pinning centers in LTG-SmBCO thin film [4], and stacking faults are effective as a/b-axis correlated pinning centers in magnetic fields [9]. Fig. 4 shows the Jc–B–h curves at B = 3 T normalized by the Jc in the self field on Ni–W, compared with those on IBAD and MgO. Firstly we focus at B//c, Jc/Jcs.f. in film on Ni–W and IBAD reached 0.1, which value is higher than that on MgO. From Table 2, the nisland of LTG-SmBCO films are almost similar values with on other substrates, while LTG-SmBCO on Ni–W and IBAD had near du value compared with that of MgO. Even under a high magnetic field, the dislocation of the grain boundaries act as c-axis correlated pinning centers. Furthermore, we speculate that pinning centers with a large size play the dominant pinning centers. Namely, Sm-rich low-Tc phases would be the pinning centers. Therefore we suggest that the difference of Jc/Jcs.f. at B//c results from effects of Sm-rich low-Tc phases. Secondly we focus the Jc/Jcs.f. at B//ab. The Jc/Jcs.f. of the film on Ni–W showed the higher value at B//ab, which is compared with that on IBAD and MgO. We speculate that the stacking faults may be affected by the growth conditions, because the surface roughness of interfaced with superconducting layer and buffer layer grown on the substrate were difference between the single crystalline substrate and metal substrate. It is reported that the peak of Jc at B//ab was lowered by the decrease of stacking faults in MOD-YBCO film [10]. We suggest that the difference of Jc/Jcs.f. at B//ab are attributed by the stacking faults acting as a/b-axis correlated pinning centers. In addition, we consider 3D-PC for a wide angle range. Based on above discussion, we concluded that the Jc– B–h curves of the LTG-SmBCO on various substrates would be affected by pinning centers such as dislocations of grain boundary, stacking faults, and Sm-rich low-Tc phases. These pinning centers
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ducting properties of LTG-SmBCO films on various substrates, we found that the Jc–B of the PLD-SmBCO films depended on the substrate, while the substrate independent Jc–B could be obtained by using the LTG technique. The Jc–B–h curves of the LTG-SmBCO on various substrates were not the same, because they would be affected by dislocations of grain boundary, Sm-rich low-Tc phases, and stacking faults. These pinning centers in superconducting layer are affected and introduced by the surface morphology of buffer layer such as misfit, surface roughness and so on.
B//ab
B//c LTG-SmBCO/MgO LTG-SmBCO/IBAD LTG-SmBCO/Ni-W
JC (θ ) / JC
s.f.
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Acknowledgments
0.1 3 T @ 77 K
0
20
40
60
80
100
120
θ [deg.] Fig. 4. Field angular dependence of Jc at 77 K for the LTG-SmBCO films on various substrate. As indicated in the figure, h = 0° corresponds to B//c, and h = 90° to B//ab.
in superconducting layer are affected and introduced by the surface morphology in buffer layer such as misfit, surface roughness and so on. 4. Conclusion The LTG-SmBCO films with a thickness of 600 nm were fabricated on CeO2/CZO/Ni–W substrate. We confirmed that the films grew to biaxial orientation on Ni–W textured substrate as well as on IBAD tape and MgO substrate. The Jc of LTG-SmBCO film on Ni–W was higher than PLD-SmBCO film on Ni–W in magnetic field in this study. The high Jc of the LTG-SmBCO film on Ni–W was improved in magnetic field at B//c as compared with PLD-SmBCO according that the pinning force enhanced with increasing dislocations by using LTG technique. From the comparison of supercon-
This work was partly supported by Grant-in-Aid for Scientific Research (19676005) and the New Energy and Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications. References [1] M. Itoh, Y. Yoshida, Y. Ichino, M. Miura, Y. Takai, K. Matsumoto, A. Ichinose, S. Horii, M. Mukaida, Physica C 412–414 (2004) 833. [2] Y. Yoshida, K. Matsumoto, Y. Ichino, M. Itoh, A. Ichinose, S. Horii, M. Mukaida, Y. Takai, Jpn. J. Appl. Phys. 44 (2005) L129. [3] M. Miura, Y. Yoshida, Y. Ichino, Y. Takai, K. Matsumoto, A. Ichinose, S. Horii, M. Mukaida, Jpn. J. Appl. Phys. 45 (2006) L11. [4] S.AwajiM.S. Awaji, M. Namba, K. Watanabe, M. Miura, Y. Yoshida, Y. Ichino, Y. Takai, K. Mastumoto, Appl. Phys. Lett. 90 (2007) 122501. [5] M. Miura, Y. Yoshida, T. Ozaki, M. Mutoh, S. Funaki, Y. Takai, K. Matsumoto, A. Ichinose, S. Horii, M. Mukaida, S. Awaji, K. Watanabe, IEEE Trans. Appl. Supercond. 17 (2007) 3247. [6] A. Goyal, S. Kang, K.J. Leonard, P.M. Martin, A.A. Gapud, M. Varela, M. Paranthaman, A.O. Ijaduola, E.D. Specht, J.R. Thompson, D.K. Christen, S.J. Pennycook, F.A. List, Supercond. Sci. Technol. 18 (2005) 1533. [7] S.I. Kim, D.M. Feldmann, D.T. Verebelyi, C. Thieme, X. Li, A.A. Polyanskii, D.C. Larbalestier, Phys. Rev. B 71 (2005) 104501. [8] M. Miura, Y. Yoshida, Y. Ichino, T. Ozaki, Y. Takai, K. Matsumoto, A. Ichinose, S. Horii, M. Mukaida, Jpn. J. Appl. Phys. 45 (2006) L701. [9] E.D. Specht, A. Goyal, J. Li, P.M. Martin, X. Li, M.W. Rupich, Appl. Phys. Lett. 89 (2006) 162510. [10] Z.J. Chen, D.M. Feldmann, D.C. Larbalestier, T.G. Holesinger, X. Li, W. Zhang, M.W. Rupich, Appl. Phys. Lett. 91 (2007) 052508.