- Email: [email protected]
Superconducting properties in magnetic field of (Nd, Eu, Gd)1+xBa2−xCu3Oy thin films prepared by low temperature growth technique
Physica C 468 (2008) 1611–1614
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Superconducting properties in magnetic field of (Nd, Eu, Gd)1+xBa2 xCu3Oy thin films prepared by low temperature growth technique K. Inoue a,f,*, Y. Yoshida a,f, Y. Ichino a,f, Y. Takai a, K. Matsumoto b,f, A. Ichinose c,f, M. Mukaida d,f, S. Horii e,f a
Department of Energy Engineering and Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Department of Materials Science, Kyushu Institute of Technology, Sensui-cho 1-1, Tobata-ku, Kitakyushu-shi, Fukuoka 804-8550, Japan c Central Research Institute of Electric Power Industry, Yokosuka, Kanagawa 201-8511, Japan d Department of Materials Science and Engineering, Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan e Department of Applied Chemistry, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan f CREST, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan b
a r t i c l e
i n f o
Article history: Available online 7 June 2008 PACS: 74.62.Bf 74.78.Bz Keywords: (Nd, Eu, Gd)Ba2Cu3Oy Thin film Low temperature growth Critical current density Pinning
a b s t r a c t In order to achieve high irreversibility field (Birr), we fabricated (Nd, Eu, Gd)Ba2Cu3Oy (NEGBCO) thin films by pulsed laser deposition (PLD) method and low temperature growth (LTG) technique. In the LTG–NEGBCO films on MgO, critical current density (Jc) at 77 K and magnetic field B = 5 T was 0.13 MA/cm2 twice as high as that of PLD–NEGBCO/MgO and Birr at 77 K reached 11.8 T. Interestingly, the anomalous peaks on the Jc–B and Fp–B (flux pinning force density Fp = Jc B) curves at 77 K and B = 4 T were observed on the LTG–NEGBCO/MgO films. From these results, we can consider that NEGBCO films are good candidates and LTG is an effective technique for improving superconducting properties. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction For practical application of REBa2Cu3Oy (REBCO) films, enhancement of critical current density (Jc) is required. Particularly, to achieve high-Jc under high magnetic field, the enhancement of irreversibility field and improvement of flux pinning are strongly desired. One of the ways to enhance flux pinning is introduction of artificial pinning centers (APC’s) into REBCO thin films. For example, Macmaus-Driscoll et al. reported BaZrO3 in YBa2Cu3Oy (YBCO) thin films was an effective APC [1]. Previously, we have reported that Sm1+xBa2 xCu3Oy (SmBCO) thin films grown by low temperature growth (LTG) technique on MgO(1 0 0) and IBAD–YSZ substrate showed high-Jc in magnetic fields, because nanosized Sm-rich phases were formed and acted as APC’s [2]. Recently, Muralidhar et al. have reported that (Nd, Eu, Gd)1+xBa2 xCu3Oy (NEGBCO) melt textured bulks have high critical temperature (Tc) of 94 K, Jc of 7 104 A/cm2 at 77 K and 4.5 T, and an extraordinary irreversibility
* Corresponding author. Address: Department of Energy Engineering and Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan. Tel./fax: +81 52 789 3777. E-mail addresses: [email protected] (K. Inoue), [email protected]. ac.jp (Y. Yoshida). 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2008.05.257
field (Birr) of 15 T at 77 K [3]. Cai et al. indicated stress fields induced by lattice mismatch in NEGBCO thin films was related to flux pinning [4]. Honda et al. also reported that in Yb/NdBCO thin films and they argued that the stress field and crystal defects which are generated around interfaces of Yb-rich and Nd-rich domains act as pinning centers [5]. In this study, in order to improve superconducting properties due to an enhancement of Birr, we fabricated NEGBCO thin films on MgO single crystal substrates and IBAD– YSZ substrates by pulsed laser deposition (PLD) method and LTG technique. 2. Experimental NEGBCO target was prepared by standard solid-state reaction method starting with mixtures of Nd2O3, Eu2O3, Gd2O3, (purity of 99.9%), BaO2 (purity of 99%) and CuO (purity of 99.9%) powders. Appropriate amount was accurately weighed and thoroughly mixed. The mixed powder was first sintered at 850 °C. The reacted product was reground and pressed into pellets, and then sintered at 900 °C. Finally, two sintering steps were carried out at 950 °C and 990 °C for 24 h. NEGBCO thin films were deposited on MgO(1 0 0) substrates and PLD–CeO2/ion beam assisted deposition (IBAD)-yttrium stabilized
K. Inoue et al. / Physica C 468 (2008) 1611–1614
zirconium (YSZ)/Hastelloy by PLD method using KrF excimer laser. The conditions are as follows: a target–substrate distance, 60 mm; a laser repetition rate, 10 Hz; a laser energy density, 3 J/cm2; O2 pressure, 1 Torr. In the case of MgO substrate, PLD–NEGBCO thin films were deposited at a substrate temperature (Ts) of 780– 900 °C with a thickness of 500 nm. LTG technique is consisted of two steps as follows: a c-axis oriented seed layer with thickness of 100 nm and an upper layer with thickness of 400 nm on the seed layer were deposited at Ts of 870 °C and 780–840 °C, respectively. In the case of IBAD substrate, PLD–NEGBCO thin films and the seed layers were deposited at Ts of 950 °C and the upper layers were deposited at Ts of 910 °C. The crystal structure and in-plane alignment of NEGBCO films were examined by X-ray 2h/x scan and u-scan of X-ray diffraction (XRD) with Cu–K source. a-Axis grain mixed ratio of NEGBCO films were calculated from the ratio of (XRD) peak intensity ratio of 200 reflection to 005 reflection. Superconducting properties of NEGBCO films were measured using physical property measurement system (PPMS) by standard four-probe method.
100 LTG-NEGBCO/MgO
a-axis mixed ration [%]
1612
PLD-NEGBCO/MgO
80
60
40 40º C
20
0 760
790
820 850 800 Substrate Temperature [˚C]
910
3.1. Crystallinity and orientation A problem in fabricating c-axis oriented REBCO thin films by PLD method is high growth temperature which results in interdiffusion of the substrates and the films. In addition to improve the superconducting properties of REBCO by LTG technique, Itoh et al. reported that c-axis oriented SmBCO films could be fabricated at 100 °C lower Ts by LTG technique than that of the films by conventional PLD method [6]. So, we tried to fabricate c-axis oriented NEGBCO films at low temperature by LTG technique. Fig. 1a shows the a-axis grain mixed ratio of NEGBCO films on MgO grown by LTG technique and PLD method as a function of Ts. From this figure, Ts above 870 °C was required to obtain the complete c-axis oriented for PLD–NEGBCO/MgO films. On the other hand, LTG–NEGBCO/MgO films showed complete c-axis orientation in the range of 40 °C lower Ts than that of PLD–NEGBCO/MgO films. Fig. 1b shows FWHM of 005 rocking curve of NEGBCO films on MgO as a function of Ts. This figure indicates LTG–NEGBCO/MgO films also maintain good crystallinity at 40 °C lower Ts than that of PLD–NEGBCO/MgO films. Ichino et al. have suggested quasihomo epitaxial nucleation (QHEN) model to explain an orientation mechanism of REBCO [7]. Based on this model, the critical Gibbs free energy DG* of nucleation for c-axis oriented LTG–REBCO should be smaller than that of c-axis oriented PLD–NEGBCO because interface energy of LTG–REBCO get smaller than that of PLD–NEGBCO. From these results, we concluded that orientation and crystallinity of NEGBCO films were also improved by LTG technique. 3.2. Superconducting properties in magnetic field In order to clarify an effect of LTG technique on the superconducting properties of the NEGBCO film, we evaluated Birr and Jc in magnetic field of the films. Fig. 2 shows Birr for B//c of NEGBCO films. The Birr of NEGBCO/STO thin film is added for comparison [4]. Birr at 77 K of LTG–NEGBCO/MgO and PLD–NEGBCO/MgO were 11.8 T and 9.2 T, respectively. It is considered that enhancement of pinning force occurs due to improvement of Tc from 90.7 K of PLD– NEGBCO to 92.5 K of LTG–NEGBCO and introduction of pinning centers such as rare-earth rich phases. Generally, magnetic field characteristic of Jc depends on Birr, so that we measured Jc in magnetic fields for B//c at 77 K shown in Fig. 3. The Jc–B curve of NEGBCO/STO thin film is added for comparison [4]. This figure shows LTG–NEGBCO films have superior Jc in magnetic field. In the case of MgO substrate, Jc at B = 5 T of
FWHM of (005) rocking curve [deg.]
3. Results and discussion
0.4 LTG-NEGBCO/MgO PLD-NEGBCO/MgO
0. 3
0. 2
0. 1
0. 0 760
790
820
850
880
910
Substrate Temperature [ºC] Fig. 1. Orientation and crystallity of NEGBCO as a function of substrate temperature. For LTG films, substrate temperature corresponds to deposition temperature of upper layer. (a) a-Axis mixed ratio of NEGBCO films and (b) FWHM of NEGBCO 005 rocking curves.
LTG–NEGBCO were 0.13 MA/cm2 twice as high as that of PLD– NEGBCO. Interestingly, the Jc–B curve of LTG–NEGBCO/MgO exhibits a peak at B = 4 T. We suppose this peak was caused by pinning centers such as rare-earth rich phases. Furthermore, Jc of LTG–NEGBCO and PLD–NEGBCO on IBAD–YSZ substrate were 0.08 MA/cm2 and 0.028 MA/cm2 at 77 K and B = 5 T, respectively. From these results, superconducting properties of NEGBCO films were improved by LTG technique. 3.3. Pinning centers introduced by LTG growth In order to investigate a configuration of introduced pinning centers in LTG–NEGBCO films, we evaluated the angular dependence of Jc at 77 K for NEGBCO films. Fig. 4 shows Jc–B–h curves for NEGBCO films at B = 5 T. As indicated in the figure, h = 0° corresponds to B//c and h = 90° to B//ab. Jc peaks around h = 0° appeared in all of NEGBCO films. It is considered that pinning centers correlated along the c-axis exist in NEGBCO films. We reported that LTG–SmBCO films doped with nanoparticles have similar tendency of the Jc peak [8]. In addition, the LTG–NEGBCO films exhibits less variation in Jc with changing h compared with the PLD–NEGBCO. It
1613
K. Inoue et al. / Physica C 468 (2008) 1611–1614
indicates that the LTG–NEGBCO films contain isotropic pinning centers. This is similar to the case of LTG–SmBCO films [2]. Therefore, we believe that rare-earth rich phases act as 3D pinning centers in LTG–NEGBCO. Furthermore, we calculated the flux pinning force density (Fp = Jc B) from Fig. 2. Fig. 5 shows the magnetic field dependent Fp of the NEGBCO films. The vertical axis values are normalized by the maximum Fp (F p,max). Interestingly, LTG–NEGBCO films have anomalous peak at B = 4 T as well as LTG–SmBCO + nanoparticles [9,10]. In the case of LTG–SmBCO + nanoparticles, as shown in Fig. 6, low-Tc phases with x–0.12 in the film have Birr of 2–3 T at 77 K [10], and they become normal state and act as pinning centers above B = 4 T. We think that this is also the reason of anomalous peak at B = 4 T for NEGBCO films represented by cross mark in the figure. In the case of NEGBCO films, PLD–NEGBCO films with x = 0.04 have Birr of 2–3 T at 77 K in Fig. 6. It has reported that the stress field and crystal defects induced in mixed REBCO films
15 LTG-NEGBCO/MgO PLD-NEGBCO/MgO
11.8 T
PLD-NEGBCO/STO[4]
9.2 T
B [T]
10
5
B // c
0 75
80
85
90
95
1.2
T [K]
anomalous peak
Fig. 2. Irreversibility fields of NEGBCO thin films on MgO and STO [4].
LTG-NEGBCO/MgO PLD-NEGBCO/MgO
10
LTG-NEGBCO/IBAD
6
Jc [MA / cm2]
10
0.6
PLD-NEGBCO/IBAD
0.4
PLD-NEGBCO/STO[4]
10
77 K, B//c
0.8
7
Fp/Fp,max
10
1.0
5
LT G- NEGBCO/MgO PL D- NEGBCO/MgO LT G- SmBCO+nanoparticle [10]
0.2 4
0.0 0
2
4
6
8
10
B [T] 10
3
Fig. 5. Normalized pinning force density of the NEGBCO films and SmBCO + nanoparticle film [9,10] as a function of applied field. The anomalous Fp peak at 77 K and B = 4 T is represented by arrows.
77 K , B//c
10
2
0
2
4
6
8
10
B [T]
18 LTG-NEGBCO
Fig. 3. Jc–B curves of NEGBCO thin films on MgO, IBAD–YSZ, and STO [4].
LTG-SmBCO+nanoparticle[10]
15 12
B [T]
0.8
Jc /Jc(B//ab)
B// ab
LTG-NEGBCO/MgO PLD-NEGBCO/MgO LTG-NEGBCO/IBAD PLD-NEGBCO/IBAD
1.0
PLD-NEGBCO(x = 0.04) PLD-SmBCO(x = 0.12)[10]
11.8 T
9
B//c 0.6
6
0.4
3
0.2 -20
0 60
B = 5 T, 77 K 0
20
40
60
80
100
120
θ [deg.] Fig. 4. Magnetic field angular dependence of Jc at 77 K and B = 5 T for NEGBCO films.
2 ~3 T B // c
65
70
75 80 T [K]
85
90
95
Fig. 6. Irreversibility field of NEGBCO films and SmBCO films [10] with various ratios of RE/Ba substitution as a function of temperature. Cross marks represent the anomalous peaks which appear on Jc–B and Fp–B curves at 77 K.
1614
K. Inoue et al. / Physica C 468 (2008) 1611–1614
act as pinning centers [4,5]. In addition, we argue that NEGBCO phases of x–0.04 with nano-dimensions exist in the LTG–NEGBCO films and act as 3D pinning centers.
Acknowledgement This work was partly supported by Grant-in-Aid for Scientific Research (19676005).
4. Conclusion References We fabricated NEGBCO thin films on MgO single crystal substrate and IBAD–YSZ substrate by PLD and LTG technique in order to improve Birr. LTG–NEGBCO films kept complete c-axis orientation and good crystallinity at 40 °C lower Ts than that of PLD–NEGBCO films. LTG–NEGBCO films showed superior Jc in magnetic field than PLD–NEGBCO films. Additionally, Birr at 77 K of LTG–NEGBCO/ MgO reached 11.8 T. Furthermore, from Jc–B–h curves for NEGBCO films, we found that the LTG–NEGBCO films contain isotropic pinning centers. Interestingly, in the LTG–NEGBCO/MgO, the anomalous peak was observed on the Jc–B and Fp–B curves measured at 77 K and B = 4 T as well as in the LTG–SmBCO + nanoparticles. We believe NEGBCO phases of x–0.04 with nano-dimensions exist in the LTG–NEGBCO films and act as 3D pinning centers above B = 4 T. From these results, we can consider that NEGBCO films are good candidates and LTG is an effective technique for improving superconducting properties.
[1] J.L. Macmaus-Driscoll, S.R. Foltyn, Q.X. Jia, H. Wang, A. Serquis, L. Civale, B. Maiorov, M.E. Hawley, M.P. Maley, D.E. Peterson, Nat. Mater. 3 (2004) 439. [2] Y. Yoshida, K. Matsumoto, Y. Ichino, M. Itoh, A. Ichinose, S. Horii, M. Mukaida, Y. Takai, Jpn. J. Appl. Phys. 44 (2005) 129. [3] M. Muralidhar, N. Sakai, M. Nishiyama, M. Jirsa, T. Machi, M. Murakami, Appl. Phys. Lett. 82 (2003) 943. [4] C. Cai, B. Holzapfel, J. Hanisch, L. Fernandez, L. Schults, Appl. Phys. Lett. 84 (2004) 377. [5] R. Honda, Y. Ichino, Y. Yoshida, Y. Takai, K. Matsumoto, A. Ichinose, R. Kita, Physica C 426 (2005) 1005. [6] M. Itoh, Y. Yoshida, Y. Ichino, M. Miura, Y. Takai, K. Matsumoto, A. Ichinose, M. Mukaida, S. Horii, Physica C 412 (2004) 833. [7] Y. Ichino, K. Sudou, K. Miyachi, Y. Yoshida, Y. Takai, IEEE Trans. Appl. Supercond. 13 (2003) 2735. [8] S. Awaji, M. Namba, K. Watanabe, M. Miura, Y. Yoshida, Y. Ichino, K. Matsumono, Appl. Phys. 90 (2007) 122501. [9] Y. Yoshida, K. Matsumoto, M. Miura, Y. Ichino, Y. Takai, A. Ichinose, S. Horii, M. Mukaida, Jpn. J. Appl. Phys. 44 (2005) 546. [10] Y. Ichino, Y. Yoshida, M. Miura, Y. Takai, K. Matsumoto, M. Mukaida, A. Ichinose, S. Horii, S. Awaji, K. Watanabe, Physica C 463 (2007) 639.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Superconducting properties in magnetic field of (Nd, Eu, Gd)1+xBa2 xCu3Oy thin films prepared by low temperature growth technique K. Inoue a,f,*, Y. Yoshida a,f, Y. Ichino a,f, Y. Takai a, K. Matsumoto b,f, A. Ichinose c,f, M. Mukaida d,f, S. Horii e,f a
Department of Energy Engineering and Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Department of Materials Science, Kyushu Institute of Technology, Sensui-cho 1-1, Tobata-ku, Kitakyushu-shi, Fukuoka 804-8550, Japan c Central Research Institute of Electric Power Industry, Yokosuka, Kanagawa 201-8511, Japan d Department of Materials Science and Engineering, Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan e Department of Applied Chemistry, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan f CREST, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan b
a r t i c l e
i n f o
Article history: Available online 7 June 2008 PACS: 74.62.Bf 74.78.Bz Keywords: (Nd, Eu, Gd)Ba2Cu3Oy Thin film Low temperature growth Critical current density Pinning
a b s t r a c t In order to achieve high irreversibility field (Birr), we fabricated (Nd, Eu, Gd)Ba2Cu3Oy (NEGBCO) thin films by pulsed laser deposition (PLD) method and low temperature growth (LTG) technique. In the LTG–NEGBCO films on MgO, critical current density (Jc) at 77 K and magnetic field B = 5 T was 0.13 MA/cm2 twice as high as that of PLD–NEGBCO/MgO and Birr at 77 K reached 11.8 T. Interestingly, the anomalous peaks on the Jc–B and Fp–B (flux pinning force density Fp = Jc B) curves at 77 K and B = 4 T were observed on the LTG–NEGBCO/MgO films. From these results, we can consider that NEGBCO films are good candidates and LTG is an effective technique for improving superconducting properties. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction For practical application of REBa2Cu3Oy (REBCO) films, enhancement of critical current density (Jc) is required. Particularly, to achieve high-Jc under high magnetic field, the enhancement of irreversibility field and improvement of flux pinning are strongly desired. One of the ways to enhance flux pinning is introduction of artificial pinning centers (APC’s) into REBCO thin films. For example, Macmaus-Driscoll et al. reported BaZrO3 in YBa2Cu3Oy (YBCO) thin films was an effective APC [1]. Previously, we have reported that Sm1+xBa2 xCu3Oy (SmBCO) thin films grown by low temperature growth (LTG) technique on MgO(1 0 0) and IBAD–YSZ substrate showed high-Jc in magnetic fields, because nanosized Sm-rich phases were formed and acted as APC’s [2]. Recently, Muralidhar et al. have reported that (Nd, Eu, Gd)1+xBa2 xCu3Oy (NEGBCO) melt textured bulks have high critical temperature (Tc) of 94 K, Jc of 7 104 A/cm2 at 77 K and 4.5 T, and an extraordinary irreversibility
* Corresponding author. Address: Department of Energy Engineering and Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan. Tel./fax: +81 52 789 3777. E-mail addresses: [email protected] (K. Inoue), [email protected]. ac.jp (Y. Yoshida). 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2008.05.257
field (Birr) of 15 T at 77 K [3]. Cai et al. indicated stress fields induced by lattice mismatch in NEGBCO thin films was related to flux pinning [4]. Honda et al. also reported that in Yb/NdBCO thin films and they argued that the stress field and crystal defects which are generated around interfaces of Yb-rich and Nd-rich domains act as pinning centers [5]. In this study, in order to improve superconducting properties due to an enhancement of Birr, we fabricated NEGBCO thin films on MgO single crystal substrates and IBAD– YSZ substrates by pulsed laser deposition (PLD) method and LTG technique. 2. Experimental NEGBCO target was prepared by standard solid-state reaction method starting with mixtures of Nd2O3, Eu2O3, Gd2O3, (purity of 99.9%), BaO2 (purity of 99%) and CuO (purity of 99.9%) powders. Appropriate amount was accurately weighed and thoroughly mixed. The mixed powder was first sintered at 850 °C. The reacted product was reground and pressed into pellets, and then sintered at 900 °C. Finally, two sintering steps were carried out at 950 °C and 990 °C for 24 h. NEGBCO thin films were deposited on MgO(1 0 0) substrates and PLD–CeO2/ion beam assisted deposition (IBAD)-yttrium stabilized
K. Inoue et al. / Physica C 468 (2008) 1611–1614
zirconium (YSZ)/Hastelloy by PLD method using KrF excimer laser. The conditions are as follows: a target–substrate distance, 60 mm; a laser repetition rate, 10 Hz; a laser energy density, 3 J/cm2; O2 pressure, 1 Torr. In the case of MgO substrate, PLD–NEGBCO thin films were deposited at a substrate temperature (Ts) of 780– 900 °C with a thickness of 500 nm. LTG technique is consisted of two steps as follows: a c-axis oriented seed layer with thickness of 100 nm and an upper layer with thickness of 400 nm on the seed layer were deposited at Ts of 870 °C and 780–840 °C, respectively. In the case of IBAD substrate, PLD–NEGBCO thin films and the seed layers were deposited at Ts of 950 °C and the upper layers were deposited at Ts of 910 °C. The crystal structure and in-plane alignment of NEGBCO films were examined by X-ray 2h/x scan and u-scan of X-ray diffraction (XRD) with Cu–K source. a-Axis grain mixed ratio of NEGBCO films were calculated from the ratio of (XRD) peak intensity ratio of 200 reflection to 005 reflection. Superconducting properties of NEGBCO films were measured using physical property measurement system (PPMS) by standard four-probe method.
100 LTG-NEGBCO/MgO
a-axis mixed ration [%]
1612
PLD-NEGBCO/MgO
80
60
40 40º C
20
0 760
790
820 850 800 Substrate Temperature [˚C]
910
3.1. Crystallinity and orientation A problem in fabricating c-axis oriented REBCO thin films by PLD method is high growth temperature which results in interdiffusion of the substrates and the films. In addition to improve the superconducting properties of REBCO by LTG technique, Itoh et al. reported that c-axis oriented SmBCO films could be fabricated at 100 °C lower Ts by LTG technique than that of the films by conventional PLD method [6]. So, we tried to fabricate c-axis oriented NEGBCO films at low temperature by LTG technique. Fig. 1a shows the a-axis grain mixed ratio of NEGBCO films on MgO grown by LTG technique and PLD method as a function of Ts. From this figure, Ts above 870 °C was required to obtain the complete c-axis oriented for PLD–NEGBCO/MgO films. On the other hand, LTG–NEGBCO/MgO films showed complete c-axis orientation in the range of 40 °C lower Ts than that of PLD–NEGBCO/MgO films. Fig. 1b shows FWHM of 005 rocking curve of NEGBCO films on MgO as a function of Ts. This figure indicates LTG–NEGBCO/MgO films also maintain good crystallinity at 40 °C lower Ts than that of PLD–NEGBCO/MgO films. Ichino et al. have suggested quasihomo epitaxial nucleation (QHEN) model to explain an orientation mechanism of REBCO [7]. Based on this model, the critical Gibbs free energy DG* of nucleation for c-axis oriented LTG–REBCO should be smaller than that of c-axis oriented PLD–NEGBCO because interface energy of LTG–REBCO get smaller than that of PLD–NEGBCO. From these results, we concluded that orientation and crystallinity of NEGBCO films were also improved by LTG technique. 3.2. Superconducting properties in magnetic field In order to clarify an effect of LTG technique on the superconducting properties of the NEGBCO film, we evaluated Birr and Jc in magnetic field of the films. Fig. 2 shows Birr for B//c of NEGBCO films. The Birr of NEGBCO/STO thin film is added for comparison [4]. Birr at 77 K of LTG–NEGBCO/MgO and PLD–NEGBCO/MgO were 11.8 T and 9.2 T, respectively. It is considered that enhancement of pinning force occurs due to improvement of Tc from 90.7 K of PLD– NEGBCO to 92.5 K of LTG–NEGBCO and introduction of pinning centers such as rare-earth rich phases. Generally, magnetic field characteristic of Jc depends on Birr, so that we measured Jc in magnetic fields for B//c at 77 K shown in Fig. 3. The Jc–B curve of NEGBCO/STO thin film is added for comparison [4]. This figure shows LTG–NEGBCO films have superior Jc in magnetic field. In the case of MgO substrate, Jc at B = 5 T of
FWHM of (005) rocking curve [deg.]
3. Results and discussion
0.4 LTG-NEGBCO/MgO PLD-NEGBCO/MgO
0. 3
0. 2
0. 1
0. 0 760
790
820
850
880
910
Substrate Temperature [ºC] Fig. 1. Orientation and crystallity of NEGBCO as a function of substrate temperature. For LTG films, substrate temperature corresponds to deposition temperature of upper layer. (a) a-Axis mixed ratio of NEGBCO films and (b) FWHM of NEGBCO 005 rocking curves.
LTG–NEGBCO were 0.13 MA/cm2 twice as high as that of PLD– NEGBCO. Interestingly, the Jc–B curve of LTG–NEGBCO/MgO exhibits a peak at B = 4 T. We suppose this peak was caused by pinning centers such as rare-earth rich phases. Furthermore, Jc of LTG–NEGBCO and PLD–NEGBCO on IBAD–YSZ substrate were 0.08 MA/cm2 and 0.028 MA/cm2 at 77 K and B = 5 T, respectively. From these results, superconducting properties of NEGBCO films were improved by LTG technique. 3.3. Pinning centers introduced by LTG growth In order to investigate a configuration of introduced pinning centers in LTG–NEGBCO films, we evaluated the angular dependence of Jc at 77 K for NEGBCO films. Fig. 4 shows Jc–B–h curves for NEGBCO films at B = 5 T. As indicated in the figure, h = 0° corresponds to B//c and h = 90° to B//ab. Jc peaks around h = 0° appeared in all of NEGBCO films. It is considered that pinning centers correlated along the c-axis exist in NEGBCO films. We reported that LTG–SmBCO films doped with nanoparticles have similar tendency of the Jc peak [8]. In addition, the LTG–NEGBCO films exhibits less variation in Jc with changing h compared with the PLD–NEGBCO. It
1613
K. Inoue et al. / Physica C 468 (2008) 1611–1614
indicates that the LTG–NEGBCO films contain isotropic pinning centers. This is similar to the case of LTG–SmBCO films [2]. Therefore, we believe that rare-earth rich phases act as 3D pinning centers in LTG–NEGBCO. Furthermore, we calculated the flux pinning force density (Fp = Jc B) from Fig. 2. Fig. 5 shows the magnetic field dependent Fp of the NEGBCO films. The vertical axis values are normalized by the maximum Fp (F p,max). Interestingly, LTG–NEGBCO films have anomalous peak at B = 4 T as well as LTG–SmBCO + nanoparticles [9,10]. In the case of LTG–SmBCO + nanoparticles, as shown in Fig. 6, low-Tc phases with x–0.12 in the film have Birr of 2–3 T at 77 K [10], and they become normal state and act as pinning centers above B = 4 T. We think that this is also the reason of anomalous peak at B = 4 T for NEGBCO films represented by cross mark in the figure. In the case of NEGBCO films, PLD–NEGBCO films with x = 0.04 have Birr of 2–3 T at 77 K in Fig. 6. It has reported that the stress field and crystal defects induced in mixed REBCO films
15 LTG-NEGBCO/MgO PLD-NEGBCO/MgO
11.8 T
PLD-NEGBCO/STO[4]
9.2 T
B [T]
10
5
B // c
0 75
80
85
90
95
1.2
T [K]
anomalous peak
Fig. 2. Irreversibility fields of NEGBCO thin films on MgO and STO [4].
LTG-NEGBCO/MgO PLD-NEGBCO/MgO
10
LTG-NEGBCO/IBAD
6
Jc [MA / cm2]
10
0.6
PLD-NEGBCO/IBAD
0.4
PLD-NEGBCO/STO[4]
10
77 K, B//c
0.8
7
Fp/Fp,max
10
1.0
5
LT G- NEGBCO/MgO PL D- NEGBCO/MgO LT G- SmBCO+nanoparticle [10]
0.2 4
0.0 0
2
4
6
8
10
B [T] 10
3
Fig. 5. Normalized pinning force density of the NEGBCO films and SmBCO + nanoparticle film [9,10] as a function of applied field. The anomalous Fp peak at 77 K and B = 4 T is represented by arrows.
77 K , B//c
10
2
0
2
4
6
8
10
B [T]
18 LTG-NEGBCO
Fig. 3. Jc–B curves of NEGBCO thin films on MgO, IBAD–YSZ, and STO [4].
LTG-SmBCO+nanoparticle[10]
15 12
B [T]
0.8
Jc /Jc(B//ab)
B// ab
LTG-NEGBCO/MgO PLD-NEGBCO/MgO LTG-NEGBCO/IBAD PLD-NEGBCO/IBAD
1.0
PLD-NEGBCO(x = 0.04) PLD-SmBCO(x = 0.12)[10]
11.8 T
9
B//c 0.6
6
0.4
3
0.2 -20
0 60
B = 5 T, 77 K 0
20
40
60
80
100
120
θ [deg.] Fig. 4. Magnetic field angular dependence of Jc at 77 K and B = 5 T for NEGBCO films.
2 ~3 T B // c
65
70
75 80 T [K]
85
90
95
Fig. 6. Irreversibility field of NEGBCO films and SmBCO films [10] with various ratios of RE/Ba substitution as a function of temperature. Cross marks represent the anomalous peaks which appear on Jc–B and Fp–B curves at 77 K.
1614
K. Inoue et al. / Physica C 468 (2008) 1611–1614
act as pinning centers [4,5]. In addition, we argue that NEGBCO phases of x–0.04 with nano-dimensions exist in the LTG–NEGBCO films and act as 3D pinning centers.
Acknowledgement This work was partly supported by Grant-in-Aid for Scientific Research (19676005).
4. Conclusion References We fabricated NEGBCO thin films on MgO single crystal substrate and IBAD–YSZ substrate by PLD and LTG technique in order to improve Birr. LTG–NEGBCO films kept complete c-axis orientation and good crystallinity at 40 °C lower Ts than that of PLD–NEGBCO films. LTG–NEGBCO films showed superior Jc in magnetic field than PLD–NEGBCO films. Additionally, Birr at 77 K of LTG–NEGBCO/ MgO reached 11.8 T. Furthermore, from Jc–B–h curves for NEGBCO films, we found that the LTG–NEGBCO films contain isotropic pinning centers. Interestingly, in the LTG–NEGBCO/MgO, the anomalous peak was observed on the Jc–B and Fp–B curves measured at 77 K and B = 4 T as well as in the LTG–SmBCO + nanoparticles. We believe NEGBCO phases of x–0.04 with nano-dimensions exist in the LTG–NEGBCO films and act as 3D pinning centers above B = 4 T. From these results, we can consider that NEGBCO films are good candidates and LTG is an effective technique for improving superconducting properties.
[1] J.L. Macmaus-Driscoll, S.R. Foltyn, Q.X. Jia, H. Wang, A. Serquis, L. Civale, B. Maiorov, M.E. Hawley, M.P. Maley, D.E. Peterson, Nat. Mater. 3 (2004) 439. [2] Y. Yoshida, K. Matsumoto, Y. Ichino, M. Itoh, A. Ichinose, S. Horii, M. Mukaida, Y. Takai, Jpn. J. Appl. Phys. 44 (2005) 129. [3] M. Muralidhar, N. Sakai, M. Nishiyama, M. Jirsa, T. Machi, M. Murakami, Appl. Phys. Lett. 82 (2003) 943. [4] C. Cai, B. Holzapfel, J. Hanisch, L. Fernandez, L. Schults, Appl. Phys. Lett. 84 (2004) 377. [5] R. Honda, Y. Ichino, Y. Yoshida, Y. Takai, K. Matsumoto, A. Ichinose, R. Kita, Physica C 426 (2005) 1005. [6] M. Itoh, Y. Yoshida, Y. Ichino, M. Miura, Y. Takai, K. Matsumoto, A. Ichinose, M. Mukaida, S. Horii, Physica C 412 (2004) 833. [7] Y. Ichino, K. Sudou, K. Miyachi, Y. Yoshida, Y. Takai, IEEE Trans. Appl. Supercond. 13 (2003) 2735. [8] S. Awaji, M. Namba, K. Watanabe, M. Miura, Y. Yoshida, Y. Ichino, K. Matsumono, Appl. Phys. 90 (2007) 122501. [9] Y. Yoshida, K. Matsumoto, M. Miura, Y. Ichino, Y. Takai, A. Ichinose, S. Horii, M. Mukaida, Jpn. J. Appl. Phys. 44 (2005) 546. [10] Y. Ichino, Y. Yoshida, M. Miura, Y. Takai, K. Matsumoto, M. Mukaida, A. Ichinose, S. Horii, S. Awaji, K. Watanabe, Physica C 463 (2007) 639.