- Email: [email protected]
Relationship between vortex pinning properties and microstructure in Ba–Nb–O-doped YBa2Cu3Oy and ErBa2Cu3Oy films
Physica C 494 (2013) 158–162
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Relationship between vortex pinning properties and microstructure in Ba–Nb–O-doped YBa2Cu3Oy and ErBa2Cu3Oy films Masakazu Haruta a,⇑, Keisuke Saura a, Natsuto Fujita a, Yuta Ogura a, Ataru Ichinose b, Toshihiko Maeda a, Shigeru Horii a a b
School of Environmental Science and Engineering, Kochi University of Technology, Kami-shi, Kochi 782-8502, Japan Central Research Institute of Electric Power Industry, Yokosuka, Kanagawa 240-0196, Japan
a r t i c l e
i n f o
Article history: Received 19 January 2013 Received in revised form 15 March 2013 Accepted 9 April 2013 Available online 23 April 2013 Keywords: YBa2Cu3Oy film Pulsed laser deposition Nanorods Critical current properties Growth temperature
a b s t r a c t In-field Jcs were improved by introducing Ba–Nb–O (BNO)-nanorods in YBa2Cu3Oy (Y123) and ErBa2Cu3Oy (Er123) films. Retention of Jc against the magnetic field for the BNO-doped Er123 film was superior to that for the BNO-doped Y123 film. Sharp distribution of local critical current density originating from vortex pinning by nanorods with uniform morphology was demonstrated in the Er123 film. On the other hand, fluctuating microstructures of nanorods formed in the Y123 film prepared by the same deposition conditions. Moreover, different growth temperature dependences of nanorod morphology between the Y123 and Er123 films were clarified. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction To apply high-Tc superconductors based on REBa2Cu3Oy (RE: Y and rare-earth elements, RE123) to practical uses, it is necessary to enhance in-field critical current densities (Jcs) at the temperature of liquid nitrogen (77.3 K). Introducing a non-superconducting second phase as vortex pinning centers of nanometer size is very effective at improving in-field Jcs. In RE123 films prepared by pulsed laser deposition (PLD), MacManus-Driscoll et al. reported remarkable enhancement of Jcs in magnetic fields applied parallel to the c-axis (B//c) by introduction of nanorods of BaZrO3 (BZO) which were self-assembled and elongated along the c-axis [1]. This microstructure containing nanorods can be achieved simply by mixing BZO into the Y123 target in PLD. Since this work, many nanorod materials have been developed, such as BZO [1–7], BaNb2O6 (BNO) [8–12], BaSnO3 [13–15], RE3TaO7 [16], and BaHfO3 [17]. In order to achieve higher in-field Jcs for RE123 films with nanorods, doping levels of the nanorod material must be optimized. The influence of doping levels and types of nanorod material on in-field Jc and Tc was reported [5,18]. However, the effects of growth conditions on critical current properties for RE123 films with nanorods have not yet been clarified. Maiorov et al. reported that microstruc⇑ Corresponding author. Present address: WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Tel.: +81 22 217 5948; fax: +81 22 217 5943. E-mail address: [email protected] (M. Haruta). 0921-4534/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2013.04.044
tures of BZO embedded into Y123 films were changed by growth temperature (Ts) and repetition rate of pulsed lasers [6]. In this report, the BZO microstructure was changed from nanoparticles to nanorods with increasing Ts, resulting in remarkable enhancement of in-field Jcs under B//c. On the other hand, Ts-dependent irreversibility lines (ILs) and in-field Jcs in BNO-doped Er123 films were reported by Horii et al. [9]. The values of Jc for the BNO-doped Er123 films decreased with the increase in Ts. Moreover, Haruta et al. reported that Ts-dependent critical current properties can be controlled by the RE composition in RE123 matrices [19]. These results indicate that the morphology of nanorods depends on Ts and RE composition, and Ts-dependent nanorod morphology leads to change in critical current properties. Clarification of changes in microstructure is very important to achieving optimal Jc properties for RE123 films with nanorods. In this study, we focused on the relationship between nanorod morphology and vortex pinning properties in the different superconducting matrix films (RE = Y and Er) with BNO-nanorods. 2. Experimental details Y123 and Er123 films with and without BNO-nanorods were deposited on SrTiO3 (100) single crystal substrates by a PLD technique using a Q-switched Nd:YAG-laser with its fourth harmonics operating at the wavelength of 266 nm. The repetition rates of the flash lamp and Q-switch were 10 and 2 Hz, respectively. The stoichiometric RE123 and 5 at.%-BNO-doped-RE123 sintered targets
159
M. Haruta et al. / Physica C 494 (2013) 158–162
were used for the film deposition. The laser energy density was approximately 2 J/cm2 on the target. During deposition, the substrate temperature (Ts, or growth temperature) was kept at 850– 890 °C with oxygen pressure of 20 Pa by flowing O2 gas. Incidentally, Ts was controlled by a thermocouple mounted on a SiC heater. Thicknesses of the films were about 250 nm via the deposition for 30 min. After deposition, Ts was rapidly dropped to 450 °C, and the deposition chamber was filled with oxygen up to 0.2 atm. Subsequently, in situ oxygen annealing was performed by ramp cooling for Ts range of 450–250 °C with a cooling rate of 5 °C/min. The bi-axial orientation of the films was confirmed by X-ray diffraction analysis. In order to measure transport properties, the films were patterned into a microstrip line shape 100 lm wide and 1.5 mm long. Transport properties were measured with a fourprobe method. The transport current was always perpendicular to the direction of the applied magnetic field. Microstructures of the films were observed with a transmission electron microscope (TEM).
3. Results and discussion Magnetic field dependences of Jcs at 77.3 K under B//c for the BNO-doped Y123 and Er123 films with Ts = 860 °C are shown in Fig. 1, together with data for the BNO-free films. The Jc values for both BNO-doped films were higher than those for each BNO-free film under the higher magnetic fields. The increments of in-field Jcs indicate the effective pinning by the BNO-nanorods embedded into the RE123 matrix. The Jc retention for the BNO-doped Er123 film was better than that for the BNO-doped Y123 film. This result indicates that the BNO-doped Er123 film has more effective pinning centers under high magnetic fields as compared with the BNO-doped Y123 film. Fig. 2 shows the electric field versus the current density (E–J) characteristics for the BNO-doped Y123 film with Ts = 860 °C under 1 T (B//c). The E–J characteristics were measured in the magnetic field from 0.5 to 7 T and at various temperatures to clarify the pinning properties quantitatively. An E–J curve exhibited negative curvature at a lower temperature in a log–log plot, and the curvature changed to positive on reaching a certain temperature with increasing measurement temperature. The negative and positive curvature regions correspond to the vortex glass and liquid states, respectively, and the boundary temperature is defined as the vortex glass-liquid transition temperature (Tg). It has been reported
Fig. 1. Magnetic field dependences of Jc at 77.3 K for the BNO-doped Y123 and Er123 films with Ts = 860 °C (line + symbol plots) together with data for the BNOfree films (line plots), where solid and broken lines indicate data for the Y123 and Er123 films, respectively.
Fig. 2. E–J characteristics for the BNO-doped Y123 films with Ts = 860 °C under 1 T in the temperature range of 80.0–84.5 K. The vortex glass-liquid transition temperature (Tg) is decided as a transition point between the E–J curves of positive and negative curvature.
that these distinctive behaviors of the E–J characteristics for high-Tc superconductors can be expressed by the percolation transition model, which takes into account the statistical distribution of the local critical current density (Jcl) [20]. The distribution of Jcl reflects the spatial distribution of elemental pinning forces. According to this model, the E–J curves in the mixed state are expressed with three different pinning parameters of m, DJc, and Jcm:
E¼
8 mþ1 qFF DJc JJcm > > > mþ1 D J > c > > mþ1 <
for T < T g
qFF DJc J for T ¼ T g mþ1 DJ c > > > mþ1 mþ1 > > JþjJ cm j FF DJ c > : qmþ1 jJDcmJc j for T > T DJ c
ð1Þ
where qFF indicates flux flow resistivity. The pinning parameter m characterizes the shape of the distribution of Jcl, and m+1 indicates the slope of the E–J curve at Tg in a log–log plot. DJc and Jcm indicate the width and the minimum value of the Jcl distribution, respectively. From the E–J characteristics under each magnetic field, Tg was chosen as a boundary temperature between the E–J curves of positive and negative curvatures. Comparison of Tg of the BNO-doped Y123 and Er123 films is shown in Fig. 3a, where uncertainties of Tgs are indicated with error bars, and the average uncertainty is approximately ±0.3 K. The Tg–B curves for the BNO-doped films consist of positive curvature and linear portions with a boundary field, whereas the Tg–B curve for the BNO-free film exhibited monotonous variation. Similar Tg–B curves and irreversibility lines have been already reported in Y123 single crystals [21–23] and films [24–26] with columnar defects introduced by heavy-ion irradiation, where a Tg–B curve and an irreversibility line are essentially equivalent, although criteria for their decisions are different. From the reports, the characteristic behaviors of Tg–B curves in the present Y123 and Er123 films with BNO-nanorods are understood in terms of crossover from strong to weak vortex-Bose-glass states [22] and originate from effective pinning with BNO-nanorods. Here, the crossover field (Bcr) is defined as the boundary field between the positive curvature and the linear portions on the Tg–B curves. The Bcr values for the BNO-doped Y123 and Er123 were approximately 2 and 5 T, respectively, and were indicated by arrows in Fig. 3a. On the other hand, Bcr did not appear on the Tg–B curve for the BNO-free film. Incidentally, these behaviors of the Tg–B curves were qualitatively coincident
160
M. Haruta et al. / Physica C 494 (2013) 158–162
Fig. 4. Cross-sectional TEM image for (a) the BNO-doped Y123 film with Ts = 860 °C and (b) the BNO-doped Er123 film with Ts = 850 °C.
Magnetic field dependences of m for the BNO-doped Y123 and Er123 films are shown in Fig. 3b. The values of m for the BNOdoped Er123 film were larger than those for the BNO-doped Y123 film. This means that the E–J curves for the BNO-doped Er123 film are steeper than those for the BNO-doped Y123 film. It is striking that a peak of m is observed around 5 T, which corresponds to Bcr decided from the Tg–B curve in the BNO-doped Er123 film. Here, the pinning parameter m is related to the dynamic critical exponent z in the vortex glass theory [27] as m = (z–1)/2 [28]. A peak of z in the magnetic field dependence for the Y123 films with columnar defects due to the matching effect has been reported [25,26]. The peak of m in the present study is attributable to the matching effect by the effective vortex pinning with the BNOnanorods. Therefore, the vortex pinning in the BNO-doped Er123 film probably originates from the uniform microstructure of BNO-nanorods like the columnar defects. On the other hand, a peak of m due to the matching effect was not observed for the BNOdoped Y123 film with smaller m, implying the vortex pinning by the BNO-nanorods distributed microstructure. The distribution of Jcl based on the percolation transition model is expressed by the Weibull probability density function as: Fig. 3. (a) Vortex glass-liquid transition temperature, Tg, and (b) magnetic field dependences of m for the Y123 and Er123 films with and without BNO-nanorods; (c) distribution of local critical current density for the BNO-doped Y123 and Er123 films. Solid and broken lines indicate data for the Y123 and Er123 films, respectively.
with irreversibility lines derived from temperature dependences of resistivity under magnetic fields in the BNO-doped Y123 and Er123 films [19]. The values of Bcr for the BNO-doped Er123 films were higher than those for the BNO-doped Y123 film, indicating superior vortex pinning along the c-axis direction for the BNO-doped Er123 film.
PðJ cl Þ ¼
m1 m m J cl J cm J J cm : exp cl DJ c DJ c DJ c
ð2Þ
The distributions of Jcl for the BNO-doped Y123 and Er123 films at each Tg are shown in Fig. 3c, where pinning parameters were derived from fitting Eq. (1) to experimental E–J data. For the BNOdoped Er123 film with larger m, the Jcl distribution rose steeply; its curve was sharp in comparison with that for the BNO-doped Y123 film. The sharp distribution of Jcl for the BNO-doped Er123 films indicates uniform vortex pinning. On the other hand, the
M. Haruta et al. / Physica C 494 (2013) 158–162
161
in these results. It was found that, for lower Tss, distributed microstructure of nanorods in the Y-system and uniform nanorods for Er-system led to broad Jcl distribution without the matching effect and the sharp Jcl distribution with the matching effect, respectively; these microstructures were coincident with above predictions by evaluating critical current properties. Additionally, in the case of the Y-system, the nanorod morphology which is sensitive to Ts, in-field Jcs for B//c can be improved with increasing Ts due to the formation of the uninterrupted and vertical nanorods at higher Ts [19,29]. Since critical current properties are strongly affected by nanorod morphology, precise control of Ts, taking into account the type of RE element in the matrix, is very important to achieving higher in-field Jcs for RE123 films with nanorods. 4. Conclusion
Fig. 5. Ts dependences of mean (b) Lrod/Lfilm and (c) |/| for the BNO-doped Y123 and Er123 films with Ts = 850–890 °C, where Lrod, Lfilm, and |/| are the length of nanorods, thickness of the films, and tilt angle of the nanorods, respectively.
BNO-doped Y123 films exhibited broad Jcl distribution. Therefore, different microstructures of nanorods are predicted between the Y- and Er-systems. Microstructures of the BNO-doped films were revealed by cross-sectional TEM observations as shown in Fig. 4a and b as examples. Microstructures of nanorods for lower Tss were obviously different between Y- and Er-systems. From the TEM images, the degree of interruption (Lrod/Lfilm) and tilt angle (/) of nanorods were evaluated, where Lrod, Lfilm, and / are the length of nanorods, the thickness of the film, and the angle between the vertical direction against the substrate surface and the elongated direction of nanorods, respectively. Ts dependences of mean Lrod/Lfilm and |/| are shown in Fig. 5a and b, respectively, where mean Lrod and |/| are defined as the center value of the Gaussian distribution fitted to each statistical distribution. In the BNO-doped Y123 films, interrupted and tilted nanorods were formed in the lower Ts = 860 °C as shown in Fig. 4a. The value of Lrod/Lfilm increased with increasing Ts, whereas |/| decreased with increasing Ts, indicating a drastic change of the nanorod morphology from interrupted and tilted nanorods to uninterrupted and vertical nanorods with the increase in Ts for the BNO-doped Y123 films. On the other hand, in the case of the BNO-doped Er123 films, Lrod/Lfilm and |/| were almost independent of Ts, and uninterrupted and vertical nanorods were formed in the Er123 matrix even at lower Ts = 850 °C as shown in Fig. 4b. Quite different Ts-dependent changes in the morphology of nanorods between the Y- and Er-systems were demonstrated
In-field Jcs were improved by introducing BNO-nanorods in the Y123 and Er123 films prepared by Nd:YAG-PLD with Ts = 860 °C. The Jc retention against the magnetic field for the BNO-doped Er123 film was superior to that of the BNO-doped Y123 film. The vortex-Bose-glass-like behaviors emerged on the Tg–B curves for the BNO-doped Y123 and Er123 films. The vortex pinning properties of the BNO-doped films were investigated by the local critical current densities derived from the E–J characteristics based on the percolation transition model. These results suggested that the BNO-doped Er123 film had the uniform microstructure of nanorods like columnar defects, and the BNO-doped Y123 had the distributed microstructure of nanorods. The suggestion was coincident with the results of TEM observations; uninterrupted and vertical (uniform) nanorods and interrupted and tilted (distributed) nanorods were formed in the BNO-doped Er123 and Y123 films, respectively. Moreover, different changes in the nanorod morphology as function of Ts between the Y123 and Er123 films with nanorods were clarified. To achieve higher in-field Jcs for RE123-coated conductors with nanorods, optimization and precise control of Ts is necessary in addition to choosing appropriate chemical compositions of superconducting matrix and nanorod material. Acknowledgements This work was partially supported by Grants-in-Aid for Scientific Research (No. 23760020) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan; TEPCO Memorial Foundation; and Iketani Science and Technology Foundation. References [1] J.L. Macmanus-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– 443. [2] 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) 1533–1538. [3] M. Mukaida, T. Horide, R. Kita, S. Horii, A. Ichinose, Y. Yoshida, O. Miura, K. Matsumoto, K. Yamada, N. Mori, Jpn. J. Appl. Phys. 30 (2005) L952–L954. [4] M. Haruta, T. Fujiyoshi, T. Sueyoshi, K. Dezaki, D. Ichigosaki, K. Miyahara, R. Miyagawa, M. Mukaida, K. Matsumoto, Y. Yoshida, A. Ichinose, S. Horii, Supercond. Sci. Technol. 19 (2006) 803–807. [5] M. Peurla, P. Paturi, Y.P. Stepanov, H. Huhtinen, Y.Y. Tse, A.C. Bódi, J. Raittila, T. Laiho, Supercond. Sci. Technol. 19 (2006) 767–771. [6] B. Maiorov, S.A. Baily, H. Zhou, O. Ugurlu, J.A. Kennison, P.C. Dowden, T.G. Holesinger, S.R. Foltyn, L. Civale, Nat. Mater. 8 (2009) 398–404. [7] T. Ozaki, Y. Yoshida, Y. Ichino, Y. Takai, A. Ichinose, K. Matsumoto, S. Horii, M. Mukaida, Y. Takano, J. Appl. Phys. 108 (2010) 093905(5). [8] S. Horii, K. Yamada, H. Kai, A. Ichinose, M. Mukaida, T. Teranishi, R. Kita, K. Matsumoto, Y. Yoshida, J. Shimoyama, K. Kishio, Supercond. Sci. Technol. 20 (2007) 1115–1119. [9] S. Horii, H. Kai, M. Mukaida, K. Yamada, T. Teranishi, A. Ichnose, K. Matsumoto, Y. Yoshida, R. Kita, J. Shimoyama, K. Kishio, Appl. Phys. Lett. 93 (2008) 152506(3).
162
M. Haruta et al. / Physica C 494 (2013) 158–162
[10] H. Kai, S. Horii, A. Ichinose, R. Kita, K. Matsumoto, Y. Yoshida, T. Fujiyoshi, R. Teranishi, N. Mori, M. Mukaida, Supercond. Sci. Technol. 23 (2010) 025017(5). [11] M. Haruta, T. Sueyoshi, T. Fujiyoshi, M. Mukaida, H. Kai, K. Matsumoto, P. Mele, T. Maeda, S. Horii, Physica C 471 (2011) 944–946. [12] G. Ercolano, M. Bianchetti, S.C. Wimbush, S.A. Harrington, H. Wang, J.H. Lee, J.L. MacManus-Driscoll, Supercond. Sci. Technol. 24 (2011) 095012(7). [13] P. Mele, K. Matsumoto, T. Horide, A. Ichinose, M. Mukaida, Y. Yoshida, S. Horii, R. Kita, Supercond. Sci. Technol. 21 (2008) 032002(5). [14] C.V. Varanasi, J. Burke, L. Brunke, H. Wang, J.H. Lee, P.N. Barnes, J. Mater. Res. 23 (2008) 3363. [15] S. Yasunaga, M. Mukaida, A. Ichinose, S. Horii, T. Teranishi, K. Yamada, K. Matsumoto, R. Kita, Y. Yoshida, N. Mori, Physica C 468 (2008) 1858. [16] S.A. Harrington, J.H. Durrell, B. Maiorov, H. Wang, S.C. Wimbush, A. Kursumovic, J.H. Lee, J.L. MacManus-Driscoll, Supercond. Sci. Technol. 22 (2009) 022001(5). [17] H. Tobita, K. Notoh, K. Higashikawa, M. Inoue, T. Kiss, T. Kato, T. Hirayama, M. Yoshizumi, T. Izumi, Y. Shiohara, Supercond. Sci. Technol. 25 (2012) 062002(4). [18] M. Namba, S. Awaji, K. Watanabe, S. Ito, E. Aoyagi, H. Kai, M. Mukaida, R. Kita, Appl. Phys. Exp. 2 (2009) 073001(3).
[19] M. Haruta, A. Ichinose, N. Fujita, Y. Ogura, T. Nakata, T. Maeda, S. Horii, Appl. Phys. Exp. 5 (2012) 073102(3). [20] K. Yamafuji, T. Kiss, Physica C 258 (1996) 197–212. [21] L. Civale, A.D. Marwick, T.K. Worthington, M.A. Kirk, J.R. Thompson, L. KrusinErbaum, Y. Sun, J.R. Clem, F. Holtzberg, Phys. Rev. Lett. 67 (1991) 648–651. [22] L. Krusin-Elbaum, L. Civale, G. Blatter, A.D. Marwick, F. Holtzberg, C. Field, Phys. Rev. Lett. 72 (1994) 1914–1917. [23] R.J. Olsson, W.K. Kwok, L.M. Paulius, A.M. Petream, D.J. Hofman, G.W. Crabtree, Phys. Rev. B 65 (2002) 104520(6). [24] A. Mazilu, H. Safar, M.P. Maley, J.Y. Coulter, L.N. Bulaevskii, S. Foltyn, Phys. Rev. B 58 (1998) R8909–R8912. [25] T. Nojima, M. Katakura, S. Okayasu, N. Kobayashi, Physica C 378–381 (2002) 593–597. [26] T. Sueyoshi, S. Inada, T. Ueno, N. Jyodai, T. Fujiyoshi, K. Miyahara, T. Ikegami, K. Ebihara, R. Miyagawa, Y. Chimi, N. Ishikawa, Physica C 424 (2005) 153–158. [27] D.S. Fisher, M.P.A. Fisher, D.A. Huse, Phys. Rev. B 43 (1991) 130–159. [28] T. Kiss, T. Matsushita, F. Irie, Supercond. Sci. Technol. 12 (1999) 1079–1082. [29] M. Haruta, N. Fujita, T. Maeda, S. Horii, Physics Procedia 36 (2012) 1576–1581.
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Relationship between vortex pinning properties and microstructure in Ba–Nb–O-doped YBa2Cu3Oy and ErBa2Cu3Oy films Masakazu Haruta a,⇑, Keisuke Saura a, Natsuto Fujita a, Yuta Ogura a, Ataru Ichinose b, Toshihiko Maeda a, Shigeru Horii a a b
School of Environmental Science and Engineering, Kochi University of Technology, Kami-shi, Kochi 782-8502, Japan Central Research Institute of Electric Power Industry, Yokosuka, Kanagawa 240-0196, Japan
a r t i c l e
i n f o
Article history: Received 19 January 2013 Received in revised form 15 March 2013 Accepted 9 April 2013 Available online 23 April 2013 Keywords: YBa2Cu3Oy film Pulsed laser deposition Nanorods Critical current properties Growth temperature
a b s t r a c t In-field Jcs were improved by introducing Ba–Nb–O (BNO)-nanorods in YBa2Cu3Oy (Y123) and ErBa2Cu3Oy (Er123) films. Retention of Jc against the magnetic field for the BNO-doped Er123 film was superior to that for the BNO-doped Y123 film. Sharp distribution of local critical current density originating from vortex pinning by nanorods with uniform morphology was demonstrated in the Er123 film. On the other hand, fluctuating microstructures of nanorods formed in the Y123 film prepared by the same deposition conditions. Moreover, different growth temperature dependences of nanorod morphology between the Y123 and Er123 films were clarified. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction To apply high-Tc superconductors based on REBa2Cu3Oy (RE: Y and rare-earth elements, RE123) to practical uses, it is necessary to enhance in-field critical current densities (Jcs) at the temperature of liquid nitrogen (77.3 K). Introducing a non-superconducting second phase as vortex pinning centers of nanometer size is very effective at improving in-field Jcs. In RE123 films prepared by pulsed laser deposition (PLD), MacManus-Driscoll et al. reported remarkable enhancement of Jcs in magnetic fields applied parallel to the c-axis (B//c) by introduction of nanorods of BaZrO3 (BZO) which were self-assembled and elongated along the c-axis [1]. This microstructure containing nanorods can be achieved simply by mixing BZO into the Y123 target in PLD. Since this work, many nanorod materials have been developed, such as BZO [1–7], BaNb2O6 (BNO) [8–12], BaSnO3 [13–15], RE3TaO7 [16], and BaHfO3 [17]. In order to achieve higher in-field Jcs for RE123 films with nanorods, doping levels of the nanorod material must be optimized. The influence of doping levels and types of nanorod material on in-field Jc and Tc was reported [5,18]. However, the effects of growth conditions on critical current properties for RE123 films with nanorods have not yet been clarified. Maiorov et al. reported that microstruc⇑ Corresponding author. Present address: WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Tel.: +81 22 217 5948; fax: +81 22 217 5943. E-mail address: [email protected] (M. Haruta). 0921-4534/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2013.04.044
tures of BZO embedded into Y123 films were changed by growth temperature (Ts) and repetition rate of pulsed lasers [6]. In this report, the BZO microstructure was changed from nanoparticles to nanorods with increasing Ts, resulting in remarkable enhancement of in-field Jcs under B//c. On the other hand, Ts-dependent irreversibility lines (ILs) and in-field Jcs in BNO-doped Er123 films were reported by Horii et al. [9]. The values of Jc for the BNO-doped Er123 films decreased with the increase in Ts. Moreover, Haruta et al. reported that Ts-dependent critical current properties can be controlled by the RE composition in RE123 matrices [19]. These results indicate that the morphology of nanorods depends on Ts and RE composition, and Ts-dependent nanorod morphology leads to change in critical current properties. Clarification of changes in microstructure is very important to achieving optimal Jc properties for RE123 films with nanorods. In this study, we focused on the relationship between nanorod morphology and vortex pinning properties in the different superconducting matrix films (RE = Y and Er) with BNO-nanorods. 2. Experimental details Y123 and Er123 films with and without BNO-nanorods were deposited on SrTiO3 (100) single crystal substrates by a PLD technique using a Q-switched Nd:YAG-laser with its fourth harmonics operating at the wavelength of 266 nm. The repetition rates of the flash lamp and Q-switch were 10 and 2 Hz, respectively. The stoichiometric RE123 and 5 at.%-BNO-doped-RE123 sintered targets
159
M. Haruta et al. / Physica C 494 (2013) 158–162
were used for the film deposition. The laser energy density was approximately 2 J/cm2 on the target. During deposition, the substrate temperature (Ts, or growth temperature) was kept at 850– 890 °C with oxygen pressure of 20 Pa by flowing O2 gas. Incidentally, Ts was controlled by a thermocouple mounted on a SiC heater. Thicknesses of the films were about 250 nm via the deposition for 30 min. After deposition, Ts was rapidly dropped to 450 °C, and the deposition chamber was filled with oxygen up to 0.2 atm. Subsequently, in situ oxygen annealing was performed by ramp cooling for Ts range of 450–250 °C with a cooling rate of 5 °C/min. The bi-axial orientation of the films was confirmed by X-ray diffraction analysis. In order to measure transport properties, the films were patterned into a microstrip line shape 100 lm wide and 1.5 mm long. Transport properties were measured with a fourprobe method. The transport current was always perpendicular to the direction of the applied magnetic field. Microstructures of the films were observed with a transmission electron microscope (TEM).
3. Results and discussion Magnetic field dependences of Jcs at 77.3 K under B//c for the BNO-doped Y123 and Er123 films with Ts = 860 °C are shown in Fig. 1, together with data for the BNO-free films. The Jc values for both BNO-doped films were higher than those for each BNO-free film under the higher magnetic fields. The increments of in-field Jcs indicate the effective pinning by the BNO-nanorods embedded into the RE123 matrix. The Jc retention for the BNO-doped Er123 film was better than that for the BNO-doped Y123 film. This result indicates that the BNO-doped Er123 film has more effective pinning centers under high magnetic fields as compared with the BNO-doped Y123 film. Fig. 2 shows the electric field versus the current density (E–J) characteristics for the BNO-doped Y123 film with Ts = 860 °C under 1 T (B//c). The E–J characteristics were measured in the magnetic field from 0.5 to 7 T and at various temperatures to clarify the pinning properties quantitatively. An E–J curve exhibited negative curvature at a lower temperature in a log–log plot, and the curvature changed to positive on reaching a certain temperature with increasing measurement temperature. The negative and positive curvature regions correspond to the vortex glass and liquid states, respectively, and the boundary temperature is defined as the vortex glass-liquid transition temperature (Tg). It has been reported
Fig. 1. Magnetic field dependences of Jc at 77.3 K for the BNO-doped Y123 and Er123 films with Ts = 860 °C (line + symbol plots) together with data for the BNOfree films (line plots), where solid and broken lines indicate data for the Y123 and Er123 films, respectively.
Fig. 2. E–J characteristics for the BNO-doped Y123 films with Ts = 860 °C under 1 T in the temperature range of 80.0–84.5 K. The vortex glass-liquid transition temperature (Tg) is decided as a transition point between the E–J curves of positive and negative curvature.
that these distinctive behaviors of the E–J characteristics for high-Tc superconductors can be expressed by the percolation transition model, which takes into account the statistical distribution of the local critical current density (Jcl) [20]. The distribution of Jcl reflects the spatial distribution of elemental pinning forces. According to this model, the E–J curves in the mixed state are expressed with three different pinning parameters of m, DJc, and Jcm:
E¼
8 mþ1 qFF DJc JJcm > > > mþ1 D J > c > > mþ1 <
for T < T g
qFF DJc J for T ¼ T g mþ1 DJ c > > > mþ1 mþ1 > > JþjJ cm j FF DJ c > : qmþ1 jJDcmJc j for T > T DJ c
ð1Þ
where qFF indicates flux flow resistivity. The pinning parameter m characterizes the shape of the distribution of Jcl, and m+1 indicates the slope of the E–J curve at Tg in a log–log plot. DJc and Jcm indicate the width and the minimum value of the Jcl distribution, respectively. From the E–J characteristics under each magnetic field, Tg was chosen as a boundary temperature between the E–J curves of positive and negative curvatures. Comparison of Tg of the BNO-doped Y123 and Er123 films is shown in Fig. 3a, where uncertainties of Tgs are indicated with error bars, and the average uncertainty is approximately ±0.3 K. The Tg–B curves for the BNO-doped films consist of positive curvature and linear portions with a boundary field, whereas the Tg–B curve for the BNO-free film exhibited monotonous variation. Similar Tg–B curves and irreversibility lines have been already reported in Y123 single crystals [21–23] and films [24–26] with columnar defects introduced by heavy-ion irradiation, where a Tg–B curve and an irreversibility line are essentially equivalent, although criteria for their decisions are different. From the reports, the characteristic behaviors of Tg–B curves in the present Y123 and Er123 films with BNO-nanorods are understood in terms of crossover from strong to weak vortex-Bose-glass states [22] and originate from effective pinning with BNO-nanorods. Here, the crossover field (Bcr) is defined as the boundary field between the positive curvature and the linear portions on the Tg–B curves. The Bcr values for the BNO-doped Y123 and Er123 were approximately 2 and 5 T, respectively, and were indicated by arrows in Fig. 3a. On the other hand, Bcr did not appear on the Tg–B curve for the BNO-free film. Incidentally, these behaviors of the Tg–B curves were qualitatively coincident
160
M. Haruta et al. / Physica C 494 (2013) 158–162
Fig. 4. Cross-sectional TEM image for (a) the BNO-doped Y123 film with Ts = 860 °C and (b) the BNO-doped Er123 film with Ts = 850 °C.
Magnetic field dependences of m for the BNO-doped Y123 and Er123 films are shown in Fig. 3b. The values of m for the BNOdoped Er123 film were larger than those for the BNO-doped Y123 film. This means that the E–J curves for the BNO-doped Er123 film are steeper than those for the BNO-doped Y123 film. It is striking that a peak of m is observed around 5 T, which corresponds to Bcr decided from the Tg–B curve in the BNO-doped Er123 film. Here, the pinning parameter m is related to the dynamic critical exponent z in the vortex glass theory [27] as m = (z–1)/2 [28]. A peak of z in the magnetic field dependence for the Y123 films with columnar defects due to the matching effect has been reported [25,26]. The peak of m in the present study is attributable to the matching effect by the effective vortex pinning with the BNOnanorods. Therefore, the vortex pinning in the BNO-doped Er123 film probably originates from the uniform microstructure of BNO-nanorods like the columnar defects. On the other hand, a peak of m due to the matching effect was not observed for the BNOdoped Y123 film with smaller m, implying the vortex pinning by the BNO-nanorods distributed microstructure. The distribution of Jcl based on the percolation transition model is expressed by the Weibull probability density function as: Fig. 3. (a) Vortex glass-liquid transition temperature, Tg, and (b) magnetic field dependences of m for the Y123 and Er123 films with and without BNO-nanorods; (c) distribution of local critical current density for the BNO-doped Y123 and Er123 films. Solid and broken lines indicate data for the Y123 and Er123 films, respectively.
with irreversibility lines derived from temperature dependences of resistivity under magnetic fields in the BNO-doped Y123 and Er123 films [19]. The values of Bcr for the BNO-doped Er123 films were higher than those for the BNO-doped Y123 film, indicating superior vortex pinning along the c-axis direction for the BNO-doped Er123 film.
PðJ cl Þ ¼
m1 m m J cl J cm J J cm : exp cl DJ c DJ c DJ c
ð2Þ
The distributions of Jcl for the BNO-doped Y123 and Er123 films at each Tg are shown in Fig. 3c, where pinning parameters were derived from fitting Eq. (1) to experimental E–J data. For the BNOdoped Er123 film with larger m, the Jcl distribution rose steeply; its curve was sharp in comparison with that for the BNO-doped Y123 film. The sharp distribution of Jcl for the BNO-doped Er123 films indicates uniform vortex pinning. On the other hand, the
M. Haruta et al. / Physica C 494 (2013) 158–162
161
in these results. It was found that, for lower Tss, distributed microstructure of nanorods in the Y-system and uniform nanorods for Er-system led to broad Jcl distribution without the matching effect and the sharp Jcl distribution with the matching effect, respectively; these microstructures were coincident with above predictions by evaluating critical current properties. Additionally, in the case of the Y-system, the nanorod morphology which is sensitive to Ts, in-field Jcs for B//c can be improved with increasing Ts due to the formation of the uninterrupted and vertical nanorods at higher Ts [19,29]. Since critical current properties are strongly affected by nanorod morphology, precise control of Ts, taking into account the type of RE element in the matrix, is very important to achieving higher in-field Jcs for RE123 films with nanorods. 4. Conclusion
Fig. 5. Ts dependences of mean (b) Lrod/Lfilm and (c) |/| for the BNO-doped Y123 and Er123 films with Ts = 850–890 °C, where Lrod, Lfilm, and |/| are the length of nanorods, thickness of the films, and tilt angle of the nanorods, respectively.
BNO-doped Y123 films exhibited broad Jcl distribution. Therefore, different microstructures of nanorods are predicted between the Y- and Er-systems. Microstructures of the BNO-doped films were revealed by cross-sectional TEM observations as shown in Fig. 4a and b as examples. Microstructures of nanorods for lower Tss were obviously different between Y- and Er-systems. From the TEM images, the degree of interruption (Lrod/Lfilm) and tilt angle (/) of nanorods were evaluated, where Lrod, Lfilm, and / are the length of nanorods, the thickness of the film, and the angle between the vertical direction against the substrate surface and the elongated direction of nanorods, respectively. Ts dependences of mean Lrod/Lfilm and |/| are shown in Fig. 5a and b, respectively, where mean Lrod and |/| are defined as the center value of the Gaussian distribution fitted to each statistical distribution. In the BNO-doped Y123 films, interrupted and tilted nanorods were formed in the lower Ts = 860 °C as shown in Fig. 4a. The value of Lrod/Lfilm increased with increasing Ts, whereas |/| decreased with increasing Ts, indicating a drastic change of the nanorod morphology from interrupted and tilted nanorods to uninterrupted and vertical nanorods with the increase in Ts for the BNO-doped Y123 films. On the other hand, in the case of the BNO-doped Er123 films, Lrod/Lfilm and |/| were almost independent of Ts, and uninterrupted and vertical nanorods were formed in the Er123 matrix even at lower Ts = 850 °C as shown in Fig. 4b. Quite different Ts-dependent changes in the morphology of nanorods between the Y- and Er-systems were demonstrated
In-field Jcs were improved by introducing BNO-nanorods in the Y123 and Er123 films prepared by Nd:YAG-PLD with Ts = 860 °C. The Jc retention against the magnetic field for the BNO-doped Er123 film was superior to that of the BNO-doped Y123 film. The vortex-Bose-glass-like behaviors emerged on the Tg–B curves for the BNO-doped Y123 and Er123 films. The vortex pinning properties of the BNO-doped films were investigated by the local critical current densities derived from the E–J characteristics based on the percolation transition model. These results suggested that the BNO-doped Er123 film had the uniform microstructure of nanorods like columnar defects, and the BNO-doped Y123 had the distributed microstructure of nanorods. The suggestion was coincident with the results of TEM observations; uninterrupted and vertical (uniform) nanorods and interrupted and tilted (distributed) nanorods were formed in the BNO-doped Er123 and Y123 films, respectively. Moreover, different changes in the nanorod morphology as function of Ts between the Y123 and Er123 films with nanorods were clarified. To achieve higher in-field Jcs for RE123-coated conductors with nanorods, optimization and precise control of Ts is necessary in addition to choosing appropriate chemical compositions of superconducting matrix and nanorod material. Acknowledgements This work was partially supported by Grants-in-Aid for Scientific Research (No. 23760020) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan; TEPCO Memorial Foundation; and Iketani Science and Technology Foundation. References [1] J.L. Macmanus-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– 443. [2] 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) 1533–1538. [3] M. Mukaida, T. Horide, R. Kita, S. Horii, A. Ichinose, Y. Yoshida, O. Miura, K. Matsumoto, K. Yamada, N. Mori, Jpn. J. Appl. Phys. 30 (2005) L952–L954. [4] M. Haruta, T. Fujiyoshi, T. Sueyoshi, K. Dezaki, D. Ichigosaki, K. Miyahara, R. Miyagawa, M. Mukaida, K. Matsumoto, Y. Yoshida, A. Ichinose, S. Horii, Supercond. Sci. Technol. 19 (2006) 803–807. [5] M. Peurla, P. Paturi, Y.P. Stepanov, H. Huhtinen, Y.Y. Tse, A.C. Bódi, J. Raittila, T. Laiho, Supercond. Sci. Technol. 19 (2006) 767–771. [6] B. Maiorov, S.A. Baily, H. Zhou, O. Ugurlu, J.A. Kennison, P.C. Dowden, T.G. Holesinger, S.R. Foltyn, L. Civale, Nat. Mater. 8 (2009) 398–404. [7] T. Ozaki, Y. Yoshida, Y. Ichino, Y. Takai, A. Ichinose, K. Matsumoto, S. Horii, M. Mukaida, Y. Takano, J. Appl. Phys. 108 (2010) 093905(5). [8] S. Horii, K. Yamada, H. Kai, A. Ichinose, M. Mukaida, T. Teranishi, R. Kita, K. Matsumoto, Y. Yoshida, J. Shimoyama, K. Kishio, Supercond. Sci. Technol. 20 (2007) 1115–1119. [9] S. Horii, H. Kai, M. Mukaida, K. Yamada, T. Teranishi, A. Ichnose, K. Matsumoto, Y. Yoshida, R. Kita, J. Shimoyama, K. Kishio, Appl. Phys. Lett. 93 (2008) 152506(3).
162
M. Haruta et al. / Physica C 494 (2013) 158–162
[10] H. Kai, S. Horii, A. Ichinose, R. Kita, K. Matsumoto, Y. Yoshida, T. Fujiyoshi, R. Teranishi, N. Mori, M. Mukaida, Supercond. Sci. Technol. 23 (2010) 025017(5). [11] M. Haruta, T. Sueyoshi, T. Fujiyoshi, M. Mukaida, H. Kai, K. Matsumoto, P. Mele, T. Maeda, S. Horii, Physica C 471 (2011) 944–946. [12] G. Ercolano, M. Bianchetti, S.C. Wimbush, S.A. Harrington, H. Wang, J.H. Lee, J.L. MacManus-Driscoll, Supercond. Sci. Technol. 24 (2011) 095012(7). [13] P. Mele, K. Matsumoto, T. Horide, A. Ichinose, M. Mukaida, Y. Yoshida, S. Horii, R. Kita, Supercond. Sci. Technol. 21 (2008) 032002(5). [14] C.V. Varanasi, J. Burke, L. Brunke, H. Wang, J.H. Lee, P.N. Barnes, J. Mater. Res. 23 (2008) 3363. [15] S. Yasunaga, M. Mukaida, A. Ichinose, S. Horii, T. Teranishi, K. Yamada, K. Matsumoto, R. Kita, Y. Yoshida, N. Mori, Physica C 468 (2008) 1858. [16] S.A. Harrington, J.H. Durrell, B. Maiorov, H. Wang, S.C. Wimbush, A. Kursumovic, J.H. Lee, J.L. MacManus-Driscoll, Supercond. Sci. Technol. 22 (2009) 022001(5). [17] H. Tobita, K. Notoh, K. Higashikawa, M. Inoue, T. Kiss, T. Kato, T. Hirayama, M. Yoshizumi, T. Izumi, Y. Shiohara, Supercond. Sci. Technol. 25 (2012) 062002(4). [18] M. Namba, S. Awaji, K. Watanabe, S. Ito, E. Aoyagi, H. Kai, M. Mukaida, R. Kita, Appl. Phys. Exp. 2 (2009) 073001(3).
[19] M. Haruta, A. Ichinose, N. Fujita, Y. Ogura, T. Nakata, T. Maeda, S. Horii, Appl. Phys. Exp. 5 (2012) 073102(3). [20] K. Yamafuji, T. Kiss, Physica C 258 (1996) 197–212. [21] L. Civale, A.D. Marwick, T.K. Worthington, M.A. Kirk, J.R. Thompson, L. KrusinErbaum, Y. Sun, J.R. Clem, F. Holtzberg, Phys. Rev. Lett. 67 (1991) 648–651. [22] L. Krusin-Elbaum, L. Civale, G. Blatter, A.D. Marwick, F. Holtzberg, C. Field, Phys. Rev. Lett. 72 (1994) 1914–1917. [23] R.J. Olsson, W.K. Kwok, L.M. Paulius, A.M. Petream, D.J. Hofman, G.W. Crabtree, Phys. Rev. B 65 (2002) 104520(6). [24] A. Mazilu, H. Safar, M.P. Maley, J.Y. Coulter, L.N. Bulaevskii, S. Foltyn, Phys. Rev. B 58 (1998) R8909–R8912. [25] T. Nojima, M. Katakura, S. Okayasu, N. Kobayashi, Physica C 378–381 (2002) 593–597. [26] T. Sueyoshi, S. Inada, T. Ueno, N. Jyodai, T. Fujiyoshi, K. Miyahara, T. Ikegami, K. Ebihara, R. Miyagawa, Y. Chimi, N. Ishikawa, Physica C 424 (2005) 153–158. [27] D.S. Fisher, M.P.A. Fisher, D.A. Huse, Phys. Rev. B 43 (1991) 130–159. [28] T. Kiss, T. Matsushita, F. Irie, Supercond. Sci. Technol. 12 (1999) 1079–1082. [29] M. Haruta, N. Fujita, T. Maeda, S. Horii, Physics Procedia 36 (2012) 1576–1581.