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Flux pinning properties of REBa2Cu3Oy thin films with BaZrO3 nano-rods
Physica C 468 (2008) 1635–1637
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Physica C journal homepage: www.elsevier.com/locate/physc
Flux pinning properties of REBa2Cu3Oy thin films with BaZrO3 nano-rods T. Fujiyoshi a,*, M. Haruta a, T. Sueyoshi a, K. Yonekura a, M. Watanabe a, M. Mukaida b, R. Teranishi b, K. Matsumoto c, Y. Yoshida d, A. Ichinose e, S. Horii f, S. Awaji g, K. Watanabe g a
Department of Computer Science and Electrical Engineering, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan Department of Materials Science and Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0385, Japan c Department of Materials Science and Engineering, Kyushu Institute of Technology, Sensui-cho, 1-1, Tobata-ku, Kitakyushu, Fukuoka 804-8550, Japan d Department of Electrical Engineering and Computer Science, Nagoya University, Furo-cho, Chisusa-ku, Nagoya 464-8603, Japan e Electric Power Engineering Research Laboratory, Central Research Institute Electric Power Industry, Nagasaka 2-6-1, Yokosuka, Kanagawa 240-0196, Japan f Department of Applied Chemistry, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan g High Field Laboratory for Superconducting Materials, IMR, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan b
a r t i c l e
i n f o
Article history: Available online 29 May 2008 PACS: 74.25.Qt 74.25.Sv 74.78.Bz Keywords: Flux pinning Nano-rods
a b s t r a c t Flux pinning properties in ErBa2Cu3Oy and YBa2Cu3Oy thin films with BaZrO3 nano-rods prepared by PLD were measured to investigate the flux pinning mechanism. The enhancement of Jc was confirmed by the measurement of the dependence of Jc on magnetic field. The angular dependence of Jc has a broad peak at B||c-axis. This result indicates that the BaZrO3 nano-rods work as the c-axis-correlated pinning centers. The pinning parameter m was evaluated from the electric fields versus current density characteristics and the peak of m appears in the magnetic field dependence. The characteristic behavior of m is caused by the matching of the density of BaZrO3 nano-rods with that of fluxoids. It is found that the relative distribution width of the local critical current density for the ErBa2Cu3Oy film with BaZrO3 nano-rods is smaller than that of the YBa2Cu3Oy films with BaZrO3 nano-rods. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction REBa2Cu3Oy (REBCO) coated conductors are expected to be applied to superconducting machines because REBCO films indicate a high critical current density Jc at 77 K. However, the value of Jc for REBCO films at 77 K decreases rapidly with increasing the magnetic field. Flux pinning force from naturally introduced pinning centers during the fabrication process, e.g., oxygen vacancies, dislocations and grain boundaries, is not enough to achieve high-Jc at 77 K under the magnetic field. Recently, some groups have succeeded in improving Jc properties under the magnetic field using practical methods, in which the nano-crystal structures were controlled during deposition of films. For example, Y2BaCuO5, BaZrO3 and Y2O3 nano-particles were introduced into YBa2Cu3Oy thin films and bring about a remarkable improvement of Jc properties [1–6]. Among these artificial pinning centers, BaZrO3 (BZO) nano-rods are much effective as c-axis-correlated pinning centers [2,7]. The BZO nano-rods can be fabricated using a mixture target of REBCO and BZO in a pulsed laser deposition (PLD) method. Extended columns of self-assembled BZO particles are formed and aligned along the c-axis of
* Corresponding author. Tel.: +81 96 342 3849; fax: +81 96 342 3630. E-mail address: [email protected] (T. Fujiyoshi). 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2008.05.155
REBCO films. However, the comprehensive understanding of the flux pinning mechanism of REBCO films with BZO nano-rods is not enough. In this study, the flux pinning properties of REBCO films with BZO nano-rods were evaluated and the flux pinning mechanism was investigated on the basis of the percolation transition model [8,9]. ErBa2Cu3Oy (ErBCO) and YBa2Cu3Oy (YBCO) thin films containing BaZrO3 (BZO) were prepared by PLD. The enhancement of the Jc of both samples was observed in the high magnetic field region. The broad peak at B||c-axis appears in the angular dependence of Jc. The vortex glass–liquid transition temperature Tg and the pinning parameter m were evaluated from electric fields versus current density (E–J) characteristics. The pinning parameter m characterizes the shape of the distribution of the local critical current density, and corresponds to the efficiency of flux pinning. 2. Experimental The ErBCO and YBCO thin films with BZO nano-rods were prepared on SrTiO3 (1 0 0) substrates by PLD with an ArF excimer laser [7]. Sintered ErBCO and YBCO ceramic bulks containing 1.5 wt% BZO were used as targets. The substrate temperatures at the ErBCO + BZO and YBCO + BZO thin films were 750 °C and 715 °C, respectively. The laser frequency was 1 Hz and the laser energy
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T. Fujiyoshi et al. / Physica C 468 (2008) 1635–1637
was 300 mJ. During deposition, the oxygen partial pressure was fixed at 53.3 Pa. After deposition, the substrates were cooled to a room temperature without any annealing processes. The thicknesses of the ErBCO + BZO and YBCO + BZO thin films were 368 nm and 300 nm, respectively. The ErBCO + BZO and YBCO + BZO thin films have the critical temperatures Tc of 89.1 K and 89.3 K, respectively. In order to measure the transport properties, the samples were patterned with shape of 50 lm wide and 1 mm long by photolithography. To obtain good contacts, Au contact pads were deposited on the films by RF sputtering after cleaning of the film surfaces by oxygen–ion etching. The sample was mounted on a rotating sample holder with a cernox thermometer and a heater. The sample temperature was controlled by both He gas flow in a temperature variable cryostat and the heater placed on the sample holder. The sample temperature was stabilized within ±0.02 K. Magnetic fields were applied using a superconducting magnet. A magnetic field angle is defined such that B||c-axis is h = 0° and transport currents are always perpendicular to the magnetic field and the c-axis. The E–J characteristics were measured with a dc four-probe method. The value of Jc was evaluated from the transport properties with the criterion of the electric field Ec = 10 lm/cm. 3. Result and discussion The dependences of Jc on the magnetic field applied parallel to the c-axis for the ErBCO + BZO and YBCO + BZO thin films are shown in Fig. 1. The ErBCO + BZO film has higher Jc at 77.3 K and 84.0 K compared to the YBCO + BZO film. For comparison, the Jc characteristics of pure-YBCO thin film prepared on a SrTiO3 substrate by a PLD method [10] is also shown in Fig. 1. Although the pure-YBCO thin film has higher Jc than the ErBCO + BZO and YBCO + BZO films in the low magnetic field region, the values of Jc of the ErBCO + BZO and YBCO + BZO films exceed that of the pure-YBCO thin film above 4 T and 6 T, respectively. Therefore, the reduction of Jc with increasing the magnetic field becomes smaller due to the introduction of BZO into ErBCO and YBCO films. Fig. 2 shows the angular dependence of Jc at 84.0 K, 0.5 T for the ErBCO + BZO and YBCO + BZO films, where the values of Jc are normalized by the value of Jc(B||c). There are two peaks appeared at h = 0° and 90° in both samples. According to Civale et al., the main contribution of the angular dependence of Jc is associated with the effective mass anisotropy and Jc(h) peaks are added on it [11]. The peak of Jc at h = 90°, corresponding to the direction of the magnetic field parallel to the ab-plane, are contributed to the intrinsic pinning or stacking faults. The broad peak of Jc at B||c-axis indicates
Fig. 1. Dependences of Jc on the magnetic field parallel to the c-axis for ErBCO + BZO and YBCO + BZO films. The data for pure-YBCO film prepared by PLD [10] are also shown in the figure.
Fig. 2. Angular dependence of Jc for ErBCO + BZO and YBCO + BZO films at 84 K, 0.5 T.
that c-axis-correlated pinning centers have been introduced by the BZO addition. The enhancement of Jc by BZO for the ErBCO film is larger than that for the YBCO film. E–J characteristics were observed in the magnetic field at various temperatures. Fig. 3 shows the typical example of E–J characteristics for the YBCO + BZO film in the magnetic field of 0.4 T applied parallel to the c-axis. An E–J curve at a low temperature has a negative curvature in a log-log plot. As the temperature increases, E–J curves vary from a negative curvature to a positive curvature on reaching a certain temperature. This border temperature is defined as the vortex glass–liquid transition temperature Tg [12]. It has been reported that these distinctive behavior of E–J characteristics can be described by the percolation transition model which takes account of the statistical distribution of the local critical current density Jcl [8,9]. In this model, the pinning parameter m characterizes the shape of the distribution and m + 1 indicates the slope of the E–J curve at T = Tg in a log–log plot. The values of Jcm and DJc are the minimum value of Jcl and the width of distribution of Jcl, respectively. Measured E–J characteristics have are fitted to the theoretical expression based on the percolation transition
Fig. 3. E–J characteristics for YBCO + BZO films in magnetic field of 0.4 T applied parallel to the c-axis, where closed circles are experimental data and solid lines are fitted curves calculated by the percolation transition model.
T. Fujiyoshi et al. / Physica C 468 (2008) 1635–1637
model. As shown in Fig. 3, the theoretical curves are in excellent agreement with the experimental data in all measured temperature and magnetic field region.
Fig. 4. B-Tg/Tc phase diagram for ErBCO + BZO and YBCO + BZO films in the magnetic field parallel to the c-axis.
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Fig. 4 is the B-Tg/Tc phage diagram for the ErBCO + BZO and YBCO + BZO films in the magnetic field parallel to the c-axis. The values of Tg are normalized Tc of the each sample in order to eliminate the influence of Tc. The values of Tg/Tc of the ErBCO + BZO film are higher than those of the YBCO + BZO film. This result indicates that the vortex glass phase with Jc 6¼ 0 of the ErBCO + BZO film extends to a higher temperature region. Fig. 5 shows the magnetic field dependence of pinning parameter m for the ErBCO + BZO and YBCO + BZO films estimated from the observed E–J characteristics. The peaks appear around 0.8 T in both samples. The pinning parameter m characterizes the shape of the distribution of Jcl, and corresponds to the efficiency of flux pinning. Then, it is considered that these peaks of m are due to the matching of the density of BZO nano-rods with that of fluxoids. In fact, it was confirmed that the density of BZO nano-rods for the ErBCO thin film containing 1.5 wt% BZO is 0.79 T by TEM observation [13]. Similar behaviors have been also observed in the magnetic field dependence of m for the YBCO thin films with columnar defects introduced by heavy ion irradiation [14]. Therefore, the peak of m in the magnetic field dependence is a characteristic feature in the REBCO films with strong c-axis correlated pinning centers. The magnetic field dependences of the ratio of DJc to Jcm for the ErBCO + BZO and YBCO + BZO films are shown in Fig. 6. This value is connected with the relative distribution width of Jcl. Therefore, the ErBCO + BZO film has a sharp distribution Jcl compared to the YBCO + BZO. This result provably originates from the high crystalline quality of the ErBCO + BZO film which has a narrow FWHM of rocking curve of 0.067° [15]. 4. Conclusion The enhancement of Jc due to the introduction of BZO nano-rods in the high magnetic field region was observed. The peak of Jc at B||c-axis in the field angular dependence indicates that the BZO nano-rods work as the c-axis-correlated pinning centers. We found the matching effect of the density of BZO nano-rods with that of fluxoids. The relative distribution width of the local critical current density for the ErBCO + BZO film is smaller than that of the YBCO + BZO films. References
Fig. 5. Magnetic field dependence of m for ErBCO + BZO and YBCO + BZO films.
Fig. 6. Magnetic field dependences of the ratio of DJc to Jcm for ErBCO + BZO and YBCO + BZO films at T/Tg = 0.9.
[1] T. Haugan, P.N. Barnes, R. Wheeler, F. Meisenkothen, M. Sumption, Nature 430 (2004) 867. [2] 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, Nature Mater. 3 (2004) 439. [3] 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. [4] S. Kang, A. Goyal, J. Li, A.A. Gapud, P.M. Martin, L. Heatherly, J.R. Thompson, D.K. Christen, F.A. List, M. Paranthaman, D.F. Lee, Science 311 (2006) 1911. [5] K. Matsumoto, T. Horide, A. Ichinose, S. Horii, Y. Yoshida, M. Mukaida, Jpn. J. Appl. Phys. 44 (2005) L246. [6] A.A. Gapud, D.K. Kumar, S.K. Viswanathan, C. Cantoni, M. Varela, J. Abiade, S.J. Pennycook, D.K. Christen, Supercond. Sci. Technol. 18 (2005) 1502. [7] M. Mukaida, T. Horide, R. Kita, S. Horii, A. Ichinose, Y. Yoshida, O. Miura, K. Matsumoto, K. Yamada, N. Mori, Jpn. J. Appl. Phys. 44 (2005) L952. [8] K. Yamafuji, T. Kiss, Physica C 258 (1996) 197. [9] K. Yamafuji, T. Fujiyoshi, T. Kiss, Physica C 397 (2003) 132. [10] V. Boffa, T. Petrisor, L. Ciontea, U. Gambardalla, S. Barbanera, Physica C 260 (1996) 111. [11] L. Civale, B. Maiorov, A. Serquis, J.O. Willis, J.Y. Coulter, H. Wang, Q.X. Jaa, P.N. Arendt, M. Jaime, J.L. MacManus-Driscoll, M. Maley, R. Foltyn, J. Low Temp. Phys. 135 (2004) 87. [12] D.S. Fisher, M.P.A. Fisher, D.A. Huse, Phys. Rev. B 43 (1991) 130. [13] 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. [14] 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. [15] M. Mukaida, M. Ito, R. Kita, S. Horii, A. Ichinose, K. Matsumoto, Y. Yoshida, A. Saito, K. Koike, F. Hirose, S. Ohshima, Jpn. J. Appl. Phys. 43 (2005) L1623.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Flux pinning properties of REBa2Cu3Oy thin films with BaZrO3 nano-rods T. Fujiyoshi a,*, M. Haruta a, T. Sueyoshi a, K. Yonekura a, M. Watanabe a, M. Mukaida b, R. Teranishi b, K. Matsumoto c, Y. Yoshida d, A. Ichinose e, S. Horii f, S. Awaji g, K. Watanabe g a
Department of Computer Science and Electrical Engineering, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan Department of Materials Science and Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0385, Japan c Department of Materials Science and Engineering, Kyushu Institute of Technology, Sensui-cho, 1-1, Tobata-ku, Kitakyushu, Fukuoka 804-8550, Japan d Department of Electrical Engineering and Computer Science, Nagoya University, Furo-cho, Chisusa-ku, Nagoya 464-8603, Japan e Electric Power Engineering Research Laboratory, Central Research Institute Electric Power Industry, Nagasaka 2-6-1, Yokosuka, Kanagawa 240-0196, Japan f Department of Applied Chemistry, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan g High Field Laboratory for Superconducting Materials, IMR, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan b
a r t i c l e
i n f o
Article history: Available online 29 May 2008 PACS: 74.25.Qt 74.25.Sv 74.78.Bz Keywords: Flux pinning Nano-rods
a b s t r a c t Flux pinning properties in ErBa2Cu3Oy and YBa2Cu3Oy thin films with BaZrO3 nano-rods prepared by PLD were measured to investigate the flux pinning mechanism. The enhancement of Jc was confirmed by the measurement of the dependence of Jc on magnetic field. The angular dependence of Jc has a broad peak at B||c-axis. This result indicates that the BaZrO3 nano-rods work as the c-axis-correlated pinning centers. The pinning parameter m was evaluated from the electric fields versus current density characteristics and the peak of m appears in the magnetic field dependence. The characteristic behavior of m is caused by the matching of the density of BaZrO3 nano-rods with that of fluxoids. It is found that the relative distribution width of the local critical current density for the ErBa2Cu3Oy film with BaZrO3 nano-rods is smaller than that of the YBa2Cu3Oy films with BaZrO3 nano-rods. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction REBa2Cu3Oy (REBCO) coated conductors are expected to be applied to superconducting machines because REBCO films indicate a high critical current density Jc at 77 K. However, the value of Jc for REBCO films at 77 K decreases rapidly with increasing the magnetic field. Flux pinning force from naturally introduced pinning centers during the fabrication process, e.g., oxygen vacancies, dislocations and grain boundaries, is not enough to achieve high-Jc at 77 K under the magnetic field. Recently, some groups have succeeded in improving Jc properties under the magnetic field using practical methods, in which the nano-crystal structures were controlled during deposition of films. For example, Y2BaCuO5, BaZrO3 and Y2O3 nano-particles were introduced into YBa2Cu3Oy thin films and bring about a remarkable improvement of Jc properties [1–6]. Among these artificial pinning centers, BaZrO3 (BZO) nano-rods are much effective as c-axis-correlated pinning centers [2,7]. The BZO nano-rods can be fabricated using a mixture target of REBCO and BZO in a pulsed laser deposition (PLD) method. Extended columns of self-assembled BZO particles are formed and aligned along the c-axis of
* Corresponding author. Tel.: +81 96 342 3849; fax: +81 96 342 3630. E-mail address: [email protected] (T. Fujiyoshi). 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2008.05.155
REBCO films. However, the comprehensive understanding of the flux pinning mechanism of REBCO films with BZO nano-rods is not enough. In this study, the flux pinning properties of REBCO films with BZO nano-rods were evaluated and the flux pinning mechanism was investigated on the basis of the percolation transition model [8,9]. ErBa2Cu3Oy (ErBCO) and YBa2Cu3Oy (YBCO) thin films containing BaZrO3 (BZO) were prepared by PLD. The enhancement of the Jc of both samples was observed in the high magnetic field region. The broad peak at B||c-axis appears in the angular dependence of Jc. The vortex glass–liquid transition temperature Tg and the pinning parameter m were evaluated from electric fields versus current density (E–J) characteristics. The pinning parameter m characterizes the shape of the distribution of the local critical current density, and corresponds to the efficiency of flux pinning. 2. Experimental The ErBCO and YBCO thin films with BZO nano-rods were prepared on SrTiO3 (1 0 0) substrates by PLD with an ArF excimer laser [7]. Sintered ErBCO and YBCO ceramic bulks containing 1.5 wt% BZO were used as targets. The substrate temperatures at the ErBCO + BZO and YBCO + BZO thin films were 750 °C and 715 °C, respectively. The laser frequency was 1 Hz and the laser energy
1636
T. Fujiyoshi et al. / Physica C 468 (2008) 1635–1637
was 300 mJ. During deposition, the oxygen partial pressure was fixed at 53.3 Pa. After deposition, the substrates were cooled to a room temperature without any annealing processes. The thicknesses of the ErBCO + BZO and YBCO + BZO thin films were 368 nm and 300 nm, respectively. The ErBCO + BZO and YBCO + BZO thin films have the critical temperatures Tc of 89.1 K and 89.3 K, respectively. In order to measure the transport properties, the samples were patterned with shape of 50 lm wide and 1 mm long by photolithography. To obtain good contacts, Au contact pads were deposited on the films by RF sputtering after cleaning of the film surfaces by oxygen–ion etching. The sample was mounted on a rotating sample holder with a cernox thermometer and a heater. The sample temperature was controlled by both He gas flow in a temperature variable cryostat and the heater placed on the sample holder. The sample temperature was stabilized within ±0.02 K. Magnetic fields were applied using a superconducting magnet. A magnetic field angle is defined such that B||c-axis is h = 0° and transport currents are always perpendicular to the magnetic field and the c-axis. The E–J characteristics were measured with a dc four-probe method. The value of Jc was evaluated from the transport properties with the criterion of the electric field Ec = 10 lm/cm. 3. Result and discussion The dependences of Jc on the magnetic field applied parallel to the c-axis for the ErBCO + BZO and YBCO + BZO thin films are shown in Fig. 1. The ErBCO + BZO film has higher Jc at 77.3 K and 84.0 K compared to the YBCO + BZO film. For comparison, the Jc characteristics of pure-YBCO thin film prepared on a SrTiO3 substrate by a PLD method [10] is also shown in Fig. 1. Although the pure-YBCO thin film has higher Jc than the ErBCO + BZO and YBCO + BZO films in the low magnetic field region, the values of Jc of the ErBCO + BZO and YBCO + BZO films exceed that of the pure-YBCO thin film above 4 T and 6 T, respectively. Therefore, the reduction of Jc with increasing the magnetic field becomes smaller due to the introduction of BZO into ErBCO and YBCO films. Fig. 2 shows the angular dependence of Jc at 84.0 K, 0.5 T for the ErBCO + BZO and YBCO + BZO films, where the values of Jc are normalized by the value of Jc(B||c). There are two peaks appeared at h = 0° and 90° in both samples. According to Civale et al., the main contribution of the angular dependence of Jc is associated with the effective mass anisotropy and Jc(h) peaks are added on it [11]. The peak of Jc at h = 90°, corresponding to the direction of the magnetic field parallel to the ab-plane, are contributed to the intrinsic pinning or stacking faults. The broad peak of Jc at B||c-axis indicates
Fig. 1. Dependences of Jc on the magnetic field parallel to the c-axis for ErBCO + BZO and YBCO + BZO films. The data for pure-YBCO film prepared by PLD [10] are also shown in the figure.
Fig. 2. Angular dependence of Jc for ErBCO + BZO and YBCO + BZO films at 84 K, 0.5 T.
that c-axis-correlated pinning centers have been introduced by the BZO addition. The enhancement of Jc by BZO for the ErBCO film is larger than that for the YBCO film. E–J characteristics were observed in the magnetic field at various temperatures. Fig. 3 shows the typical example of E–J characteristics for the YBCO + BZO film in the magnetic field of 0.4 T applied parallel to the c-axis. An E–J curve at a low temperature has a negative curvature in a log-log plot. As the temperature increases, E–J curves vary from a negative curvature to a positive curvature on reaching a certain temperature. This border temperature is defined as the vortex glass–liquid transition temperature Tg [12]. It has been reported that these distinctive behavior of E–J characteristics can be described by the percolation transition model which takes account of the statistical distribution of the local critical current density Jcl [8,9]. In this model, the pinning parameter m characterizes the shape of the distribution and m + 1 indicates the slope of the E–J curve at T = Tg in a log–log plot. The values of Jcm and DJc are the minimum value of Jcl and the width of distribution of Jcl, respectively. Measured E–J characteristics have are fitted to the theoretical expression based on the percolation transition
Fig. 3. E–J characteristics for YBCO + BZO films in magnetic field of 0.4 T applied parallel to the c-axis, where closed circles are experimental data and solid lines are fitted curves calculated by the percolation transition model.
T. Fujiyoshi et al. / Physica C 468 (2008) 1635–1637
model. As shown in Fig. 3, the theoretical curves are in excellent agreement with the experimental data in all measured temperature and magnetic field region.
Fig. 4. B-Tg/Tc phase diagram for ErBCO + BZO and YBCO + BZO films in the magnetic field parallel to the c-axis.
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Fig. 4 is the B-Tg/Tc phage diagram for the ErBCO + BZO and YBCO + BZO films in the magnetic field parallel to the c-axis. The values of Tg are normalized Tc of the each sample in order to eliminate the influence of Tc. The values of Tg/Tc of the ErBCO + BZO film are higher than those of the YBCO + BZO film. This result indicates that the vortex glass phase with Jc 6¼ 0 of the ErBCO + BZO film extends to a higher temperature region. Fig. 5 shows the magnetic field dependence of pinning parameter m for the ErBCO + BZO and YBCO + BZO films estimated from the observed E–J characteristics. The peaks appear around 0.8 T in both samples. The pinning parameter m characterizes the shape of the distribution of Jcl, and corresponds to the efficiency of flux pinning. Then, it is considered that these peaks of m are due to the matching of the density of BZO nano-rods with that of fluxoids. In fact, it was confirmed that the density of BZO nano-rods for the ErBCO thin film containing 1.5 wt% BZO is 0.79 T by TEM observation [13]. Similar behaviors have been also observed in the magnetic field dependence of m for the YBCO thin films with columnar defects introduced by heavy ion irradiation [14]. Therefore, the peak of m in the magnetic field dependence is a characteristic feature in the REBCO films with strong c-axis correlated pinning centers. The magnetic field dependences of the ratio of DJc to Jcm for the ErBCO + BZO and YBCO + BZO films are shown in Fig. 6. This value is connected with the relative distribution width of Jcl. Therefore, the ErBCO + BZO film has a sharp distribution Jcl compared to the YBCO + BZO. This result provably originates from the high crystalline quality of the ErBCO + BZO film which has a narrow FWHM of rocking curve of 0.067° [15]. 4. Conclusion The enhancement of Jc due to the introduction of BZO nano-rods in the high magnetic field region was observed. The peak of Jc at B||c-axis in the field angular dependence indicates that the BZO nano-rods work as the c-axis-correlated pinning centers. We found the matching effect of the density of BZO nano-rods with that of fluxoids. The relative distribution width of the local critical current density for the ErBCO + BZO film is smaller than that of the YBCO + BZO films. References
Fig. 5. Magnetic field dependence of m for ErBCO + BZO and YBCO + BZO films.
Fig. 6. Magnetic field dependences of the ratio of DJc to Jcm for ErBCO + BZO and YBCO + BZO films at T/Tg = 0.9.
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