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Correlated pinning behavior in ErBa2Cu3Oy films with BaZrO3 nano-rods
Physica C 469 (2009) 1404–1409
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Physica C journal homepage: www.elsevier.com/locate/physc
Correlated pinning behavior in ErBa2Cu3Oy films with BaZrO3 nano-rods M. Namba a,*, S. Awaji a, K. Watanabe a, S. Ito a, E. Aoyagi a, H. Kai b, M. Mukaida b, R. Kita c a b c
High Field Laboratory for Superconducting Materials, IMR, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan Shizuoka University, 3-5-1, Johoku, Hamamatsu, Shizuoka-ken 812-8581, Japan
a r t i c l e
i n f o
Article history: Available online 30 May 2009 PACS: 74.25.Qt 74.25.Sv Keywords: Critical current density c-Axis-correlated pinning Irreversibility field Nano-rod
a b s t r a c t In REBa2Cu3Oy (RE123) films with BaZrO3 (BZO), the columnar shaped BZO precipitates, called nano-rods, are formed as the c-axis-correlated pinning centers. In order to understand the vortex pinning properties of RE123 films with BZO nano-rods, we measured the resistivity q and critical current density Jc of three different films, i.e. the Er123 films with 0, 1.5 and 3.5 wt% BZO prepared by pulsed laser deposition. We found that the peak related to the c-axis-correlated pinning in the angular dependence of Jc became large in a low field region but small in a high magnetic field with increasing BZO concentration. In addition, the irreversibility field at B||c-axis increased also with increasing BZO concentration, although the dip in the angular dependence of q and the peak of the Jc at B||c-axis decreased in the high magnetic field near the irreversible field. It is suggested that the BZO addition is effective to the enhancement of Jc in a low field region as well as the improvement of the irreversibility field. However, we have to consider the c-axiscorrelated pinning by the edge dislocations as well as the nano-rods in order to understand it in a high field region. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The films of REBa2Cu3Oy (RE = rare earth, RE123) prepared by pulsed laser deposition (PLD) using a mixture target of BaZrO3 (BZO) and RE123 have clearly the large critical current density (Jc) in a magnetic field and the high irreversibility field (Bi) for B||c-axis [1]. This is because the columnar shaped BZO precipitate, called nano-rods, can be introduced easily as the c-axis-correlated pinning centers for the RE123 film [2]. In fact, it is well known that the c-axis-correlated disorders such as columnar defects introduced by the heavy-ion irradiation [3], twin planes [4] and dislocations [5] enhance not only Jc but also Bi for B||c-axis. The effects of these c-axis-correlated disorders on transport properties are investigated by the angular dependence of the critical current density (Jc(h)) and the resistivity (q(h)). In the case of the Y123 film with columnar defects introduced by the heavy-ion irradiation parallel to the c-axis, the c-axis-correlated disorders work well at both vortex liquid and vortex solid states [3]. In the case of the Sm123 film with the edge dislocation, however, the c-axis-correlated pinning behaviors appear at both a low field and a high field near the Bi, but disappear in a certain intermediated magnetic field in the vortex solid state [5]. In addition, we found the c-axis-correlated peak of Jc(h) by BZO nano-rods was observed only in a low field region [6].
* Corresponding author. Tel.: +81 (0) 22 215 2147; fax: +81 (0) 22 215 2149. E-mail address: [email protected] (M. Namba). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.05.046
In this paper, we measured the angular dependence of transport properties for the BZO-doped Er123 films with different amount of BZO addition, and discuss the behavior of the BZO nano-rods as the c-axis-correlated pinning at the region of vortex liquid and solid. 2. Experiments The ErBa2Cu3Oy (Er123) films were grown by the pulsed ArF excimer laser deposition (PLD) on SrTiO3 single-crystal substrates [7]. We used an Er123 target with 1.5 wt% (3.8 mol%) BaZrO3 (BZO) and 3.5 wt% (8.9 mol%)BZO additions. The critical temperature Tc values are 88.8 K for the 1.5BZO-doped Er123 film and 88.0 K for the 3.5BZO-doped Er123 film, respectively. Here, Tc of the Er123 film without BZO additions (pure Er123 film) as a reference sample is 90.2 K. Tc decreased with the BZO addition. For the microstructure observation, cross-sectional- and planeview images of the BZO doped films were studied by transmission electron microscopy (TEM). The transport properties of all samples were measured by the four-probe method after patterning a microbridge shape with 30 lm in width and 2 mm in length by photolithography. The thicknesses measured by the laser microscope were about 700 nm for the 1.5BZO-doped Er123 film and 400 nm for the 3.5BZO-doped Er123 film, respectively. For the transport measurements at low temperature, the sample temperature was controlled within ±10 mK stability by both He gas flow in a temperature-variable cryostat and a heater placed onto the sample holder. Magnetic fields to 17 T were applied using a 20 T
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superconducting magnet at the High Field Laboratory for Superconducting Materials (HFLSM), Institute for Materials Research (IMR), Tohoku University. At the further high magnetic field measurements up to 27 T, we used the 28 T hybrid magnet at the HFLSM. A magnetic field angle was defined such that B||c-axis was h = 0° and transport currents were always perpendicular to the magnetic field and c-axis. Jc was determined by the 1 lV/cm criterion. Bi was determined by the 0.1 lX cm criterion.
6.4° and 4.0°, respectively. Hence, the alignment of the BZO nano-rods became better with the increase of the BZO addition. The difference of the average tilt angles of the BZO nano-rods may influence transport properties. However, we do not discuss here because the elementary pinning force and the density of the c-axis-correlated disorders should be taken account as well.
3. Results and discussion
Fig. 2 shows the angular dependence of resistivity (q(h)) for both samples at 77.3 K. One should notice that the dip structures for both samples appear on the q(h) for B||c-axis (h = 0°). These dip structures mean that the c-axis-correlated pinning in the vortex liquid region works effectively. The depth of these dips decreases with an increase of a magnetic field. The inset of Fig. 2 shows the q(h) in 26 T. These results exhibit that the effective caxis-correlated pinning sufficiently contributes to both samples in high fields up to 26 T. The q(h) dips of both samples were much shallower than that of the Sm123 film with the edge dislocation [9] and that of the Y123 film with columnar defects introduced by the heavy-ion irradiation [3]. In addition, the depth of q(h) dips of the 1.5BZO-doped Er123 film was deeper than that of the 3.5BZOdoped Er123 film. In order to study the behavior of these dips, we compared the temperature dependence of resistivity q(T) between h = 0° and 12°, i.e. on and off the dip as shown in Fig. 3a and b. The irreversibility fields Bi obtained from these results for B||c-axis (h = 0°) at 77.3 K were 8.70 T and 10.43 T for the 1.5 and 3.5BZO-doped Er123 films, respectively. Hence, it is suggested that the Bi (77.3 K, B||c-axis) enhanced with the increase of the BZO addition as reported before [10]. We adopt the difference of the normalized resistivity between h = 0° and 12°, Dqn = qn(12°) qn(0°), where qn is the normalized resistivity by the resistance at 100 K as the normal resistance. The positive Dqn is related to the depth of the q(h) dip. Dqn takes the maximum with decreasing temperature, when the dip appears in q(h). Here, we define the maximum value of , as characteristic the normalized resistivity difference, Dqmax n parameter of the dissipation reduction by the c-axis-correlated disorders in the pinned liquid state [4]. Fig. 4 shows the applied field . For the 1.5BZO-doped Er123 film, Dqmax is aldependence of Dqmax n n increases monotonmost zero at the low field below 7 T, but Dqmax n
3.1. Microstructure Fig. 1 shows the TEM images of the 1.5 and 3.5BZO-doped Er123 films. As seen in the plane-view images (Fig. 1a and b) and the cross-sectional view images (Fig. 1c and d), the precipitate with a columnar structure in nano-scale can be found. These results represent that both samples include BZO nano-rods, because BZO disperses in Er123 film as the nano-rods [8]. We estimated an average density and a diameter of the nano-rods from the plane-view, and from the cross-sectional view of the TEM images, respectively. The densities of the BZO nano-rods in the 1.5 and 3.5BZO-doped Er123 films were (8.7 ± 2.9) 1010 cm 2 and (2.5 ± 0.6) 1011 cm 2, respectively. Those correspond to the matching fields (BU) of 1.8 ± 0.6 T and 5.1 ± 1.3 T, respectively. Hence, it was found that the densities of the BZO nano-rods increased with the increase of the BZO addition. The diameter of the BZO nano-rods in the 1.5 and 3.5BZO-doped Er123 films were 7.6 ± 2.5 nm and 7.0 ± 1.5 nm, respectively. This means that the size of the BZO nano-rods is independent of the amount of the BZO addition. The volume fractions of BZO nano-rods estimated from the densities and the diameters were 3.46 mol% for the 1.5BZO-doped Er123 film and 8.15 mol% for the 1.5BZO-doped Er123 film. These values are almost consistent with the target compositions of 3.8 mol% BZO for the 1.5BZO-doped Er123 film and 8.9 mol% BZO for the 3.5BZOdoped Er123 film. As a result, we found that most BZO in the target formed the nano-rods in the film. In addition, one notes that the BZO nano-rods did not align to the c-axis of the Er123 completely. The average tilt angles of the BZO nano-rods from the c-axis direction estimated by the cross-sectional view of the TEM images (Fig. 1c and d) in the 1.5 and 3.5BZO-doped Er123 films, were
3.2. Resistivity
Fig. 1. TEM images of (a) and (c) 1.5BZO-doped Er123 film, (b) and (d) 3.5BZO-doped Er123 film. (a) and (b) are plane-view, (c) and (d) cross-sectional view.
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Fig. 2. Angular dependence of resistivity at 77.3 K for (a) 1.5BZO-doped Er123 film and (b) 3.5BZO-doped Er123 film. Insets show the angular dependence of resistivity at a high field of 26 T.
ically with increasing magnetic fields above 9 T. For the 3.5BZOdoped Er123 film, the q(h) dip appeared at 1 T, and disappears in increases monotonically with a field region of 3–11 T, but Dqmax n of the 3.5BZO-doped increasing magnetic fields above 13 T. Dqmax n Er123 film was bigger than that of the 1.5BZO-doped Er123 film in the low field region below 3 T, however smaller in the high field region above 9 T. In the case of Y123 films with the columnar defects by the heavy-ion irradiation parallel to the c-axis [3], the vais proportional to the density of the columnar defects. lue of Dqmax n It is supposed that the dip of the q(h) at the low field is originated correlates with the density from the BZO nano-rods because Dqmax n in a high field region shows of the BZO nano-rods. However, Dqmax n a negative correlation with the density of the BZO nano-rods. This means that the dip of the q(h) at the high field comes from the c-
Fig. 3. Comparison of the resistivity between h = 0° and 12° for (a) 1.5BZO-doped Er123 film and (b) 3.5BZO-doped Er123 film.
axis-correlated disorders rather than BZO nano-rods. The inset of between the pure Er123 film Fig. 4 shows the comparison of Dqmax n of the as a reference sample and the BZO-doped Er123 films. Dqmax n both BZO-Er123 films was much smaller than that of the pure Er123 film, where the edge dislocation works as the c-axis-correlated pinning. If the BZO addition may reduce the c-axis-correlated pinning by the edge dislocations still remaining slightly, it is reasonable that the edge dislocations become effective as the c-axiscorrelated pinning at the high field, which is much larger than the matching field of the nano-rods in the BZO-doped Er123 films. 3.3. Critical current density The angular dependence of the critical current density (Jc(h)) at 77.3 K and various magnetic fields is shown in Fig. 5. Peaks at h = 0°
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Fig. 4. Magnetic field dependence of Dqmax at various temperatures. Inset shows n the comparison to the pure Er123 film.
(B||c-axis) based on the c-axis-correlated disorders and at h = 90° (B||ab-plane) based on the effective mass anisotropy and/or the intrinsic pinning [11,12] are observed for both samples. The c-axis peaks of Jc(h) for both samples were higher than the Jc(90°) in the low field region below 1–2 T. Hence, it is suggested that the BZO nano-rods play a very strong role of the c-axis-correlated pinning at the low field region. In order to discuss the effect of the BZO addition, we define the peak height of Jc(h) at h = 0° by the ratio of the Jc at B||c-axis to the c min value becomes unity minimum Jc value, J cc =J min c . Note that J c =J c when the peak at B||c-axis disappears. Fig. 6 represents the field at various temperatures. For both samples, dependence of J cc =J min c value is larger than unity in the low field below their the J cc =J min c matching fields but decreases once with increasing field in the intermediate field. For further increasing field, it increases again. value of the 3.5BZO-doped Er123 In a low field region, the J cc =J min c film is larger than that of the 1.5BZO-doped Er123 film. However, between the 1.5 and 3.5BZO-doped this relationship of J cc =J min c Er123 film in a high field is opposite to that in a low field. Hence, values the peak of Jc(0°) is by the BZO nano-rods because the J cc =J min c at the low fields correlate with the density of the nano-rods. On the other hand, in the high field region near the irreversibility field, becomes small with the increase of the BZO nano-rods denJ cc =J min c and J cc =J min of the both BZO-doped Er123 films at the sity. Dqmax c n high field region were much smaller than that of the pure Er123 film (not shown here). Therefore, the c-axis peak of Jc(h), which is characterized by J cc =J min c , also suggests that the c-axis-correlated pinning behavior in a high field region is related with the edge dislocations. Finally, the pattern diagrams of the c-axis-correlated pinning property were described at the vortex liquid state by the resistivity measurement and at the vortex solid state by the critical current density measurement as shown in Fig. 7. Here, we determined the irreversibility line Bi from the temperature dependence of resistivity and Bc2 from the midpoint of Tc in a field at h = 0°. We also define the onset temperature, Tk, of the c-axis-correlated pinning becomes effective by the temperature where the Dqn becomes zero. The grayscale in the vortex solid state corresponds values as shown in the legend in Fig. 6. The to the J cc =J min c crossed-squares in Fig. 7 are the point of measurement. The strong
Fig. 5. Angular dependence of Jc at 77.3 K for (a), (b) 1.5BZO-doped Er123 film and for (c), (d) 3.5BZO-doped Er123 film. (a), (c) Linear and (b), (d) logarithmic plots for the focusing low-Jc region.
c-axis-correlated pinning behaviors are obtained near the matching field and irreversibility field for both samples. When the den-
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Fig. 6. Peak height of Jc(h) at B||c-axis as a function of magnetic field for (a) 1.5BZOdoped Er123 film and (b) 3.5BZO-doped Er123 film.
sity of the BZO nano-rods increased, the c-axis-correlated pinning in the vortex solid region became strong at the low field, but weak at the high field near the irreversibility field. These results suggest that the c-axis-correlated pinning behavior by the edge dislocations appears in the high field above the matching field of the nano-rods. However, the irreversibility field improved with the increase of BZO nano-rods densities in comparison with that of the pure Er123 film, although the edge dislocation pinning becomes effective near the irreversibility field. This discrepancy may be understood by the additional effect of the edge dislocation to the nano-rods pinning. Since the pinning force through the elastic interaction of the vortices is dominant in the high field region above a matching field, the additional c-axis-correlated pinning such as the edge dislocations can assist to the whole flux pinning properties.
Fig. 7. Schematic vortex state diagram at B||c-axis proposed in this study for (a) 1.5BZO-doped Er123 film, (b) 3.5BZO-doped Er123 film. BU is the matching field of the BZO nano-rods. The grayscale corresponds to legends of peak height of Jc(h) in Fig. 6.
4. Summary We explored the c-axis-correlated pinning behavior by the TEM microstructure, the transport resistivity and the critical current density Jc as a function of magnetic field, temperature and field angles for the BZO-doped Er123 films. The densities of the BZO nanorods were increase, but the diameters of the BZO nano-rods were almost same with the increase of BZO additions in the target. The BZO nano-rods aligned with increasing the BZO nano-rods density. From the detailed measurement of the dips of the angular dependence of the resistivity and the peaks of the angular dependence of the critical current density at h = 0°, we found that the c-axiscorrelated pinning enhanced in the low field region but decreased in the high field region with the increase of the BZO nano-rods density in the both vortex liquid and solid state. It is considered that
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the edge dislocations influence as the additional c-axis-correlated pinning behavior in the high field region. Acknowledgments This work was supported by a Grant-in-Aid for JSPS Fellows (20 6749). 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. [2] 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. [3] M. Namba, S. Awaji, K. Watanabe, T. Nojima, S. Okayasu, Physica C 468 (2008) 1652.
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[4] S. Awaji, M. Namba, K. Watanabe, J. Phys. Conf. Ser. 51 (2006) 271. [5] S. Awaji, M. Namba, K. Watanabe, M. Miura, Y. Yoshida, Y. Ichino, Y. Takai, K. Matsumoto, Appl. Phys. Lett. 90 (2007) 122501. [6] M. Namba, S. Awaji, K. Watanabe, T. Nojima, S. Okayasu, M. Miura, Y. Ichino, Y. Yoshida, Y. Takai, T. Horide, P. Mele, K. Matsumoto, APEX 1 (2008) 031703. [7] 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 (2004) L1623. [8] K. Yamada, A. Ichinose, Y. Shingai, K. Matsumoto, Y. Yoshida, S. Horii, R. Kita, S. Toh, K. Kaneko, N. Mori, M. Mukaida, Physica C 445-448 (2006) 660. [9] S. Awaji, M. Namba, K. Watanabe, M. Miura, Y. Ichinino, Y. Yoshida, Y. Takai, K. Matsumoto, IEEE Trans. Appl. Supercond. 17 (2007) 3656. [10] M. Peurla, H. Huhtinen, M.A. Shakhov, K. Traito, Yu.P. Stepanov, M. Safonchik, P. Paturi, Y.Y. Tse, R. Palai, R. Laiho, Phys. Rev. B 75 (2007) 184524. [11] L. Civale, B. Maiorov, J.L. MacManus-Driscoll, H. Wang, T.G. Holesinger, S.R. Foltyn, A. Serquis, R.N. Arendt, IEEE Trans. Appl. Supercond. 15 (2005) 2808. [12] M. Tachiki, S. Takahashi, Solid State Commun. 70 (1989) 291.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Correlated pinning behavior in ErBa2Cu3Oy films with BaZrO3 nano-rods M. Namba a,*, S. Awaji a, K. Watanabe a, S. Ito a, E. Aoyagi a, H. Kai b, M. Mukaida b, R. Kita c a b c
High Field Laboratory for Superconducting Materials, IMR, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan Shizuoka University, 3-5-1, Johoku, Hamamatsu, Shizuoka-ken 812-8581, Japan
a r t i c l e
i n f o
Article history: Available online 30 May 2009 PACS: 74.25.Qt 74.25.Sv Keywords: Critical current density c-Axis-correlated pinning Irreversibility field Nano-rod
a b s t r a c t In REBa2Cu3Oy (RE123) films with BaZrO3 (BZO), the columnar shaped BZO precipitates, called nano-rods, are formed as the c-axis-correlated pinning centers. In order to understand the vortex pinning properties of RE123 films with BZO nano-rods, we measured the resistivity q and critical current density Jc of three different films, i.e. the Er123 films with 0, 1.5 and 3.5 wt% BZO prepared by pulsed laser deposition. We found that the peak related to the c-axis-correlated pinning in the angular dependence of Jc became large in a low field region but small in a high magnetic field with increasing BZO concentration. In addition, the irreversibility field at B||c-axis increased also with increasing BZO concentration, although the dip in the angular dependence of q and the peak of the Jc at B||c-axis decreased in the high magnetic field near the irreversible field. It is suggested that the BZO addition is effective to the enhancement of Jc in a low field region as well as the improvement of the irreversibility field. However, we have to consider the c-axiscorrelated pinning by the edge dislocations as well as the nano-rods in order to understand it in a high field region. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The films of REBa2Cu3Oy (RE = rare earth, RE123) prepared by pulsed laser deposition (PLD) using a mixture target of BaZrO3 (BZO) and RE123 have clearly the large critical current density (Jc) in a magnetic field and the high irreversibility field (Bi) for B||c-axis [1]. This is because the columnar shaped BZO precipitate, called nano-rods, can be introduced easily as the c-axis-correlated pinning centers for the RE123 film [2]. In fact, it is well known that the c-axis-correlated disorders such as columnar defects introduced by the heavy-ion irradiation [3], twin planes [4] and dislocations [5] enhance not only Jc but also Bi for B||c-axis. The effects of these c-axis-correlated disorders on transport properties are investigated by the angular dependence of the critical current density (Jc(h)) and the resistivity (q(h)). In the case of the Y123 film with columnar defects introduced by the heavy-ion irradiation parallel to the c-axis, the c-axis-correlated disorders work well at both vortex liquid and vortex solid states [3]. In the case of the Sm123 film with the edge dislocation, however, the c-axis-correlated pinning behaviors appear at both a low field and a high field near the Bi, but disappear in a certain intermediated magnetic field in the vortex solid state [5]. In addition, we found the c-axis-correlated peak of Jc(h) by BZO nano-rods was observed only in a low field region [6].
* Corresponding author. Tel.: +81 (0) 22 215 2147; fax: +81 (0) 22 215 2149. E-mail address: [email protected] (M. Namba). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.05.046
In this paper, we measured the angular dependence of transport properties for the BZO-doped Er123 films with different amount of BZO addition, and discuss the behavior of the BZO nano-rods as the c-axis-correlated pinning at the region of vortex liquid and solid. 2. Experiments The ErBa2Cu3Oy (Er123) films were grown by the pulsed ArF excimer laser deposition (PLD) on SrTiO3 single-crystal substrates [7]. We used an Er123 target with 1.5 wt% (3.8 mol%) BaZrO3 (BZO) and 3.5 wt% (8.9 mol%)BZO additions. The critical temperature Tc values are 88.8 K for the 1.5BZO-doped Er123 film and 88.0 K for the 3.5BZO-doped Er123 film, respectively. Here, Tc of the Er123 film without BZO additions (pure Er123 film) as a reference sample is 90.2 K. Tc decreased with the BZO addition. For the microstructure observation, cross-sectional- and planeview images of the BZO doped films were studied by transmission electron microscopy (TEM). The transport properties of all samples were measured by the four-probe method after patterning a microbridge shape with 30 lm in width and 2 mm in length by photolithography. The thicknesses measured by the laser microscope were about 700 nm for the 1.5BZO-doped Er123 film and 400 nm for the 3.5BZO-doped Er123 film, respectively. For the transport measurements at low temperature, the sample temperature was controlled within ±10 mK stability by both He gas flow in a temperature-variable cryostat and a heater placed onto the sample holder. Magnetic fields to 17 T were applied using a 20 T
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superconducting magnet at the High Field Laboratory for Superconducting Materials (HFLSM), Institute for Materials Research (IMR), Tohoku University. At the further high magnetic field measurements up to 27 T, we used the 28 T hybrid magnet at the HFLSM. A magnetic field angle was defined such that B||c-axis was h = 0° and transport currents were always perpendicular to the magnetic field and c-axis. Jc was determined by the 1 lV/cm criterion. Bi was determined by the 0.1 lX cm criterion.
6.4° and 4.0°, respectively. Hence, the alignment of the BZO nano-rods became better with the increase of the BZO addition. The difference of the average tilt angles of the BZO nano-rods may influence transport properties. However, we do not discuss here because the elementary pinning force and the density of the c-axis-correlated disorders should be taken account as well.
3. Results and discussion
Fig. 2 shows the angular dependence of resistivity (q(h)) for both samples at 77.3 K. One should notice that the dip structures for both samples appear on the q(h) for B||c-axis (h = 0°). These dip structures mean that the c-axis-correlated pinning in the vortex liquid region works effectively. The depth of these dips decreases with an increase of a magnetic field. The inset of Fig. 2 shows the q(h) in 26 T. These results exhibit that the effective caxis-correlated pinning sufficiently contributes to both samples in high fields up to 26 T. The q(h) dips of both samples were much shallower than that of the Sm123 film with the edge dislocation [9] and that of the Y123 film with columnar defects introduced by the heavy-ion irradiation [3]. In addition, the depth of q(h) dips of the 1.5BZO-doped Er123 film was deeper than that of the 3.5BZOdoped Er123 film. In order to study the behavior of these dips, we compared the temperature dependence of resistivity q(T) between h = 0° and 12°, i.e. on and off the dip as shown in Fig. 3a and b. The irreversibility fields Bi obtained from these results for B||c-axis (h = 0°) at 77.3 K were 8.70 T and 10.43 T for the 1.5 and 3.5BZO-doped Er123 films, respectively. Hence, it is suggested that the Bi (77.3 K, B||c-axis) enhanced with the increase of the BZO addition as reported before [10]. We adopt the difference of the normalized resistivity between h = 0° and 12°, Dqn = qn(12°) qn(0°), where qn is the normalized resistivity by the resistance at 100 K as the normal resistance. The positive Dqn is related to the depth of the q(h) dip. Dqn takes the maximum with decreasing temperature, when the dip appears in q(h). Here, we define the maximum value of , as characteristic the normalized resistivity difference, Dqmax n parameter of the dissipation reduction by the c-axis-correlated disorders in the pinned liquid state [4]. Fig. 4 shows the applied field . For the 1.5BZO-doped Er123 film, Dqmax is aldependence of Dqmax n n increases monotonmost zero at the low field below 7 T, but Dqmax n
3.1. Microstructure Fig. 1 shows the TEM images of the 1.5 and 3.5BZO-doped Er123 films. As seen in the plane-view images (Fig. 1a and b) and the cross-sectional view images (Fig. 1c and d), the precipitate with a columnar structure in nano-scale can be found. These results represent that both samples include BZO nano-rods, because BZO disperses in Er123 film as the nano-rods [8]. We estimated an average density and a diameter of the nano-rods from the plane-view, and from the cross-sectional view of the TEM images, respectively. The densities of the BZO nano-rods in the 1.5 and 3.5BZO-doped Er123 films were (8.7 ± 2.9) 1010 cm 2 and (2.5 ± 0.6) 1011 cm 2, respectively. Those correspond to the matching fields (BU) of 1.8 ± 0.6 T and 5.1 ± 1.3 T, respectively. Hence, it was found that the densities of the BZO nano-rods increased with the increase of the BZO addition. The diameter of the BZO nano-rods in the 1.5 and 3.5BZO-doped Er123 films were 7.6 ± 2.5 nm and 7.0 ± 1.5 nm, respectively. This means that the size of the BZO nano-rods is independent of the amount of the BZO addition. The volume fractions of BZO nano-rods estimated from the densities and the diameters were 3.46 mol% for the 1.5BZO-doped Er123 film and 8.15 mol% for the 1.5BZO-doped Er123 film. These values are almost consistent with the target compositions of 3.8 mol% BZO for the 1.5BZO-doped Er123 film and 8.9 mol% BZO for the 3.5BZOdoped Er123 film. As a result, we found that most BZO in the target formed the nano-rods in the film. In addition, one notes that the BZO nano-rods did not align to the c-axis of the Er123 completely. The average tilt angles of the BZO nano-rods from the c-axis direction estimated by the cross-sectional view of the TEM images (Fig. 1c and d) in the 1.5 and 3.5BZO-doped Er123 films, were
3.2. Resistivity
Fig. 1. TEM images of (a) and (c) 1.5BZO-doped Er123 film, (b) and (d) 3.5BZO-doped Er123 film. (a) and (b) are plane-view, (c) and (d) cross-sectional view.
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Fig. 2. Angular dependence of resistivity at 77.3 K for (a) 1.5BZO-doped Er123 film and (b) 3.5BZO-doped Er123 film. Insets show the angular dependence of resistivity at a high field of 26 T.
ically with increasing magnetic fields above 9 T. For the 3.5BZOdoped Er123 film, the q(h) dip appeared at 1 T, and disappears in increases monotonically with a field region of 3–11 T, but Dqmax n of the 3.5BZO-doped increasing magnetic fields above 13 T. Dqmax n Er123 film was bigger than that of the 1.5BZO-doped Er123 film in the low field region below 3 T, however smaller in the high field region above 9 T. In the case of Y123 films with the columnar defects by the heavy-ion irradiation parallel to the c-axis [3], the vais proportional to the density of the columnar defects. lue of Dqmax n It is supposed that the dip of the q(h) at the low field is originated correlates with the density from the BZO nano-rods because Dqmax n in a high field region shows of the BZO nano-rods. However, Dqmax n a negative correlation with the density of the BZO nano-rods. This means that the dip of the q(h) at the high field comes from the c-
Fig. 3. Comparison of the resistivity between h = 0° and 12° for (a) 1.5BZO-doped Er123 film and (b) 3.5BZO-doped Er123 film.
axis-correlated disorders rather than BZO nano-rods. The inset of between the pure Er123 film Fig. 4 shows the comparison of Dqmax n of the as a reference sample and the BZO-doped Er123 films. Dqmax n both BZO-Er123 films was much smaller than that of the pure Er123 film, where the edge dislocation works as the c-axis-correlated pinning. If the BZO addition may reduce the c-axis-correlated pinning by the edge dislocations still remaining slightly, it is reasonable that the edge dislocations become effective as the c-axiscorrelated pinning at the high field, which is much larger than the matching field of the nano-rods in the BZO-doped Er123 films. 3.3. Critical current density The angular dependence of the critical current density (Jc(h)) at 77.3 K and various magnetic fields is shown in Fig. 5. Peaks at h = 0°
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Fig. 4. Magnetic field dependence of Dqmax at various temperatures. Inset shows n the comparison to the pure Er123 film.
(B||c-axis) based on the c-axis-correlated disorders and at h = 90° (B||ab-plane) based on the effective mass anisotropy and/or the intrinsic pinning [11,12] are observed for both samples. The c-axis peaks of Jc(h) for both samples were higher than the Jc(90°) in the low field region below 1–2 T. Hence, it is suggested that the BZO nano-rods play a very strong role of the c-axis-correlated pinning at the low field region. In order to discuss the effect of the BZO addition, we define the peak height of Jc(h) at h = 0° by the ratio of the Jc at B||c-axis to the c min value becomes unity minimum Jc value, J cc =J min c . Note that J c =J c when the peak at B||c-axis disappears. Fig. 6 represents the field at various temperatures. For both samples, dependence of J cc =J min c value is larger than unity in the low field below their the J cc =J min c matching fields but decreases once with increasing field in the intermediate field. For further increasing field, it increases again. value of the 3.5BZO-doped Er123 In a low field region, the J cc =J min c film is larger than that of the 1.5BZO-doped Er123 film. However, between the 1.5 and 3.5BZO-doped this relationship of J cc =J min c Er123 film in a high field is opposite to that in a low field. Hence, values the peak of Jc(0°) is by the BZO nano-rods because the J cc =J min c at the low fields correlate with the density of the nano-rods. On the other hand, in the high field region near the irreversibility field, becomes small with the increase of the BZO nano-rods denJ cc =J min c and J cc =J min of the both BZO-doped Er123 films at the sity. Dqmax c n high field region were much smaller than that of the pure Er123 film (not shown here). Therefore, the c-axis peak of Jc(h), which is characterized by J cc =J min c , also suggests that the c-axis-correlated pinning behavior in a high field region is related with the edge dislocations. Finally, the pattern diagrams of the c-axis-correlated pinning property were described at the vortex liquid state by the resistivity measurement and at the vortex solid state by the critical current density measurement as shown in Fig. 7. Here, we determined the irreversibility line Bi from the temperature dependence of resistivity and Bc2 from the midpoint of Tc in a field at h = 0°. We also define the onset temperature, Tk, of the c-axis-correlated pinning becomes effective by the temperature where the Dqn becomes zero. The grayscale in the vortex solid state corresponds values as shown in the legend in Fig. 6. The to the J cc =J min c crossed-squares in Fig. 7 are the point of measurement. The strong
Fig. 5. Angular dependence of Jc at 77.3 K for (a), (b) 1.5BZO-doped Er123 film and for (c), (d) 3.5BZO-doped Er123 film. (a), (c) Linear and (b), (d) logarithmic plots for the focusing low-Jc region.
c-axis-correlated pinning behaviors are obtained near the matching field and irreversibility field for both samples. When the den-
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Fig. 6. Peak height of Jc(h) at B||c-axis as a function of magnetic field for (a) 1.5BZOdoped Er123 film and (b) 3.5BZO-doped Er123 film.
sity of the BZO nano-rods increased, the c-axis-correlated pinning in the vortex solid region became strong at the low field, but weak at the high field near the irreversibility field. These results suggest that the c-axis-correlated pinning behavior by the edge dislocations appears in the high field above the matching field of the nano-rods. However, the irreversibility field improved with the increase of BZO nano-rods densities in comparison with that of the pure Er123 film, although the edge dislocation pinning becomes effective near the irreversibility field. This discrepancy may be understood by the additional effect of the edge dislocation to the nano-rods pinning. Since the pinning force through the elastic interaction of the vortices is dominant in the high field region above a matching field, the additional c-axis-correlated pinning such as the edge dislocations can assist to the whole flux pinning properties.
Fig. 7. Schematic vortex state diagram at B||c-axis proposed in this study for (a) 1.5BZO-doped Er123 film, (b) 3.5BZO-doped Er123 film. BU is the matching field of the BZO nano-rods. The grayscale corresponds to legends of peak height of Jc(h) in Fig. 6.
4. Summary We explored the c-axis-correlated pinning behavior by the TEM microstructure, the transport resistivity and the critical current density Jc as a function of magnetic field, temperature and field angles for the BZO-doped Er123 films. The densities of the BZO nanorods were increase, but the diameters of the BZO nano-rods were almost same with the increase of BZO additions in the target. The BZO nano-rods aligned with increasing the BZO nano-rods density. From the detailed measurement of the dips of the angular dependence of the resistivity and the peaks of the angular dependence of the critical current density at h = 0°, we found that the c-axiscorrelated pinning enhanced in the low field region but decreased in the high field region with the increase of the BZO nano-rods density in the both vortex liquid and solid state. It is considered that
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the edge dislocations influence as the additional c-axis-correlated pinning behavior in the high field region. Acknowledgments This work was supported by a Grant-in-Aid for JSPS Fellows (20 6749). 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. [2] 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. [3] M. Namba, S. Awaji, K. Watanabe, T. Nojima, S. Okayasu, Physica C 468 (2008) 1652.
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[4] S. Awaji, M. Namba, K. Watanabe, J. Phys. Conf. Ser. 51 (2006) 271. [5] S. Awaji, M. Namba, K. Watanabe, M. Miura, Y. Yoshida, Y. Ichino, Y. Takai, K. Matsumoto, Appl. Phys. Lett. 90 (2007) 122501. [6] M. Namba, S. Awaji, K. Watanabe, T. Nojima, S. Okayasu, M. Miura, Y. Ichino, Y. Yoshida, Y. Takai, T. Horide, P. Mele, K. Matsumoto, APEX 1 (2008) 031703. [7] 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 (2004) L1623. [8] K. Yamada, A. Ichinose, Y. Shingai, K. Matsumoto, Y. Yoshida, S. Horii, R. Kita, S. Toh, K. Kaneko, N. Mori, M. Mukaida, Physica C 445-448 (2006) 660. [9] S. Awaji, M. Namba, K. Watanabe, M. Miura, Y. Ichinino, Y. Yoshida, Y. Takai, K. Matsumoto, IEEE Trans. Appl. Supercond. 17 (2007) 3656. [10] M. Peurla, H. Huhtinen, M.A. Shakhov, K. Traito, Yu.P. Stepanov, M. Safonchik, P. Paturi, Y.Y. Tse, R. Palai, R. Laiho, Phys. Rev. B 75 (2007) 184524. [11] L. Civale, B. Maiorov, J.L. MacManus-Driscoll, H. Wang, T.G. Holesinger, S.R. Foltyn, A. Serquis, R.N. Arendt, IEEE Trans. Appl. Supercond. 15 (2005) 2808. [12] M. Tachiki, S. Takahashi, Solid State Commun. 70 (1989) 291.