Superconducting Properties of BaHfO3-doped Nd1+xBa2-xCu3Oy Films Prepared by Alternating-targets Technique

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ScienceDirect Physics Procedia 58 (2014) 154 – 157

26th International Symposium on Superconductivity, ISS 2013

Superconducting properties of BaHfO3-doped Nd1+xBa2-xCu3Oy films prepared by alternating-targets technique Y. Sawanoa, Y. Yoshidaa, *, Y. Ichinoa, K. Matsumotob, S. Awajic a Department of Energy Engineering and Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Department of Materials Science and Engineering, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu 804-8550, Japan c High Field Laboratory for Superconducting Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan

b

Abstract In order to clarify the influence of RE/Ba substitution on superconducting properties on REBCO films in magnetic fields, we fabricated BHO-doped Nd1+xBa2-xCu3Oy films in which Nd composition were controlled by PLD method using alternating-targets technique. The Tc of NdBCO(x = 0.01, 0.08) films were about 92 K. For the field applied parallel to the c-axis, a Jc and a characteristic magnetic field of the maximum pinning force of the NdBCO(x = 0.08) film increased compared with those of the NdBCO(x = 0.01) film. Additionally, a Jc of a BHO-doped NdBCO(x = 0.08) film improved for all applied angles of magnetic field in 1 T. The forms of Jc-θ curves for the BHO-NdBCO films with x = 0.01 and 0.08 were similar form. Therefore, it is considered that the reason of Jc increase in the BHO-doped NdBCO(x = 0.08) film is introduction of three-dimensional pinning centers of fine Nd/Ba solid solutions and the solid solution do not influence on the BHO growth. © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of the ISS 2013 Program Committee. Peer-review under responsibility of the ISS 2013 Program Committee Keywords: Nd1+xBa2-xCu3Oy ; BaHfO3 ; Nd/Ba substitution ; pulsed laser deposition ; thin film ; critical current density

1. Introduction In order to apply a REBa2Cu3Oy(REBCO, RE : rare earth elements) thin film in high magnetic field applications such as magnetic resonance imaging (MRI), superconducting magnetic energy storage (SMES) and nuclear fusion reactor, it is necessary that the REBCO thin film have a high critical current density (Jc) in magnetic fields. For the

* Corresponding author. Tel.: +81-52-789-5417; fax: +81-52-789-5418. E-mail address: [email protected]

1875-3892 © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the ISS 2013 Program Committee doi:10.1016/j.phpro.2014.09.031

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Y. Sawano et al. / Physics Procedia 58 (2014) 154 – 157 pure NdBCO x = 0.01 x = 0.08

pure-NdBCO BHO-doped NdBCO

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NdBCO+BHO [2.6 vol.%] x = 0.01 x = 0.08

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Fig.1 Nd composition of NdBa2Cu3Oy system dependence of the Tc

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Fig.2 Magnetic fields dependence of Jc for the fields applied parallel to the c-axis of the NdBCO films

improvement of the critical current density under magnetic fields, an introduction of artificial pinning centers (APCs) into the REBCO thin film is very effective. One of the APCs is BaMO3 (BMO, M=Hf, Zn, Sn). It is well known that the BMO grows up into a form of nanorod along the c-axis direction of a REBCO thin film. Therefore it shows a strong pinning force in magnetic fields applied parallel to the c-axis direction of the REBCO thin films. In particular, it has been reported that BaHfO3 (BHO)-doped SmBa2Cu3Oy have strong pinning force not only at 77 K but also at 40 K and 20 K [1]. Additionally, fine RE/Ba solid solution also acts as APC. When RE3+ and Ba2+ ions have similar sizes, RE/Ba solid solutions are easily formed. The size of Nd3+ ion is closer to that of Ba2+ ion among RE3+ ions [2]. The RE/Ba substitution regions with a form of fine particle show an isotropic pinning force in magnetic field for all applied angle. For high magnetic field application, reduction of the anisotropy of Jc should be effective. In this study, we fabricated the BHO-doped Nd1+xBa2-xCu3Oy (NdBCO) thin films on LaAlO3 (LAO) substrates with various Nd compositions in order to improve the superconducting properties in magnetic fields and clarify the effect of Nd/Ba substitution. 2. Experimental procedure We deposited the NdBCO thin films on LAO substrates by pulsed laser deposition (PLD) method with a KrF excimer laser (λ=248nm). The substrate temperature was fixed to 940˚C. The laser energy density was 2.0 J/cm2 and repetition frequency is 10 Hz. The distance between target and substrate was 70 mm. The partial pressure of oxygen was 0.8 Torr during the deposition. We used alternating-targets (ALT) technique to introduce BHO into NdBCO. In the ALT technique, two or more targets are repeatedly exchanged to obtain a desired composition. We fixed the BHO content was approximately 2.6 vol.%. We prepared some targets with various Nd compositions to control the Nd composition in the NdBCO films. The thickness of the NdBCO films fixed to 130-160 nm. The crystal structure and the crystallinity of the NdBCO films were analyzed by X-ray θ-2θ diffraction. The thickness and the atomic composition of the films were analyzed by inductively coupled plasma emission spectrometry. The Jc and Tc values were measured by DC four-probe method with a physical property measurement system (PPMS). 3. Results and discussion All the pure-NdBCO films and the BHO-doped NdBCO films showed c-axis orientation and cube-on-cube inplane texture. Fig. 1 shows the Nd composition of NdBa2Cu3O system dependence of the Tc. Tc of the pureNdBCO(x = 0.01, 0.08) films and the BHO-doped NdBCO(x = 0.01, 0.08) films were 90 K or higher. In contrast, Tc of the pure-NdBCO(x =0.12) film and the BHO-doped NdBCO(x = 0.12) film were low. Similar tendency have been reported in a bulk of pure-NdBCO against Nd/Ba substitution [2]. Therefore, the excessive Nd/Ba substitution decreased the Tc of the NdBCO(x = 0.12) films. In general, Tcs of BMO-doped REBCO films were lower than that of the pure-REBCO [3,4]. From Fig. 1, however, the Tcs of the BHO-doped NdBCO films was almost similar to those of the pure-NdBCO films in each Nd composition of NdBa2Cu3Oy system. A few groups have been reported that the Tc of the BMO-doped REBCO film is

Y. Sawano et al. / Physics Procedia 58 (2014) 154 – 157

1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -20

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䚹 degree [ ] degree [ ] Fig. 3 Magnetic field angular dependence of Jc for (a)BHO-doped NdBCO(x = 0.01, x = 0.08) films and (b)pureNdBCO(x = 0.01, 0.08) films

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Fig. 4 AFM images of surface morphology of (a) pure-NdBCO(x = 0.01) film, (b) pure-NdBCO(x = 0.08) film, (c) BHO-doped NdBCO(x = 0.01) film and (d) BHO-doped NdBCO(x = 0.08) film. similar to that of the pure-REBCO film [1,5] and these reports are similar to our result. It is not clarified the reason why the Tc of the BHO-doped NdBCO film did not decrease, yet. From Fig. 1, the result suggests only that the Nd composition is not related to the decrease of Tc for the BHO-doped NdBCO films. Fig. 2 shows magnetic field dependences of Jcs in the pure-NdBCO(x = 0.01, 0.08) and the BHO-doped NdBCO(x = 0.01, 0.08) films for the field applied parallel to the c-axis of the films. The Jcs of the NdBCO(x = 0.08) film were higher than that of the NdBCO(x = 0.01) films in all magnetic fields. The maximum pinning force (FpMAX) of the BHO-doped NdBCO(x = 0.08) film reached 24.0 GN/m3 at 1 T and that of the BHO-doped NdBCO(x = 0.01) film was 14.5 GN/m3 in 1.5 T. As the reason of these increase, Nd/Ba substitution would affect as pinning centers in the BHO-doped NdBCO(x = 0.08) film. Fig.3(a) shows the magnetic field angular dependence of Jc for the BHO-doped NdBCO(x = 0.01, x = 0.08) films. The Jc of the films significantly increased in B//c. This means that the BHO in the films grew up into a form of nanorod. Values of Jc(B//c)/Jc(B//ab) and the full width at half maximum (FWHM) of the BHO-doped NdBCO films were roughly equivalent in 1 T. The value of the Jc(B//c)/Jc(B//ab) and the FWHM are approximately 2 and 48ǀ, respectively. Namely, there is no difference between the forms of Jc-θ curves of the BHO-doped NdBCO(x = 0.01) film and that of the BHO-doped NdBCO(x = 0.08). Therefore, Nd composition could not affect the form of the BHO nanorod. On the other hand, the Jc of the BHO-doped NdBCO(x = 0.08) film increased for all angles of applied magnetic field compared with that of the BHO-doped NdBCO(x = 0.01) film in 1 T. This reason is considered that Nd/Ba substitution acts as three-dimensional pinning centers in the BHO-doped NdBCO(x = 0.08) film. The minimum Jc in the Jc-θ of the BHO-doped NdBCO(x = 0.08) in 1 T was 0.5 MA/cm2 and the value was higher than that of the BHO-doped NdBCO(x = 0.01) of 0.25 MA/cm2. In contrast, in 5 T, no increase of the Jc was observed. Nd/Ba did not act as three-dimensional pinning centers in 5 T and there is no defference between the forms of Jc-θ curves of the BHO-doped NdBCO films. We think that number of the Nd/Ba substitutions is small. Fig.3(b) shows the magnetic field angular dependence of Jc for the pure-NdBCO(x = 0.01, 0.08) film. The Jc of the pure-NdBCO(x = 0.08) film was higher for all angles of applied magnetic field than that of the pure-NdBCO(x =

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0.01) film such as the BHO-doped NdBCO film. Particularly, the Jc(B//c) of the pure-NdBCO(x = 0.08) films was superior to that of the pure-NdBCO(x = 0.01) film. For this behavior, there are two reasons. The first possible reason is changes in the number of the dislocation in the NdBCO film. Figs. 4(a) and (b) show AFM images of surface morphology of the pure-NdBCO(x = 0.01, 0.08) films. Spiral growth was observed in these films. However, the number of the spiral dislocations in the pure-NdBCO(x = 0.01) film was smaller than that of the pure-NdBCO(x = 0.08) film. Therefore, there was no increase of the Jc in B//c in the pure-NdBCO(x = 0.01) film. The second possible reason is Nd/Ba substitution. It is reported that Nd/Ba substitution affect as c-axis correlated pinning centers in NdBCO bulks and NdBCO films on RABiTS [6,7]. However, the number of the dislocations of the BHO-NdBCO(x = 0.01) film is similar to that of the BHONdBCO(x = 0.08) film from Figs. 4 (c) and (d). These results suggest that the difference of the number of the dislocations between the films was disappeared by doping the BHO. Thus, it is considered that there was no difference for the Jc(B//c)/Jc(B//ab) of the BHO-doped NdBCO(x = 0.01, 0.08) films. 4. Conclusion We fabricated the pure-NdBCO films and the BHO-doped NdBCO films with various Nd/Ba compositions on LAO substrates by alternating-targets technique and clarified the effect of Nd/Ba substitution. Jc of the BHO-doped NdBCO(x = 0.08) film improved compared with that of the BHO-doped NdBCO(x = 0.01) films in all magnetic field applied parallel to the c-axis of the film and Jc-θ curve also improved for all applied angles of magnetic field in 1 T. From this result, Nd/Ba substitution affects as three-dimensional pinning centers in the BHO-doped NdBCO(x = 0.08) film. In addition, there is no different between the forms of the Jc-θ curves for the BHO-doped NdBCO(x = 0.01, x = 0.08) films. From this behavior, we speculate that the Nd/Ba substitution give no effect for the form of BHO nanorods. Tcs of the BHO-doped NdBCO films were almost similar to those of the pure-NdBCO films in each Nd composition of NdBa2Cu3Oy system. The reason why the Tc of the BHO-doped NdBCO films did not decrease compared with pure-NdBCO films is not clarified yet. From Nd composition dependence of the Tc, the result suggests only that the Nd composition is not related to the decrease of Tc for the BHO-doped NdBCO films. Acknowledgement This work was partly supported by a Grant-in Aid for Scientific Research (23226014, 19676005, 252889358). References [1] A. Tsuruta, Y. Yoshida, Y. Ichino, A. Ichinose, K. Matsumoto, S. Awaji. Flux pinning properties at low temperatures in BaHfO3 doped SmBa2Cu3Oy films. IEEE Trans. Appl. Supercond. 2013; 23; 8001104 [2] M. Murakami, N. Sakai, T. Higuchi, S. I. Yoo. Melt-processed light rare earth element-Ba-Cu-O. Supercond. Sci. Techonol. 1996; 9; 1015-1032 [3] A. Ichinose, K. Naoe, T. Horide, K. Matsumoto, R. Kita, M. Mukaida, et al. Microstructures and critical current densities of YBCO films films containing structure-controlled BaZrO3 nanorods. Supercond. Sci. Technol. 2007; 20; 1144 [4] A. Tsuruta, Y. Yoshida, Y. Ichino, A. Ichinose, K. Matsumoto, S. Awaji. Flux pinning properties and microstructures of multilayered films consisting of Sm1.04Ba1.96Cu3Oy layers and BaSnO3-doped Sm1.04Ba1.96Cu3Oy layers. Jpn. J. Appl. Phys. 2013; 52; 010201 [5] A. O. Ijaduola, S. H. Wee, A. Goyal, P. M. Martin, F. Li, J. R. Thompson, et al. Critical currents, magnetic relaxation and pinning in NdBa2Cu3O7-δ films with BaZrO3-generated columnar defects. Supercond. Sci. Techonol. 2012; 25; 045013 [6] T. Egi, J. G. Wen, K. Kuroda, H. Unoki, N. Koshizuka. High critical-current density of Nd(Ba,Nd)2Cu3O7-δ. Appl. Phys. Lett. 1995; 67; 2406-2408 [7] S. H. Wee, A. Goyal, J. Li, Y. L. Zuev, S. Cook, L. Heatherly. The incorporation of nanoscale columnar defects comprised of self-assembled BaZrO3 nanodots to improve the flux pinning and critical current density of NdBa2Cu3O7-δ films grown on RABiTS. Supercond. Sci. Technol. 2007; 20; 789-793