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Anisotropic properties of RuSr2GdCu2O8 thin films grown on SrTiO3(1 0 0) and (1 1 0) substrates
Physica C 388–389 (2003) 437–438 www.elsevier.com/locate/physc
Anisotropic properties of RuSr2GdCu2O8 thin films grown on SrTiO3(1 0 0) and (1 1 0) substrates Daisuke Serita, Takahiro Kato *, Kazuki Yamanaka, Yuh Yamada, Shugo Kubo Department of Materials Science, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan
Abstract For the examination of physical property dependence on the crystalline orientation direction in RuSr2 GdCu2 O8 (Ru1212), in which superconductivity and a magnetic order coexsit, we have tried crystalline-orientation-controlled growth of Ru1212 thin films on SrTiO3 (STO)(1 0 0) and (1 1 0) substrates. Ru1212(0 0 1) thin films have grown epitaxially on STO(1 0 0) substrates, whereas Ru1212(1 1 0) or (1 0 3) films grown on STO(1 1 0) substrates. In the latter growth, we have confirmed that Ru1212(1 0 3) phase exists using four-circle X-ray diffractometry. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Oxide superconductor; Thin film; Epitaxial growth
1. Introduction RuSr2 GdCu2 O8 (Ru1212) synthesized by Bauernfeined et al. for the first time has a similar crystal structure to that of REBa2 Cu3 O7d (RE123, RE: rare earth element), which is constructed by triple-layered perovskite cells [1]. In the Ru1212 crystal structure, RE and Ba sites in the RE123 structure are replaced by Gd and Sr respectively, and CuO chains in RE123 are modified to RuO6 octahedrons. Ru1212 is very interesting material because of the coexistence of superconductivity and magnetic order at high temperatures [2]. Physical properties as well as crystal structure have been examined, however, using powder samples, owing to the difficulty of preparing single crystal samples phase diagrammatically. It is known that RE123 thin film growth directions can be controlled by substrate materials and their directions and film growth conditions [3–5], because RE123 unit cell is constructed basically by a simple perovskite structure. We consider therefore that Ru1212 thin film growth can also be controlled by epitaxy. Here
*
Corresponding author. E-mail address: [email protected] (T. Kato).
we report the results on epitaxal growth of Ru1212 thin films for the purpose of examining physical property dependence on the crystalline orientation.
2. Experimental Ru1212 thin films were deposited using a reactive evaporation technique. The apparatus used for deposition is equipped with two electron beam guns and effusion cells. It is necessary that Ru, Gd and Cu metals are evaporated using electron beam guns owing to their vapor pressure characteristics. Consequently Ru metal and GdCu alloy sources were used in this experiment. The alloy source composition of Gd4:6 Cu gives film Gd:Cu stoichiometry. Sr was evaporated using an effusion cell. 5% ozonized oxygen gas was injected into the chamber as an oxidant. The ozonized oxygen pressure was about 1 104 Torr during the deposition process. Substrates used were SrTiO3 (STO), which has a good lattice matching with Ru1212. Two kinds of STO substrate crystal orientation were used, i.e., (1 0 0) and (1 1 0) substrates. The substrate temperature was fixed at 900 °C. The deposition rate and film thickness were /s and 1000 A , respectively. Some samples about 1 A were post-annealed in oxygen gas, nitrogen gas or air
0921-4534/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(02)02564-9
STO(220) (206) or(220)
STO(110)
atmosphere for the purpose of improving the film crystallinity. X-ray diffraction (XRD) analysis was performed to verify the quality of epitaxy and to determine phases in the films and lattice constants. Four-circle X-ray diffractometry was used to identify the film growth crystalline direction, particularly for films on STO(1 1 0) substrates.
(103) or(1 10)
D. Serita et al. / Physica C 388–389 (2003) 437–438
Intensity(a.u.)
438
3. Results and discussion 2 (degree)
Ru1212 thin films deposited on STO(1 0 0) substrates grow in the c-axis orientation, as is shown in the top XRD pattern (‘‘as-grown’’) of Fig. 1. Some (0 0 n) diffraction lines, however, did not appear in as-grown thin films. This is assumed to result from insufficient substrate temperature of 900 °C and unoptimized oxygen gas pressure during deposition. As-grown Ru1212 thin film were, therefore, post-annealed and structural changes in thin films were examined in these reduction and oxidation atmospheres. Post-annealing conditions were 1000 °C and 2 h in each atmosphere. The lattice structure change in air atmosphere observed by XRD is shown in the bottom of Fig. 1. In post-annealed thin films in oxygen and nitrogen gas atmospheres, each diffraction line becomes slightly clear as compared with that of the as-grown one, whereas in post-annealing in air atmosphere all (0 0 n) diffraction lines (n ¼ 1–7) can be seen clearly. In addition, each diffraction line of the post-annealed thin film in air atmosphere shifts to higher angle position, which means that the c-axis lattice constant becomes shortened and the structure of the airannealed sample becomes close to that of the Ru1212 bulk sample. The structure of the thin films annealed in nitrogen gas, which is a reduction atmosphere, showed a small change and that annealed in air was improved largely, suggesting the presence of a very small amount
Fig. 2. XRD pattern for a film on STO(1 1 0).
of oxygen effectively determines the lattice structure of Ru1212 thin films. Fig. 2 shows the XRD pattern of Ru1212 thin film epitaxially grown on an STO(1 1 0) substrate. Here two possibilities are present for indexing the diffraction lines; one is (1 1 0) and the other is (1 0 3). For the determination of the orientation using four-circle X-ray diffractometry, assuming the diffraction line to be (1 0 3), the thin film sample surface was inclined by 45° along the v-axis and the 2h=h angle was set at the position from the (0 0 6) diffraction line. As a result of the inplane rotation scan (/ scan), the Ru1212 thin film growth direction on an STO(1 1 0) substrate is concluded to be the (1 0 3) orientation. The temperature dependence of electrical resistivity for the Ru1212 thin films prepared in this experiment is similar to that of bulk Ru1212. The resistivity values for the thin films in this experiment are, however, two order of magnitude large than those of bulk samples, and superconductivity has not been obtained in Ru1212 thin film samples at present. In summary, for the demonstration of the possibility of oriented growth in Ru1212 thin films, we have tried Ru1212 deposition on STO(1 0 0) and (1 1 0) substrates. Ru1212(0 0 1) and (1 0 3) films are proved to grow epitaxially on STO(1 0 0) and (1 1 0) substrates, respectively.
References
Fig. 1. XRD patterns for as-grown and air-annealed films on STO(1 0 0).
[1] L. Bauernfeind, W. Widder, H.F. Braun, Physica C 254 (1995) 151. [2] C. Bernhard, J.L. Tallon, Ch. Niedermayer, Th. Blasius, A. Stronach, E.J. Ansaldo, Phys. Rev. B 59 (1999) 14099. [3] Y. Enomoto, T. Murakami, M. Suzuki, K. Moriwaki, Jpn. J. Appl. Phys. 26 (1987) L1248. [4] S. Kubo, Y. Enomoto, T. Iwata, in: K. Kajimura, H. Hayakawa (Eds.), Advances in Superconductivity III, Springer, Tokyo, 1991, p. 559. [5] M. Mukaida, S. Miyazawa, Jpn. J. Appl. Phys. 31 (1992) 3317.
Anisotropic properties of RuSr2GdCu2O8 thin films grown on SrTiO3(1 0 0) and (1 1 0) substrates Daisuke Serita, Takahiro Kato *, Kazuki Yamanaka, Yuh Yamada, Shugo Kubo Department of Materials Science, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan
Abstract For the examination of physical property dependence on the crystalline orientation direction in RuSr2 GdCu2 O8 (Ru1212), in which superconductivity and a magnetic order coexsit, we have tried crystalline-orientation-controlled growth of Ru1212 thin films on SrTiO3 (STO)(1 0 0) and (1 1 0) substrates. Ru1212(0 0 1) thin films have grown epitaxially on STO(1 0 0) substrates, whereas Ru1212(1 1 0) or (1 0 3) films grown on STO(1 1 0) substrates. In the latter growth, we have confirmed that Ru1212(1 0 3) phase exists using four-circle X-ray diffractometry. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Oxide superconductor; Thin film; Epitaxial growth
1. Introduction RuSr2 GdCu2 O8 (Ru1212) synthesized by Bauernfeined et al. for the first time has a similar crystal structure to that of REBa2 Cu3 O7d (RE123, RE: rare earth element), which is constructed by triple-layered perovskite cells [1]. In the Ru1212 crystal structure, RE and Ba sites in the RE123 structure are replaced by Gd and Sr respectively, and CuO chains in RE123 are modified to RuO6 octahedrons. Ru1212 is very interesting material because of the coexistence of superconductivity and magnetic order at high temperatures [2]. Physical properties as well as crystal structure have been examined, however, using powder samples, owing to the difficulty of preparing single crystal samples phase diagrammatically. It is known that RE123 thin film growth directions can be controlled by substrate materials and their directions and film growth conditions [3–5], because RE123 unit cell is constructed basically by a simple perovskite structure. We consider therefore that Ru1212 thin film growth can also be controlled by epitaxy. Here
*
Corresponding author. E-mail address: [email protected] (T. Kato).
we report the results on epitaxal growth of Ru1212 thin films for the purpose of examining physical property dependence on the crystalline orientation.
2. Experimental Ru1212 thin films were deposited using a reactive evaporation technique. The apparatus used for deposition is equipped with two electron beam guns and effusion cells. It is necessary that Ru, Gd and Cu metals are evaporated using electron beam guns owing to their vapor pressure characteristics. Consequently Ru metal and GdCu alloy sources were used in this experiment. The alloy source composition of Gd4:6 Cu gives film Gd:Cu stoichiometry. Sr was evaporated using an effusion cell. 5% ozonized oxygen gas was injected into the chamber as an oxidant. The ozonized oxygen pressure was about 1 104 Torr during the deposition process. Substrates used were SrTiO3 (STO), which has a good lattice matching with Ru1212. Two kinds of STO substrate crystal orientation were used, i.e., (1 0 0) and (1 1 0) substrates. The substrate temperature was fixed at 900 °C. The deposition rate and film thickness were /s and 1000 A , respectively. Some samples about 1 A were post-annealed in oxygen gas, nitrogen gas or air
0921-4534/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(02)02564-9
STO(220) (206) or(220)
STO(110)
atmosphere for the purpose of improving the film crystallinity. X-ray diffraction (XRD) analysis was performed to verify the quality of epitaxy and to determine phases in the films and lattice constants. Four-circle X-ray diffractometry was used to identify the film growth crystalline direction, particularly for films on STO(1 1 0) substrates.
(103) or(1 10)
D. Serita et al. / Physica C 388–389 (2003) 437–438
Intensity(a.u.)
438
3. Results and discussion 2 (degree)
Ru1212 thin films deposited on STO(1 0 0) substrates grow in the c-axis orientation, as is shown in the top XRD pattern (‘‘as-grown’’) of Fig. 1. Some (0 0 n) diffraction lines, however, did not appear in as-grown thin films. This is assumed to result from insufficient substrate temperature of 900 °C and unoptimized oxygen gas pressure during deposition. As-grown Ru1212 thin film were, therefore, post-annealed and structural changes in thin films were examined in these reduction and oxidation atmospheres. Post-annealing conditions were 1000 °C and 2 h in each atmosphere. The lattice structure change in air atmosphere observed by XRD is shown in the bottom of Fig. 1. In post-annealed thin films in oxygen and nitrogen gas atmospheres, each diffraction line becomes slightly clear as compared with that of the as-grown one, whereas in post-annealing in air atmosphere all (0 0 n) diffraction lines (n ¼ 1–7) can be seen clearly. In addition, each diffraction line of the post-annealed thin film in air atmosphere shifts to higher angle position, which means that the c-axis lattice constant becomes shortened and the structure of the airannealed sample becomes close to that of the Ru1212 bulk sample. The structure of the thin films annealed in nitrogen gas, which is a reduction atmosphere, showed a small change and that annealed in air was improved largely, suggesting the presence of a very small amount
Fig. 2. XRD pattern for a film on STO(1 1 0).
of oxygen effectively determines the lattice structure of Ru1212 thin films. Fig. 2 shows the XRD pattern of Ru1212 thin film epitaxially grown on an STO(1 1 0) substrate. Here two possibilities are present for indexing the diffraction lines; one is (1 1 0) and the other is (1 0 3). For the determination of the orientation using four-circle X-ray diffractometry, assuming the diffraction line to be (1 0 3), the thin film sample surface was inclined by 45° along the v-axis and the 2h=h angle was set at the position from the (0 0 6) diffraction line. As a result of the inplane rotation scan (/ scan), the Ru1212 thin film growth direction on an STO(1 1 0) substrate is concluded to be the (1 0 3) orientation. The temperature dependence of electrical resistivity for the Ru1212 thin films prepared in this experiment is similar to that of bulk Ru1212. The resistivity values for the thin films in this experiment are, however, two order of magnitude large than those of bulk samples, and superconductivity has not been obtained in Ru1212 thin film samples at present. In summary, for the demonstration of the possibility of oriented growth in Ru1212 thin films, we have tried Ru1212 deposition on STO(1 0 0) and (1 1 0) substrates. Ru1212(0 0 1) and (1 0 3) films are proved to grow epitaxially on STO(1 0 0) and (1 1 0) substrates, respectively.
References
Fig. 1. XRD patterns for as-grown and air-annealed films on STO(1 0 0).
[1] L. Bauernfeind, W. Widder, H.F. Braun, Physica C 254 (1995) 151. [2] C. Bernhard, J.L. Tallon, Ch. Niedermayer, Th. Blasius, A. Stronach, E.J. Ansaldo, Phys. Rev. B 59 (1999) 14099. [3] Y. Enomoto, T. Murakami, M. Suzuki, K. Moriwaki, Jpn. J. Appl. Phys. 26 (1987) L1248. [4] S. Kubo, Y. Enomoto, T. Iwata, in: K. Kajimura, H. Hayakawa (Eds.), Advances in Superconductivity III, Springer, Tokyo, 1991, p. 559. [5] M. Mukaida, S. Miyazawa, Jpn. J. Appl. Phys. 31 (1992) 3317.