Transmission electron microscope study of

RuSr₂Gd_1.5Ce_0.5Cu₂O_10-d magneto-superconductor

Tadahiro Yokosawa, Veer Pal Singh Awana, Koji Kimoto, Eiji Takayama-Muromachi,

Maarit Karppinen¹, Hisao Yamauchi¹ and Yoshio Matsui

National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan; ¹Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama, 226-8503, Japan (MATSUI.Yoshio@nims.go.jp)

Coexistence of superconductivity and magnetism in RuSr₂(Gd,Sm,Eu)_1.6Ce_0.4Cu₂O_10-d (Ru-1222) (1) and RuSr₂GdCu₂O₈ (Ru-1212) (2) have been of tremendous interest. It was revealed by powder synchrotron X-ray and neutron diffraction that the basal and apical oxygen sites of the RuO₆ octahedra in Ru-1222 and Ru-1212 are refined as split sites, and are considered to be due to rotations (~ 13-14 degree) of the RuO₆ octahedra around the c axis. In this study, we have investigated the microstructure of RuSr₂Gd_1.5Ce_0.5Cu₂O_10-d (Ru-1222) (3), by SAED, dark-field electron microscopy, convergent-beam electron diffraction (CBED) and high-resolution TEM (4). The RuSr₂Gd_1.5Ce_0.5Cu₂O_10-d (Ru-1222) sample was synthesized through a solid-state reaction route from RuO₂, SrO₂, Gd₂O₃, CeO₂ and CuO. The block of specimen was crushed and dispersed on a carbon thin film on a Cu grid for transmission electron microscopy. The SAED and CBED patterns and conventional dark-field images were taken at room temperature using transmission electron microscopes (Hitachi: HF-3000S and HF-3000L) operated at an accelerating voltage of 300 kV. The SAED and CBED patterns were taken from specimen areas of about 100 and 8 nm, respectively. The HRTEM images were taken by high-resolution high-voltage TEM (H-1500), with 0.14nm resolution at 800kV. Figure 1(a) shows a CBED pattern taken with [310] incidence. Sharp superlattice reflections without diffuse streaks are observed at l = 2n as indicated by a white arrowhead. Figure 1 (b) shows a [310] CBED pattern taken from a different illumination area than that of Fig. 1(a). It should be noted that sharp superlattice reflections without diffuse streaks are observed at l = 2n+1 as indicated by a white arrowhead. Figures 2(a) and 2(b) show dark-field images by using the superlattice reflections at l = 2n = 4 and l = 2n+1= 3, respectively as indicated by the white arrowheads in Figs. 1(a) and 1(b). The superlattice domains are clearly seen as many bright striated areas of about 10 nm in width as indicated by white arrowheads. We constructed a model, on the basis of ordering of rotated RuO₆ octahedra about the c axis, with A- and B-centered orthorhombic superlattices (A and B superlattices) that correspond to those of the observed superlattice domains. It should be noted that the A and B superlattices are crystallographically identical to each other, being mutually related by 90 degree rotation about the c axis (the space group of A superlattice: Aeam). Similar results to those of Ru-1222 have also been confirmed in Ru-1212.

References

[1] I. Felner et al.:Phys. Rev. B 55, R3374 (1997)

[2] C. Bernhard et al.:Phys. Rev. B 59, 14099 (1999)

[3] V.P.S. Awana et al.:Physica C 249-254 378 (2002)

[4] T. Yokosawa et al.: J. Electron Microsc. In press.