Synthesis of superconducting FeSr2YCu2Oy thin films

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Abstract

We report the thin-film synthesis of the superconducting FeSr2YCu2Oy (Fe-1212) discovered recently by Shimoyama et al. (Physica C 341–348 (2000) 563). The superconducting phase was obtained by an ex situ three-step process that essentially follows the bulk synthesis process. The resultant Fe-1212 films are c-axis oriented and the lattice constants are c0=11.45 and 11.33 Å before and after oxygen loading, both of which are close to the bulk values. The best film shows Tconset∼40 K, Tcend∼10 K, and ρ(300K)∼3 mΩ cm. Attempts at in situ molecular beam epitaxy growth with atomic oxygen or ozone were unsuccessful since Fe4+, unlike the Fe3+ required, is stable in activated oxygen.

Introduction

FeSr2YCu2Oy (Fe-1212) is a new superconductor that was recently synthesized by Shimoyama et al. and has Tc∼60–70 K [1]. Before their success, it was already known that partial substitution of Fe for Cu stabilizes the so-called “Sr-based 123 phase” and that (Cu1−xFex)Sr2YCu2Oy can be synthesized in a wide compositional range of x between 0.3 and 1. However, superconductivity was obtained only for the limited range of x=0.3–0.4 [2]. The lack of superconductivity for higher Fe concentration was believed to be due to the site exchange between Fe and plane-site Cu (Cu(2)) [3] and/or insufficient hole doping [1]. Shimoyama et al. suppressed the Fe/Cu-site exchange by heat treatment in low PO2 at ∼800 °C after the sintering, and also achieved oxygen loading by subsequent annealing in high PO2 at ∼250 °C with the cation sites preserved. Thus, this three-step process led them to the discovery of superconductivity in the fully Fe-substituted (x=1) phase.

Thin-film synthesis by molecular beam epitaxy (MBE) in low oxygen pressure may have a potential to achieve in situ Fe/Cu ordering. Thin films seem to have another merit for oxygen loading that strong and uniform oxidation can be achieved by ozone [4], [5] due to the large surface area for their volume. Moreover, highly oriented crystal might be obtained due to epitaxial growth on single-crystalline substrates. In this study, we attempted to synthesize the superconducting Fe-1212 films both by in situ and ex situ processes. The in situ growth was unsuccessful at the growth temperature below 730 °C. This is probably because Fe4+ is stable in ozone or atomic oxygen, while Fe3+ is required for the formation of the Fe-1212 structure. Therefore, we prepared the superconducting Fe-1212 films with c-axis orientation via the ex situ process.

Section snippets

Experimental

The ex situ growth of the Fe-1212 films was performed via the following three-step process that essentially follows the bulk synthesis route [1]. First, we prepared the precursors on SrTiO3 (STO) or LaAlO3 substrate by electron beam co-deposition at ambient temperature with the cation ratio controlled [6]. Atomic oxygen was also supplied during the deposition to avoid the precursors reacting with atmospheric H2O and/or CO2 when taken out from the vacuum chamber. Next, we annealed the thin-film

Results and discussion

We describe the high-temperature and low-PO2 annealing step first. Fig. 1 compares X-ray diffraction (XRD) patterns of the films on STO substrates annealed at (a) Ta=770 °C, (b) Ta=795 °C, and (c) Ta=820 °C. All were measured before the final oxygen loading step. Apart from the substrate peaks, only two peaks are recognizable in (a), while several new peaks appear in (b), all of which can be indexed to 0 0 n reflections of Fe-1212. These peaks are also observed in (c), but with much lower

Summary

We prepared superconducting Fe-1212 films with c-axis orientation by a three-step annealing process composed of (1) the precursor preparation, (2) Fe-1212 phase formation and crystallization, and (3) oxygen loading. The lattice constants are c0=11.45 and 11.33 Å before and after oxygen loading, both of which are close to the bulk values. The best film shows Tconset∼40 K, Tcend∼10 K, and ρ(300K)∼3 mΩ cm. Attempts at MBE growth with atomic oxygen or ozone were unsuccessful below 730 °C.

Acknowledgements

The authors are grateful to Prof. J. Shimoyama and Mr. T. Hinouchi of University of Tokyo for helpful suggestions and fruitful discussions. They also acknowledge Dr. H. Sato, Dr. S. Karimoto, and Dr. H. Shibata for stimulating discussions, and Dr. H. Takayanagi and Dr. S. Ishihara for their support and encouragement. H.Y. thanks Mr. H. Ichikawa for assistance in XRD and resistivity measurements.

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