Synthesis, Structures, and Physical Properties of Yttrium-Doped Strontium Manganese Oxide Films
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Synthesis, Structures, and Physical Properties of Yttrium-Doped Strontium Manganese Oxide Films Andrew J. Francis and Paul A. Salvador Carnegie Mellon University, Department of Materials Science and Engineering, Pittsburgh, PA, 15213-3890 ABSTRACT Cubic strontium manganese oxide is an end-member of the colossal magnetoresistive (CMR) family of manganese-based perovskites, Ln1-xAExMnO3. Because normal synthesis conditions lead to the 4-H hexagonal polymorph, high-pressure conditions are typically used to obtain the cubic perovskite polymorph. In this work, we describe the synthesis and structural/physical characterization of the cubic perovskite form of the high-alkaline-earth containing phases of Y1-xSrxMnO3 (x ≥ 0.7) as epitaxial thin films. Thin films of various stoichiometries were grown on single-crystal perovskite substrates SrTiO3, NdGaO3, and LaAlO3 using pulsed laser deposition. After optimizing deposition conditions, the perovskite polymorph is obtained using PLD at 800ºC and 10-100 mTorr O2 for x=1, 0.9, 0.8, and 0.7, as demonstrated by x-ray diffraction. Epitaxial growth was determined to be cube-on-cube. Electrical property measurements demonstrated insulating behavior and no metal-insulator transition or magnetoresistive behavior, similar to related stable compounds. INTRODUCTION Thin film deposition has been widely studied in recent years as a synthetic avenue for the development of materials having interesting properties such as superconductivity and magnetoresistance [1]. Thin film vapor deposition techniques, like chemical vapor deposition (CVD) or physical vapor deposition (PVD), transfer material to a substrate as gas or vapor phase. This changes greatly the kinetic pathways and thermodynamic states available to a given system, as compared to the bulk material, and allows for the synthesis of thin films as metastable phases, often at reduced temperatures and pressures [1]. The colossal magnetoresistive (CMR) materials, Ln1-xAExMnO3, have been well characterized owing to their interesting and complex magnetotransport properties [2]. Materials in this chemical family typically exist in the bulk as one of three general polymorphs: a nonclose packed hexagonal phase, a close packed hexagonal perovskite, and a close packed cubic perovskite. For compositions with small Ln cations and large AE cations, the pseudocubic perovskite phase is metastable with respect to the hexagonal phases, and high-pressure or multistep processing is required to obtain it [3-5]. As a consequence, materials such as those in the Y1-xSrxMnO3 system, for which the cubic perovskite is metastable (for x≥0.7 and x≤0.3), have not been widely studied as cubic perovskites, the basic structure of CMR materials. The work here investigates thin film processing as a possible route for stabilizing the metastable perovskite phase of compounds in this system. It is well known that the chemical nature of the Ln and AE cations has been observed to influence the electronic/magnetic interactions owing to three effects: the overall charge state, t
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