The Nature of Surface Oxides on Magnesium Diboride
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The Nature of Surface Oxides on Magnesium Diboride Chandana Meegoda, Yu. Paderno1, Michael Trenary Department of Chemistry, University of Illinois at Chicago, 845 W. Taylor Street, Chicago, IL 60607, USA. 1 Institute for Problems of Materials Science, Academy of Sciences of Ukraine, Kiev 252680, Ukraine. ABSTRACT Surface oxides present on polycrystalline MgB2 were characterized by high-resolution x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). X-ray diffraction (XRD) measurements were used to determine the MgB2 phases. XRD line broadening analysis reveals a grain size of 40 nm. XPS results show that MgO and B2O3 are the major surface oxides. Auger spectra provide further evidence of the presence of MgO. The B 1s and Mg 2p peaks have been used to quantify the amount of the surface oxides. INTRODUCTION The recent discovery of superconductivity at 39 K in magnesium diboride (MgB2) by Akimitsu [1] and co-workers has stimulated a great deal of interest in this compound. MgB2 possesses a simple hexagonal AlB2 type structure and has the highest Tc for a non-oxide and non-C60-based compound. In contrast to high temperature superconductors (HTSC), MgB2 has shown lower anisotropy, large coherence lengths, and transparency of the grain boundary to current flow. Isotope effect studies have indicated phonon mediated BCS-type superconductivity [2]. The large critical current density (Jc~ 106 A/cm2) demonstrated by this compound offers the promise of future practical applications [3]. Since its discovery, MgB2 has been synthesized as powder, wires, sub millimeter single crystals, and thin films [4, 5, 6]. Even MgB2 nanowires have been formed in a very recent study [7]. In spite of the tremendous research activity in this field, little attention has been directed towards the properties of MgB2 surfaces. Point contact measurements have indicated that the superconductivity may be weakened in surface layers [8]. Thus, it is fundamentally important to carefully characterize the surface and to understand the surface chemistry of this new class of superconductor. This study is aimed at exploring surface oxides formed on MgB2 when it is exposed to ambient atmosphere. The surfaces were characterized with x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy. EXPERIMENTAL The polycrystalline MgB2 samples used for our study were in the form of discs with a diameter of approximately 8 mm. The samples used in our study have a critical temperature Tc of 39 K as revealed by magnetic susceptibility measurements. They were ultrasonically cleaned with methylene chloride and methanol before doing UHV studies. Even though these samples were not mirror polished, their surfaces were relatively scratch free. Phase analysis with XRD was performed on a Siemens automated powder diffractometer using Cu Kα radiation. XPS data were collected using a high-resolution monochromatic Al-Kα x-ray source (15 keV, 25 mA emission current, VSW MX10 with 700 mm Rowland circle monochromator) and a 150 mm E5.11.1
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