Thermal Stability of Defects in Substrates for Multiferroic Materials
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Thermal Stability of Defects in Substrates for Multiferroic Materials Shehnaz Jeddy1, Mary Ellen Zvanut1, Brian Einstein Lassiter2, Gregg M. Janowski3, and Leonard J. Brillson4 1 Physics, University of Alabama at Birmingham, Birmingham, AL, 35294 2 Materials Science and Engineering, University of Idaho, Moscow, ID, 83844 3 Materials Science and Engineering, University of Alabama at Birmingham, Birmingham, AL, 35294 4 Department of Physics and Center for Materials Research, Ohio State University, Columbus, OH, 43210 ABSTRACT Strontium Titanate (STO) substrates were studied by electron paramagnetic resonance (EPR) spectroscopy to assess possible changes incurred by deposition of multiferroic thin films. To this effect, STO was vacuum annealed at pressures of 10-5 Torr for one hour at temperatures in the range of 200 – 500 0 C. EPR spectra, measured before and after each anneal, revealed changes in the amount of three different defects, Cr3+, Fe3+ and an ironoxygen vacancy complex, Fe3+Vo. The latter was used to monitor the diffusion of oxygen. EPR analysis showed that Fe3+Vo increases from its as-grown value, suggesting that a charged oxygen species is mobile in the substrate under film deposition conditions. Coupled with a subsequent O2 anneal showing minimal change in the Fe3+Vo signal, the data indicate a loss of oxygen from the sample during vacuum annealing. As charged oxygen vacancies may affect the substrate as well as the substrate/ thin film interface, these results are important for understanding the behavior of multiferroic devices built on STO substrates. INTRODUCTION Multiferroics (MF) are are materials exhibiting several different properties including ferromagnetism, ferroelectricity, and ferroelasticity. They exhibit great potential for applications which include spintronics and information storage [1]. Bismuth ferrite, a rhombohedral perovskite, bismuth magnetite, a monoclinic perovskite and yittrium magnetite, a hexagonal perovskite are a few examples. Strontium titanate (STO) is a diamagnetic insulator with a band gap of 3.2 eV and a dielectric constant as high as 300. At room temperature STO is a cubic perovskite with titanium at the center of the cube surrounded by six oxygens and strontium at the eight corners.The material is thought to be a good choice as a substrate for multiferroic thin films. When transitional metal impurities, like the Cr and Fe, enter the lattice they substitute isoelectronically for the Ti4+ ions, and when neutral, are referred to by the ionic notation, Fe4+ and Cr4+. However iron can also be associated with a near neighbor oxygen vacancies and form an Fe-Vo complex defect [2, 3]. These impurities often assume different charge states in the lattice and create charged defect centers. Of critical importance to film growth is that oxygen vacancies are known to be mobile at temperatures as low as room temperature [4], and recent investigations have shown that defects affect substrate behavior and impact MF
thin film properties. Our study focuses on the charge sta
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