Strain relaxation defects in perovskite oxide superlattices
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Michael D. Biegalski and Hans M. Christen Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Chengyu Song National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California 94720
Craig R. Dearden Department of Chemical Engineering and Materials Science, University of California–Davis, Davis, California 95616
Nigel D. Browninga) Department of Chemical Engineering and Materials Science, University of California–Davis, Davis, California 95616; Department of Molecular and Cellular Biology, University of California–Davis, Davis, California 95616
Yayoi Takamurab) Department of Chemical Engineering and Materials Science, University of California–Davis, Davis, California 95616 (Received 26 September 2011; accepted 24 January 2012)
This paper reports on the defect structures formed upon strain relaxation in pulsed laser-deposited complex oxide superlattices consisting of the ferromagnetic metal, La0.67Sr0.33MnO3, and the antiferromagnetic insulator, La0.67Sr0.33FeO3. Atomic resolution scanning transmission electron microscopy and electron energy loss spectroscopy were used to characterize the structure and chemistry of the defects. For thinner superlattices, strain relaxation occurs through the formation of 2-D stacking faults, whereas for thicker superlattices, the prolonged thermal exposure during film growth leads to the formation of nanoflowers and cracks/pinholes to reduce the overall strain energy. I. INTRODUCTION
Current address: Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352 b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.42
(FM) metal La0.67Sr0.33MnO3 (LSMO) and the antiferromagnetic (AFM) insulator La0.67Sr0.33FeO3 (LSFO) and investigate the interplay between epitaxial strain and the presence of structural defects on their multifunctional properties. LSMO is a candidate material for spintronic and magnetic data storage applications due to its high-degree of spin polarization and colossal magnetoresistant properties. The coincident metal/insulator and FM/paramagnetic transitions originate from the double exchange mechanism which involves hopping of electrons along Mn3+—O2-— Mn4+ chains,7,8 whereas electron-phonon interactions through cooperative Jahn-Teller distortions also play an important role.9 On the other hand, LSFO is a G-type antiferromagnet in which the overlapping Fe 3d and O 2p orbitals influence the AFM superexchange interaction.10,11 Previously, we showed using soft x-ray magnetic spectroscopy and photoemission electron microscopy that the FM and AFM properties of the superlattice depend strongly on the sublayer thickness and that a robust spin-flop coupling occurs at the interfaces of a superlattice composed of six unit cells of LSFO and six unit cells of LSMO, repeated 10 times (referred to as [6LSFO][6LSMO]10.2,12,13 The strength of the spin-flop coupling was observed to overcome the pinning effect of
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