The Effect of Changing Epitaxial Strain on Colossal Magnetoresistance Thin Films
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The Effect of Changing Epitaxial Strain on Colossal Magnetoresistance Thin Films Darren Dale1, Aaron Fleet2, J.D. Brock2, Y. Suzuki1 1 Department of Materials Science and Engineering, Cornell University, Ithaca NY, 14853 2 School of Applied and Engineering Physics, Cornell University, Ithaca NY, 14853 ABSTRACT We have studied the properties of epitaxial La(1-x)SrxMnO3 (x=0.3, 0.5) epitaxial thin films grown on BaTiO3 and SrTiO3. Significant modifications of properties are observed in magnetization and resistivity versus temperature experiments. These modifications occur at temperatures corresponding to structural phase transitions in the substrate. The x=0.5 composition is particularly sensitive to changes in the epitaxial strain state, exhibiting a direct correlation between changes in strain, magnetization and resistivity. Strain can also be induced in BaTiO3 by the inverse piezoelectric effect, which results in as much as a 13% decrease in the resistivity of the film. INTRODUCTION The observation of “colossal” magnetoresistance (CMR) near the Curie temperature in doped manganese perovskites recently has renewed interest in this class of materials and has led to the study of its complex phase diagram. The structural sensitivity of the magnetic and electronic properties of these CMR materials makes them of fundamental as well as technological interest. In the past five years, many groups have focused on the strain dependence of the magnetic and electronic properties of CMR materials [1-8]. In addition, these materials are now recognized as a prototype condensed matter system for the study of correlated degrees of freedom: charge, spin, lattice, and orbital ordering [9-11]. Doped manganate magnetic properties depend most specifically on the Mn-O-Mn bond angle, rather than the bond length [1, 9]. Jahn-Teller distortions give rise to polarons above Tc, while below Tc the Mn-O bonds are of equal length, although tilting of octahedra results in a non-cubic unit cell [10]. An additional complexity to the ground state of this system of highly correlated electrons is the existence of competing phases in the material [11, 12]. It has been proposed that the interplay of ferromagnetic metallic, antiferromagnetic metallic, paramagnetic insulating, and charge-ordered insulating phases gives rise to a dynamic CMR effect, accounting for the large MR effect when double exchange arguments alone can not [12,13]. The focus of this study is the coupling of a CMR thin film to an externally controlled strain. We have grown La(1-x)SrxMnO3 (LSMO) films on ‹001› SrTiO3 (STO) and BaTiO3 (BTO) substrates via pulsed laser deposition. BTO undergoes numerous phase transitions as a function of temperature [14]. BTO is cubic above 410K, tetragonal above 290K, orthorhombic above 190K (monoclinic in a pseudo-cubic setting) and rhombohedral at lower temperatures. A BTO substrate thus provides a surface lattice that can be dynamically changed, altering the strain state of the epitaxial film. STO is used as a control, although it has a cubic-
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