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Title:Heteroepitaxial Interfaces Modify Collective States in Complex Oxides
Author(s):Warusawithana, Maitri Priyadharshana
Department / Program:Physics
Discipline:Physics
Degree:Ph.D.
Genre:Dissertation
Subject(s):Heteropitaxial
oxides
diffraction
reflectivity
rutherford
backscattering
dielectric
kinetics
dipole
Oscillation
rheed
arrhenius
magnetoresistance
interfacial
morphology
Abstract:Complex oxides exhibit a wide range of collective phenomena in isostructural phases. Titanates and manganites are two examples where various members of their groups have close enough lattice constants that strained superlattices involving both kinds of materials grow epitaxially. In this work aggregate response and interface effects of heteroepitaxial digital superlattices (DSLs) with thickness of each constituent approaching the ultimate molecular layer limit is discussed. The samples were assembled by Ozone assisted atomic layer by layer molecular beam epitaxy. In dielectric DSLs consisting of titanate phases - BaTiO3, SrTiO3 and CaTiO3, the sequence of interfaces built into the structure controlled the symmetry of the superlattice. Stacking architectures modified the dielectric and ferroelectric properties of the constituents giving rise to novel material properties. Asymmetric strain fields built into the films via the stacking sequence led to an effective polarizing field, Ep, which polarized the samples at all temperatures. Sensitive pyrocurrent measurements indicated the onset of a hysteretic response below a temperature, Tx. The P(E) loops measured were not only displaced along the E-axis due to Ep, but also displaced along the P-axis below Tx, indicating an offset polarization. Size of the high-T dipole units were found to be ~10 unit cells while further growth of correlations occurred below Tx. In manganite films a collectively ordered surface state was observed along with a sharp surface morphology transition due to a <0.002 offset in stoichiometry between Mn and A-site atoms. Consequences of this slight composition imbalance on electronic transport and magnetic order were discussed under the formalism of the double exchange interaction. By constructing DSLs with LaMnO3, SrMnO3 and CaMnO3, electronic properties in ordered pseudo alloys having average doping, x=⅓ was compared with random alloy La⅔Sr⅓MnO3 samples. Effects due to strain disorder and charge disorder/localization was discussed. Magnetic order of manganite layers near a titanate interfaces was also probed using x-ray magnetic circular dichroism, x-ray resonant magnetic scattering and electronic transport measurements in manganite-titanate DSLs. The measurements indicated a progressive reduction in magnetic order as the interface was approached consistent with the premature loss of magnetoresistance previously observed in magnetic tunnel junctions.
Issue Date:2005
Type:Text
Language:English
URI:http://hdl.handle.net/2142/35201
Rights Information:2005 Warusawithana ©
Date Available in IDEALS:2012-11-11


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