Abstract
This dissertation is devoted to understanding the La0.7Sr0.3MnO3 (LSMO)/ PbZr0.2Ti0.8O3 (PZT) magnetoelectric interface through synchrotron x-ray absorption and other techniques. The word magnetoelectric (ME) describes the coupling effects in certain materials that exhibit a change of the magnetic (electric) order with the change of the electric (magnetic) field. Materials that are ME could potentially advance current technology. Faster, more sensitive, and more energy efficient devices can be built with ME materials as compared to the present systems. Practical ME materials are essential for the realization of this advancement. Single phase ME materials are rare and the known few do not have strong coupling effects at ambient temperatures. Bilayer (or multilayer) systems, however, provide a feasible alternative because they sometimes exhibit ME coupling effects at the interface(s). As an example of multilayer systems, ME coupling effects were previously reported by Vaz et. al. in between ferromagnetic LSMO and ferroelectric PZT, where the Mn valence changed with the varying external electric field. Through the use of a programmable shutter, small thickness gradients were created in both LSMO and PZT layers grown by pulsed laser deposition. A few flat samples were also grown for comparison. These samples were characterized by synchrotron methods including fluorescence mapping, micro x-ray diffraction (µXRD), x-ray absorption near edge spectroscopy (XANES), x-ray magnetic circular dichroism (XMCD), and photoemission electron microscopy (PEEM) as well as non-synchrotron based lab techniques such as atomic force microscopy (AFM) and scanning transmission electron microscopy (STEM). Wedge samples were locally smooth over experimental spot sizes and had uniform thickness gradients. Interfaces were sharp, exhibiting epitaxial growth. The magnetization increased with LSMO thickness. Mn valence in LSMO is associated with the LSMO magnetization states and was extensively studied in this dissertation. With the increase of LSMO thickness, Mn valence increased. A depth dependent valence model was developed to fit the LSMO thickness dependent valence results. The Mn charges were found to rearrange at the LSMO/PZT interface, and the effects of the ferroelectric polarization and polar interface on this interfacial valence were theoretically treated. In order to understand the effects of PZT on LSMO, Mn valence was measured at varied PZT thickness. Mn valence was found to be smaller when the PZT thickness was under 65 nm and decreased with decreasing PZT thickness in the PZT thickness region of 0 to 65 nm. Piezoresponse force microscopy (PFM) showed a transformation in PZT from polydomain to monodomain with
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