Jinling Zhou

Date of Graduation

2016

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Mikel Holcom

Committee Co-Chair

David Lederman

Committee Member

Edward Flagg

Committee Member

Leonardo Golubovic

Committee Member

Kenneth Showalter

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 decreasing thickness. Charges within the LSMO layer drawn to the interface to screen the PZT surface charge should theoretically vary with PZT domain structures, and would lead to a PZT thickness dependent Mn valence. This ferroelectric modulation of Mn valence charge was confirmed by measuring Mn valence in locally poled PZT. These discoveries agree with a charge modulated interfacial ME coupling mechanism. This thickness dependence study additionally indicates that thin PZT is preferred to avoid in-plane ferroelectric domain orientations and to maximize the coupling effect with LSMO. In order to understand the relation of the magnetic domains to the ferroelectric domains on the microscopic scale appropriate for devices, linear and circular dichroic images were taken by PEEM at both the Mn and Ti absorption L-edges. At these interfaces, uncompensated spins were first seen in images taken with circularly polarized x-rays at the Ti absorption L-edge. These spins preferred to orient perpendicular to the LSMO ferromagnetic direction, consistent with magnetic biquadratic coupling.

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