Self-Assembled Epitaxial Multiferroic Oxide Nanostructures Grown by Pulsed Electron Deposition

Comes, Ryan, Engineering Physics - School of Engineering and Applied Science, University of Virginia
Wolf, Stuart, Department of Materials Science and Engineering, University of Virginia

Multiferroic oxide nanocomposites thin films composed of a ferrimagnetic spinel oxide nanopillar and ferroelectric perovskite oxide matrix offer magnetoelectric coupling between the matrix and pillar, which can be used in spintronic devices for logic and memory applications. CoFe2O4 (CFO) offers unique properties as a magnetoelectric material due to its large magnetoelastic response under stain. Previous work has shown that when CFO is co-deposited with BiFeO3 (BFO), nanostructured phase segregation occurs, with CFO pillars forming in a BFO matrix. This work has also shown that electrical control of the magnetic anisotropy in the CFO pillar is possible via switching polarization of the ferroelectric BFO matrix. However potential spintronic device schemes require the ability to control the growth of these materials to pattern the locations where CFO pillars form, which has thus far been illusory.
This dissertation has focused on the templated growth of multiferroic complex oxide films and nanostructures via pulsed electron deposition (PED). The relatively new PED technique is similar to the more traditional pulsed laser deposition (PLD) approach, but with some key differences in growth kinetics. Uniform epitaxial CFO, BFO and La0.72Sr0.28MnO3 films have been grown on various substrates via the PED technique. All three epitaxial growth modes have been demonstrated in these materials, representing the first such result for complex oxide films grown via PED. Spontaneously formed epitaxial CFO-BFO nanocomposites have also been grown via PED and demonstrate unusual strain conditions in the CFO pillars that is not seen in composites grown via PLD. Analysis of the magnetic anisotropy of CFO-BFO nanocomposites using vibrating sample magnetometry showed that the magnetic properties what is expected given the strain demonstrated in the pillars. Finally, patterning of the growth of CFO-BFO nanocomoposites has been demonstrated for the first time through the patterning of nanoscale CFO islands on the surface of the substrate to serve as a template for the composite. Electron and scanning probe microscopy analysis has demonstrated that these patterned composites exhibit similar crystal quality and multiferroic properties as the spontaneously formed unpatterned films.

PHD (Doctor of Philosophy)
CoFe2O4, BiFeO3, Multiferroic, Directed self-assembly, Nanocomposite, Complex oxide, Epitaxial thin film, Pulsed electron deposition
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