Structure, magnetism and multiferroicity in self-assembled oxide nanocomposites

Abstract

The route to enhanced functionality in electronic and magnetic devices is often through materials engineering and the use of new materials structures. Oxides, in particular, exhibit a wide range of highly tunable properties due to the interplay of lattice, orbital, charge and spin degrees of freedom. Recently, a new paradigm for epitaxy has been studied, where two oxide phases self-assembled into a vertical columnar morphology, with epitaxially strained interfaces perpendicular to the substrate. Through appropriate materials selection and strain tuning, the interfaces in these vertically aligned nanocomposites exhibit exciting properties such as high conduction at interfaces, enhanced ferroelectricity and magnetoelectric coupling, which are often absent or occur at a lower magnitude in single phase materials. In particular, magnetoelectric multiferroics, materials that exhibit two or more ferroic orders (such as ferromagnetism and ferroelectricity) and also exhibit electric field control of magnetism, have been widely explored, due to their utility in realizing novel low power multifunctional devices. Few materials exhibit robust room temperature multiferroicity, and thus vertical nanocomposites such as BiFeO₃-CoFe₂O₄ (BFO-CFO) which consist of magnetic CFO pillars in a matrix of ferroelectric BFO coupled via strain provide an exciting path to create artificial magnetoelectric multiferroics. In this thesis, we explore the magnetic, multiferroic and magnetoelectric properties of BFOCFO nanocomposites. Exploiting the rich strain tunability of BFO, we utilize different ways to modulate the structure of BFO in the BFO-CFO nanocomposites. Using different crystal substrates, we demonstrate that the presence of CFO offers additional parameters by which to tune the structure of BFO. In order to enable reliable device use, we need to understand and control the various interactions in BFO-CFO system. We demonstrate that composition tuning is an effective way to systematically tune the anisotropy of the magnetic pillars, thereby controlling their magnetostatic interactions. We probe the magnetoelectric coupling between the BFO and CFO phases by using Scanning Probe microscopy. By demonstrating tunability of the ferroelectric and magnetic phase of BFO-CFO nanocomposites and exploring the quantification of magnetoelectric coupling at the nanoscale, this thesis could enable intelligent design and optimization of the multiferroic and magnetoelectric properties in oxide nanocomposites.