Two phase nanocomposites comprising of magnetic spinel pillars in a ferroelectric perovskite matrix have been widely studied over the last decade for their potential application as room temperature magnetoelectric multiferroics in low power memory and logic devices. Such nanocomposites grow by self-assembly, a desirable trait for low cost, high throughput production. A unique feature of the nanocomposites is the epitaxial vertical interface between the phases, permitting the strain between the phases to be sustained over large film thicknesses with the advantage of larger interface area relative to laminar films. This feature is very useful in modulating the properties and functionality of the composite. For example, the strain allows for an indirect coupling between the magnetic and ferroelectric properties of the individual phases, which can be utilized for electrically assisted switching of magnetic information stored in the pillars. The strained interface might also serve as a high conduction pathway for use in solid oxide fuel cells. In essence, two distinct properties can be coupled through the strain to yield a variety of multifunctional devices.
We use combinatorial pulsed laser deposition to grow the nanocomposites. Using BiFeO3 as the ferroelectric phase and CoFe2O4, NiFe2O4 and MgFe2O4 as the ferrimagnetic phase grown on a single crystal strontium titanate substrate, we have studied a number of magnetic and ferroelectric properties of the nanocomposites. The epitaxial strain in the magnetic spinel phase dramatically influences the magnetic anisotropy in the highly magnetoelastic CoFe2O4. Switching of the magnetic state of individual pillars under the influence of applied electric fields has been observed. The variation in anisotropy of the pillars through changes in composition has also been studied.
Significant control of the growth process has been achieved, as demonstrated by our ability to template the nanocomposites, thus allowing the growth of uniformly spaced pillars within the ferroelectric matrix, which retain the properties of as grown nanocomposites. Integration of the composites onto silicon has been achieved in collaboration with Charles Ahn’s group at Yale.