Complex perovskite oxides exhibit a rich spectrum of functional responses,
including magnetism, ferroelectricity, highly correlated electron behavior,
superconductivity, etc. There exists a small set of materials which
exhibit multiple order parameters; these are known as multiferroics.
Using our work in the field of ferroelectrics and ferromagnetics as the
background, we are now exploring such materials, as epitaxial thin films
as well as nanocomposites. Specifically, we are studying the role of thin
film growth, heteroepitaxy and processing on the magnitude of the coupling
between the order parameters. In single phase multiferroic perovskites,
such as BiFeO3, we have found significant
enhancements in magnetism and
ferroelectricity compared to bulk. Detailed measurements indicate that
the enhancement in magnetism is due to a mixed
Fe+2/Fe+3 state
in the films. A very exciting new development has been the discovery of
the formation of spontaneously assembled nanostructures consisting of
a ferromagnetic phase embedded in a ferroelectric matrix that exhibit very
strong coupling between the two order parameters. This involves
3-dimensional heteroepitaxy between the substrate, the matrix perovskite
phase and spinel phase that is embedded as single crystalline pillars in
this matrix. This epitaxial coupling is critical and is responsible for
the significantly higher magnetoelectric coupling and magnetic anisotropy
in such vertical heterostructures compared to a conventional heterostructure.
This work is supported by the UMD-MRSEC and by the ONR under a MURI program.