Multiferroic materials based on artificial thin film heterostructures

We discuss the use of thin film heterostructures for the generation of artificial multiferroic or magnetoelectric material systems. In this regard the transition metal oxides BiFeO3 and BiCrO3 are of particular interest. By using laser molecular beam epitaxy we have grown epitaxial thin films of these materials on SrTiO3 substrates in a two-dimensional layer-by-layer growth mode. The films show small surface roughness and excellent crystallographic quality with mosaic spread below 0.04. In contrast to the bulk materials, both BiFeO3 and BiCrO3 films have tetragonal crystal structure due to epitaxial coherency strain. The magnetic properties have been studied by SQUID magnetometry. In contrast to a previous report on BiFeO3 we find only a small saturation magnetization below 0.05 B/formula unit both for BiFeO3 and BiCrO3, which can be explained by a canting of the antiferromagnetically aligned Fe and Cr spins, respectively. Furthermore, no indication for a strain induced enhancement of magnetization was found.