Multiferroic thin films

Multiferroic materials are both ferroelectric and ferromagnetic. This combination opens up new applications, as the magnetization can be addressed with an electric field and the polarization by a magnetic field. However, most multiferroic materials either have low polarizations and/or low transition temperatures, thus limiting their potential use in novel devices. We report on high quality epitaxial films of two candidate materials, namely BiFeO$_{3}$ and BiMnO$_{3}$. Both films have narrow X-ray rocking curves of around 0.04\r{ } and are highly insulating with resistivities in excess of 10$^{9}$ Ohm cm. The BiMnO$_{3}$ films are ferromagnetic (2.2 $\mu _{B}$/Mn) and the BiFeO$_{3}$ films show a very weak ferromagnetic magnetization of $<$0.05 $\mu _{B}$/Fe, in contrast to recent claims of an epitaxial enhancement to 1 $\mu _{B}$/Fe (Wang \textit{et al}., Science \textbf{299}, 1719 (2003)). In BiFeO$_{3}$ films we record a dielectric constant of 80 at room temperature. Interestingly, this figure is 2500 for BiMnO$_{3}$ suggesting good ferroelectric properties.