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AX: Intrinsic multiferroic BiFeO₃ epitaxial films and nanostructures

BiFeO₃ is the canonical intrinsic multiferroic perovskite material with large ferroelectric Curie temperature T_C ~ 1100 K,  record ferroelectric polarization of 95 µC/cm² at room temperature and a Néel antiferromagnetic ordering temperature of  T_N=673 K. It is therefore an excellent playground for studying the properties of perovskite multiferroics that possess simultaneously two order parameters at room temperature. Additionally, many interactions via various mechanisms, such as the exchange bias, occur when BiFeO₃ is interfaced in heterostructures with other functional oxides, e.g. ferromagnetic La_0.7Sr_0.3MnO₃.

An essential prerequisite for gaining insight from studying the physical properties of multiferroic BiFeO₃ is the ability to grow either large single crystals of phase pure BiFeO₃ or epitaxial thin films. The latter is even more important as thin films have more technological relevance. Moreover, epitaxy and strain engineering have lately been proven to be a powerful tool in tailoring the physical properties of functional perovskite thin films.

We grow phase pure epitaxial BiFeO₃ films by pulsed-laser deposition on various single crystal substrates, such as (100), (111)-oriented SrTiO₃, (110) orientated DyScO₃, etc. On different substrates, as well as by varying the thickness of the films or the growth conditions, various strain states in the films can be achieved. The strain states lead to a formation of different ferroelectric/ferroelastic domain patterns in the BiFeO₃ films. These patterns are studied by piezoresponse force microscopy and transmission electron microscopy. Ferromagnetic half metallic La_0.7Sr_0.3MnO₃and metallic SrRuO₃ can be employed as electrodes. Furthermore, the influence of cation substitutions on the Bi-site and Fe-site with La and Mn, respectively, is investigated, being expected to have significant impact on the electronic properties of the epitaxial films.

We also fabricate nanostructures of BiFeO₃ by both a top-down and a bottom-up approach. By means of focused ion beam our phase pure epitaxial films can be patterned in a predesigned way. A bottom-up method can be carried out by the use of stencil masks attached to the substrates during the deposition of BiFeO₃. The influence of the size of the structures on the ferroelectric domain stability and switching as well as the piezoelectric properties is investigated.

Principal Investigator

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