MS-12-O-1737 Thickness dependent atomic structure and microsctructures of supertetragonal multiferroic BiFeO3 thin films

BiFeO3 thin films grown on LaAlO3 substrates exhibit a giant c/a ratio driven by the in-plane epitaxial stress imposed by the substrate leading to a so called supertetragonal phase. Previous structural studies [1] have shown that this behavior applies to film thicknesses ranging from few unit cells to several tens of nanometers. Deeper analysis of the thicker films reveals the coexistence of a mixture of phases which are most probably promoted by the stress relaxation as they are not observed in the thinner ones [2]. Despite numerous studies on this topic, the real atomic structure of BiFeO3 remains under debate.

We revisit the atomic structure and microstructure of these supertetragonal phases of highly strained epitaxial BiFeO3 thin films. Quantitative atomic resolution scanning transmission electron microscopy is used to directly image the atomic positions. Electron energy loss spectroscopy is further employed to reveal subtle electronic structure features.

The monoclinic Cm phase suggested by electron diffraction and predicted by ab initio calculations is evidenced by annular bright field imaging (fig.1). The relative positions of Bi, Fe and O atoms in the BiFeO3 unit cell have also been probed to compare them with the structural models proposed in the literature by ab initio calculations [3] confirming the reorganization of the unit-cell with the transformation of the oxygen octahedron in a square-based pyramid; this structure being nano-twinned in thicker films (fig. 2). Monochromated EELS experiments have subsequently been carried out to investigate the O-K and Fe-L23 edges. For the thinner films, the O-K fine structures experience changes from the interface to the surface of the film. Multilinear fit of the data set with specific fingerprints was employed in order to map the fine structures. The map reveals a modification of the crystal field resulting in a distortion of FeO5 pyramids described above by the underlying symmetry imposed by the LAO substrate.

Interpreted in a framework of antiferrodistortive distortions coupling with the substrate, these results point towards a phase near the interface closer to the P4mm purely tetragonal phase [4].

Our results emphasize the need for quantitative microscopy to investigate the subtle structure of these complex functional materials.

[1] H. Béa et al., Phys. Rev. Lett., 102 (2009), 217603.

[2] I.C. Infante et al., Phys. Rev. Lett., 107 (2011), 237601.

[3] O. Diéguez et al., Phys. Rev. B., 83 (2011), 0940105.

[4] F. Pailloux et al. Phys Rev B, to be published