Optically Induced Picosecond Timescale Oxygen Octahedral Rotations in Multiferroic BiFeO₃ Thin Films

Structural effects originating from above bandgap optical excitation provide a route for ultrafast control of ferroelectricity and magnetism in multiferroic materials. The electrical and magnetic properties of multiferroic BiFeO₃ depend on structural distortions arising from the rotations and tilts of FeO₆ octahedra. We report an x-ray free-electron laser diffraction study designed to probe the dynamics of oxygen octahedral rotation (OOR) in a 35 nm BiFeO₃ film epitaxially deposited on a (001) SrTiO₃ substrate following a 3.1 eV optical pump pulse. The diffraction measurement recorded the time dependence of the reciprocal space location and intensities of half-integer reflections (0.5 0.5 2.5) and (0.5 1.5 2.5). Both reflections shifted to a lower out-of-plane wavevector following the optical excitation indicating that there is a transient photoinduced lattice expansion. The intensity of the (0.5 0.5 2.5) reflection depends on the in-plane OOR angles, providing a straightforward way to determine the tilt angle. The intensities of (0.5 1.5 2.5) reflections depend on both in-plane and out-of-plane OOR angles. A model for the dependence of the intensity on the in-plane and out-of-plane OOR angles was used to interpret the diffraction results.