Coherent acoustic phonons and ultrafast carrier dynamics in hetero-epitaxial BaTiO₃-BiFeO₃ films and nanorods

The desire for multifunctional devices has driven significant research toward exploring multiferroics, where the coupling between electric, magnetic, optical, and structural order parameters can provide new functionality. While BiFeO₃ is a well studied multiferroic, recent research has shown that the addition of BaTiO₃ can improve the material properties. In this study, we perform time resolved differential reflectivity measurements of (1 - x)BaTiO₃-(x)BiFeO₃, with x = 0.725 epitaxial films grown on a lanthanum strontium manganite (LSMO) layer with a strontium titanate (STO) substrate, and BaTiO₃-BiFeO₃ nano-rods grown on Pt/Si. Our time resolved measurements can be used to probe dynamical properties on picosecond time scales. Information obtained is vital towards the design and understanding of high speed devices that incorporate multiferroic materials. We find that the contribution to the transient reflectivity response of a BaTiO₃-BiFeO₃ film due to carrier dynamics can be quantitatively explained by the diffusion of the photoexcited carriers away from the surface and that the ambipolar diffusion constant is below 1-2 cm² s^-1. At low temperature and for thicker films, in addition to a response from carrier dynamics, we also observed oscillations in the transient reflectivity with periods of 30-37 ps. We attribute these oscillations to propagating coherent acoustic phonons in the films. In the BaTiO₃-BiFeO₃ nano-rods samples only, we observed an additional oscillation with frequency in the range of 8 GHz. This is close to a theoretically predicted magnon frequency, but the strength of this oscillation only had a very weak magnetic field dependence. Another explanation for this feature could be multiple reflections of the acoustic phonons at the Pt interfaces (due to the large acoustic impedance mismatch).