Keywords:Multiferroic materials, Time-resolved differential reflectivity, Coherent acoustic phonons
Abstract: The improved coupling between electric, magnetic, optical, and structural order parameters found in multiferroic BaTiO3-BiFeO3, generate the desire to explore novel functionalities in these lead-free systems. In order to obtain information to develop high speed multifunctional devices, we performed time-resolved differential reflectivity measurements of (1-x) BaTiO3 -(x) BiFeO3, with x = 0.725 and BaTiO3-BiFeO3 nanorods. We report a quantitative study of ultrafast carrier dynamics in BaTiO3-BiFeO3, by diffusion of the photoexcited carriers away from the surface and that the ambipolar diffusion constant is below 1-2 cm2/s. We also report the detection of coherent acoustic phonons(CP) in both film and nanorod samples at lower temperatures (100 K). In the film sample, we estimated the CP frequency to be ~ 27 GHz. In the Nanorod sample, we observed a higher frequency oscillation and a lower frequency oscillation with a frequency of ~ 33 GHz and 8 GHz. We attributed Higher frequency oscillations to be coherent acoustic phonons and we explain two probable origins for the lower frequency case. The lower frequency is close to a theoretically predicted magnon frequency in BFO, 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.