A Computational study of Magnetic Field-controlled Acoustic Coherent Phonon Generation and Propagation in Ferromagnetic GaMnAs

We present a theoretical study of magnetic field-controlled generation and propagation of coherent longitudinal acoustic phonons (CLAP) generated by ultrafast laser pulses in a Ferromagnetic Ga_1-xMn_xAs (x=0.082) thin film grown on an intrinsic GaAs substrate subjected to an in-plane static magnetic field varying between 2-10T. Electron-phonon deformation potential coupling is considered for the generation of the CLAP at the surface as well as at the interface for a pump energy of 3.1 eV with their propagation being governed by a semi-classical wave equation. The field-induced variation in the bandstructure, dielectric functions, and, hence, the CLAP amplitude are calculated using an 8-band Pidgeon-Brown model including the effects of magnetic, Mn impurities. The calculated differential reflectivity at the probe energy 1.55 eV agrees well with experimental time-resolved differential reflectivity measurements and suggests that a substantial contribution to the signal comes from the interface. The magnetic field dependence of the reflectivity amplitude arises from the Seraphin coefficients.