Coherent magnons and acoustic phonons were impulsively excited and probed in thin films of the room temperature multiferroic Bi1-x-yDy xLayFeO3 using femtosecond laser pulses. The elastic moduli of rhombohedral, tetragonal, and rare-earth doped BiFeO 3 were determined from acoustic-mode frequencies in conjunction with spectroscopic ellipsometry. A weak ferromagnetic order, induced alternately by magnetization in the growth direction or by tetragonality, created a magnon oscillation at 75 GHz, indicative of a Dzyaloshinskii-Moriya interaction energy of 0.31 meV. © 2013 American Physical Society.