  Understanding ultrafast magnetization dynamics on the microscopic level is of
strong current interest due to the potential for applications in information
storage. In recent years, the spin-lattice coupling has been recognized to be
essential for ultrafast magnetization dynamics. Magnetoelectric multiferroics
of type II possess intrinsic correlations among magnetic sublattices and
electric polarization (P) through spin-lattice coupling, enabling fundamentally
coupled dynamics between spins and lattice. Here we report on ultrafast
magnetization dynamics in a room-temperature multiferroic hexaferrite
possessing ferrimagnetic and antiferromagnetic sublattices, revealed by
time-resolved resonant x-ray diffraction. A femtosecond above-bandgap
excitation triggers a coherent magnon in which the two magnetic sublattices
entangle and give rise to a transient modulation of P. A novel microscopic
mechanism for triggering the coherent magnon in this ferrimagnetic insulator
based on the spin-lattice coupling is proposed. Our finding opens up a novel
but general pathway for ultrafast control of magnetism.
