  The emergence of magnetism in quantum materials creates a platform to realize
spin-based phenomena with applications in spintronics, magnetic memory, and
quantum information science. A key to unlocking new functionalities in these
materials is the identification of enhanced coupling routes between spins and
other microscopic degrees of freedom. We demonstrate coherent coupling between
optical phonons and magnetism in the layered topological insulator MnBi2Te4 [1]
enabling magnetization modulation speeds orders of magnitude greater than
traditional control modalities such as strain and magnetic fields. Ultrafast
optical pulses are used to excite coherent phonons, which in turn modulate the
magnetization at THz frequencies, detected via transient Kerr rotation.
Together with Raman spectroscopy and ultrafast electron diffraction, we reveal
that magnetic control arises from strong coupling between optical phonons and
the exchange interaction, mediated by nonequilibrium carriers. In light of the
intimate connection between magnetism and topology, our work marks an important
step towards achieving sub-picosecond control of magnetic topological phases,
creating new opportunities for the coherent manipulation of quantum matter.
