  The emergence of magnetism in quantum materials creates a platform to realize
spin-based applications in spintronics, magnetic memory, and quantum
information science. A key to unlocking new functionalities in these materials
is the discovery of tunable coupling between spins and other microscopic
degrees of freedom. We present evidence for interlayer magnetophononic coupling
in the layered magnetic topological insulator MnBi2Te4. Employing magneto-Raman
spectroscopy, we observe anomalies in phonon scattering intensities across
magnetic field-driven phase transitions, despite the absence of discernible
static structural changes. This behavior is a consequence of a magnetophononic
wave-mixing process that allows for the excitation of zone-boundary phonons
that are otherwise 'forbidden' by momentum conservation. Our microscopic model
based on density functional theory calculations reveals that this phenomenon
can be attributed to phonons modulating the interlayer exchange coupling.
Moreover, signatures of magnetophononic coupling are also observed in the time
domain through the ultrafast excitation and detection of coherent phonons
across magnetic transitions. In light of the intimate connection between
magnetism and topology in MnBi2Te4, the magnetophononic coupling represents an
important step towards coherent on-demand manipulation of magnetic topological
phases.
