Phononic control of magnetism in a topological insulator*

Magnetism in topological materials can give rise to unconventional topological phases that exhibit intriguing behavior such as the quantized magnetoelectric effect and the quantum anomalous Hall effect. The key to harnessing these properties in device applications lies in the ability to control magnetism. In this work, we demonstrate for the first time, the optical control of magnetism in a topological insulator. This is achieved in the recently discovered magnetic topological insulator MnBi₂Te₄.

First, using Raman spectroscopy, we detect that optical phonons strongly modulate the exchange interaction, resulting in a renormalization of phonon energies and lifetimes across the magnetic transition. These changes are the largest observed in any two-dimensional material to date. We exploit this strong coupling to optically engineer the spin Hamiltonian, through ultrafast modulation of the exchange interaction by coherent phonons. Using a time-resolved Kerr effect probe, we detect a resultant coherent modulation of the magnetization. Our work thus unlocks a novel mode of control of magnetism, offering new opportunities in the coherent manipulation of topological materials.