Phonon-mediated magnetic order control of bilayer antiferromagnets*

We study theoretically the effects of non-linear phonon processes after the ultra-fast photo-excitation of an infrared-active mode in the magnetic order of bilayer antiferromagnets. As a prototypical example, we consider bilayer CrI3, a van der Waals crystal composed of ferromagnetic (FM) layers with out-of-plane magnetization. Between the layers, the exchange coupling is antiferromagnetic (AFM) in the ground state, as determined experimentally, and such coupling is connected with the relative stacking between the layers. We employ group theory methods to determine the nature of the phonons modes and their allowed non-linear coupling terms in the AFM groundstate. Then, we use first-principles calculations to determine their frequencies and coupling strengths. We then use those results as inputs for the phonon equations of motion after photo-excitation and find that the Raman modes involving relative movement between the layers have the potential to influence the interlayer exchange interaction. Finally, we comment on the feasibility of the experimental detection of the changes in the magnetic order.