Crossover between Linear and Nonlinear Magno-phononics in FePS₃

Control of magnetism and other physical properties of materials with coherent phonon excitations has been firmly established to be an efficient and unique avenue. Coherent phonons are typically excited through either linear or nonlinear pathway, while simultaneous activation and control of a single mode in both pathways remain elusive. Here, we demonstrate that Raman-active phonons can be concurrently linearly and quadratically driven by hybridizing with a magnon in a van der Waals antiferromagnet FePS₃. By driving the magnon-phonon hybrid, also dubbed magnon polaron, with an intense broadband terahertz pulse, we not only observe the interference between the linear magnetic-dipole and nonlinear excitation pathways, but also achieve control of the weight of different channels by tuning either terahertz field strength or polarization. Moreover, utilizing the nonlinear excitation pathway, we are able to induce a nonequilibrium magnetic state with 2.5 millisecond lifetime. Our findings unlock a wide range of possibilities of phonon-mediated manipulation of material properties, ranging from generation of metastable magnetic states via displacive phonon excitation to phonon-Floquet engineering.