Giant phonon-induced effective magnetic fields in 4f paramagnets

Ultrafast control of magnetic order can conventionally be achieved through opto-magnetic effects, such as the inverse Faraday effect. Here, we theoretically describe a vibrational analog of this mechanism, in which coherently excited infrared-active phonons replace photons in the scattering process, and which is mediated through spin-phonon coupling. In particular, we show that circularly driven phonon modes in the rare-earth trihalide CeCl3 generate giant effective magnetic fields acting on the paramagnetic 4f spins. We compute the coherent phonon dynamics in response to the excitation by a terahertz pulse using a combination of phenomenological modeling and density functional theory calculations. We find that effective magnetic fields of up to 100 tesla can possibly be generated that polarize the spins for experimentally accessible pulse energies. The direction of induced magnetization can be inverted by reversing the polarization of the laser pulse. This phono-magnetic effect offers a new route to achieving control over and switching of magnetic order at terahertz frequencies.