Time-reversal symmetry (TRS) is pivotal for materials optical, magnetic,
topological, and transport properties. Chiral phonons, characterized by atoms
rotating unidirectionally around their equilibrium positions, generate dynamic
lattice structures that break TRS. Here we report that coherent chiral phonons,
driven by circularly polarized terahertz light pulses, can polarize the
paramagnetic spins in CeF3 like a quasi-static magnetic field on the order of 1
Tesla. Through time-resolved Faraday rotation and Kerr ellipticity, we found
the transient magnetization is only excited by pulses resonant with phonons,
proportional to the angular momentum of the phonons, and growing with magnetic
susceptibility at cryogenic temperatures, as expected from the spin-phonon
coupling model. The time-dependent effective magnetic field quantitatively
agrees with that calculated from phonon dynamics. Our results may open a new
route to directly investigate mode-specific spin-phonon interaction in
ultrafast magnetism, energy-efficient spintronics, and non-equilibrium phases
of matter with broken TRS