Chiral phononics: Controlling magnetic order with phonon angular momentum*

Chiral phonons are circularly polarized lattice vibrations, in which the atoms move along circular orbits around their equilibrium positions in a solid. In dielectric materials, the orbital motions of the ions generate atomistic charge currents and therefore a magnetic moment carried by the chiral phonon [1,2]. In this talk, I show that chiral phonons are able to induce giant effective magnetic fields in solids when driven with an ultrashort laser pulse, and I present two examples of how these fields can be utilized to control the magnetic order of materials. First, we demonstrate that chiral phonons in paramagnetic rare-earth trihalides are able to create effective magnetic fields of potentially up to 100 T that polarize the spins in the material [3,4], a prediction that has been successfully verified by a recent experiment [5]. Second, we predict that the effective magnetic field produced by chiral phonons can rectify spin precession in antiferromagnets, leading to a canting of the spins and the generation of a quasistatic magnetization. As a result, a light-induced weak ferromagnetic state is transiently created in the solid [6]. Our results represent a new way of inducing hidden spin configurations that are not accessible in the equilibrium.