Chiral phonon inverse Faraday and Barnett effects

The Einstein-de Haas effect conventionally describes the coupling of magnetization and mechanical rotation due to angular momentum conservation. On ultrashort timescales, such as during the sudden demagnetization of a material following the excitation with a laser pulse, the ultrafast Einstein-de Haas effect describes the coupling of spin and orbital angular momentum to the angular momentum of circularly polarized (chiral) phonons and strain waves. Here, we showcase the inverse mechanism: chiral phonon modes, driven by an ultrashort terahertz pulse, can induce a magnetization in both nonmagnetic and magnetic materials. We use a combination of phenomenological modeling and density functional theory calculations to simulate the coherent excitation of chiral phonons, which generate real and effective magnetic fields within the material, which subsequently produces a magnetization. We show that magnetizations of up to several Bohr magneton can possibly be induced. This mechanism can be seen as a phonon Barnett effect, the inverse of the Einstein-de Haas effect. At the same time, it can be considered a phonon analog of the inverse Faraday effect, in which circularly polarized (chiral) light induces a magnetization in materials. Phonon inverse Faraday/Barnett effects provide a new avenue to control the magnetic order of materials on ultrafast timescales.