Chiral Phonon-Induced Spin Polarization

Controlling spins on very short time scales may enable novel non-equilibrium magnetic physics as well as ultrafast spintronics for energy-efficient information processing. Chiral phonons, where atoms rotation unidirectionally around the equilibrium position inside crystalline lattice, break time reversal symmetry and are expected to directly couple with magnetization. We excited chiral phonons using circular-polarized terahertz pulses and observed a prominent transient magneto-optic Kerr effect in rare-earth trihalides. The effective magnetic field needed to polarize the paramagnetic spins is on the order of 1 T under very moderate pump fluence of 0.2 mJ/cm2. The temperature dependence of the spin dynamics indicates the magnetic field indeed is generated by the phonons, as opposed to a pure electromagnetic inverse Faraday effect. Our result may open a new route to investigate spin-phonon interaction in ultrafast magnetism and chiral transport properties for quantum devices.