Phononic switching of magnetization by the ultrafast Barnett effect.

The historic Barnett effect describes how an inertial body with otherwise zero net magnetic moment acquires spontaneous magnetization when mechanically spinning. Breakthrough experiments have recently shown that an ultrashort laser pulse destroys the magnetization of an ordered ferromagnet within hundreds of femtoseconds, with the spins losing angular momentum to circularly polarized optical phonons as part of the ultrafast Einstein-de Haas effect. However, the prospect of using such high-frequency vibrations of the lattice to reciprocally switch magnetization in a nearby magnetic medium has not yet been experimentally explored. Here we show that the spontaneous magnetization gained temporarily by means of the ultrafast Barnett effect, through the resonant excitation of circularly polarized optical phonons in a paramagnetic substrate, can be used to permanently reverse the magnetic state of a heterostructure mounted atop the said substrate. With the handedness of the phonons steering the direction of magnetic switching, the ultrafast Barnett effect offers a selective and potentially universal method for exercising ultrafast non-local control over magnetic order.