  The Barnett effect, discovered more than a century ago, 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 temporarily gained via the ultrafast
Barnett effect, through the resonant excitation of circularly-polarized optical
phonons in paramagnetic substrates, can be used to permanently reverse the
magnetic state of the substrate-mounted heterostructure. 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.
