  The emergence of two-dimensional (2D) layered magnetic materials has opened
an exciting playground for both fundamental studies of magnetism in 2D and
explorations of spinbased applications. Remarkable properties, including spin
filtering in magnetic tunnel junctions and gate control of magnetic states,
have recently been demonstrated in 2D magnetic materials. While these studies
focus on the static properties, dynamic magnetic properties such as excitation
and control of spin waves have remained elusive. Here we excite spin waves and
probe their dynamics in antiferromagnetic CrI3 bilayers by employing an
ultrafast optical pump/magneto-optical Kerr probe technique. We identify
sub-terahertz magnetic resonances under an in-plane magnetic field, from which
we determine the anisotropy and interlayer exchange fields and the spin damping
rates. We further show tuning of antiferromagnetic resonances by tens of
gigahertz through electrostatic gating. Our results shed light on magnetic
excitations and spin dynamics in 2D magnetic materials, and demonstrate their
unique potential for applications in ultrafast data storage and processing.
