  Magnetic van der Waals materials provide an ideal playground for exploring
the fundamentals of low-dimensional magnetism and open new opportunities for
ultrathin spin processing devices. The Mermin-Wagner theorem dictates that as
in reduced dimensions isotropic spin interactions cannot retain long-range
correlations; the order is stabilized by magnetic anisotropy. Here, using
ultrashort pulses of light, we demonstrate all-optical control of magnetic
anisotropy in the two-dimensional van der Waals antiferromagnet NiPS$_3$.
Tuning the photon energy in resonance with an orbital transition between
crystal-field split levels of the nickel ions, we demonstrate the selective
activation of a sub-THz two-dimensional magnon mode. The pump polarization
control of the magnon amplitude confirms that the activation is governed by the
instantaneous magnetic anisotropy axis emergent in response to photoexcitation
of orbital states with a lowered symmetry. Our results establish pumping of
orbital resonances as a universal route for manipulating magnetic order in
low-dimensional (anti)ferromagnets.
