  Van der Waals (vdW) magnets have rapidly emerged as a fertile playground for
novel fundamental physics and exciting applications. Despite the impressive
developments over the past few years, technical limitations pose a severe
challenge to many other potential breakthroughs. High on the list is the lack
of suitable experimental tools for studying spin dynamics on atomically thin
samples. Here, Raman scattering techniques are employed to observe directly the
low-lying magnon (~1 meV) even in bilayer NiPS3. The unique advantage is that
it offers excellent energy resolutions far better on low-energy sides than most
inelastic neutron spectrometers can offer. More importantly, with appropriate
theoretical analysis, the polarization dependence of the Raman scattering by
those low-lying magnons also provides otherwise hidden information on the
dominant spin-exchange scattering paths for different magnons. By comparing
with high-resolution inelastic neutron scattering data, these low-energy Raman
modes are confirmed to be indeed of magnon origin. Because of the different
scattering mechanisms involved in inelastic neutron and Raman scattering, this
new information is fundamental in pinning down the final spin Hamiltonian. This
work demonstrates the capability of Raman spectroscopy to probe the genuine
two-dimensional spin dynamics in atomically-thin vdW magnets, which can provide
novel insights that are obscured in bulk spin dynamics.
