  Two-dimensional (2D) magnetic materials have attracted much recent interest
with unique properties emerging at the few-layer limit. Beyond the reported
impacts on the static magnetic properties, the effects of reducing the
dimensionality on the magnetization dynamics are also of fundamental interest
and importance for 2D device development. In this report, we investigate the
spin dynamics in atomically-thin antiferromagnetic FePS3 of varying layer
numbers using ultrafast pump-probe spectroscopy. Following the absorption of an
optical pump pulse, the time evolution of the antiferromagnetic order parameter
is probed by magnetic linear birefringence. We observe a strong divergence in
the demagnetization time near the N\'eel temperature. The divergence can be
characterized by a power-law dependence on the reduced temperature, with an
exponent decreasing with sample thickness. We compare our results to
expectations from critical slowing down and a two-temperature model involving
spins and phonons, and discuss the possible relevance of spin-substrate phonon
interactions.
