  Phase transitions are emergent phenomena where microscopic interactions drive
a disordered system into a collectively ordered phase. Near the boundary
between two phases, the system can exhibit critical, scale-invariant behavior.
Here, we report on a second-order phase transition accompanied by critical
behavior in a system of warm cesium spins driven by linearly-polarized light.
The ordered phase exhibits macroscopic magnetization when the interactions
between the spins become dominant. We measure the phase diagram of the system
and observe the collective behavior near the phase boundaries, including
power-law dependence of the magnetization and divergence of the susceptibility.
Out of equilibrium, we observe a critical slow-down of the spin response time
by two orders of magnitude, exceeding five seconds near the phase boundary.
This work establishes a controlled platform for investigating equilibrium and
nonequilibrium properties of magnetic phases.
