  We report on the temperature evolution of the polarization-dependent Raman
spectrum of exfoliated MoI$_3$, a van der Waals material with a "true
one-dimensional" crystal structure that can be exfoliated to individual atomic
chains. The temperature evolution of several Raman features reveals anomalous
behavior suggesting a phase transition of a magnetic origin. Theoretical
considerations indicate that MoI$_3$ is an easy-plane antiferromagnet with
alternating spins along the dimerized chains and with inter-chain helical spin
ordering. The calculated frequencies of the phonons and magnons are consistent
with the interpretation of the experimental Raman data. The obtained results
shed light on the specifics of the phononic and magnonic states in MoI$_3$ and
provide a strong motivation for future study of this unique material with
potential for spintronic device applications.
