  Although the magnetic order is suppressed by a strong magnetic frustration,
it is maintained but appears in complex order forms such as a cycloid or spin
density wave in weakly frustrated systems. Herein, we report a weakly
magnetic-frustrated two-dimensional van der Waals material CrPSe$_3$.
Polycrystalline CrPSe$_3$ was synthesized at an optimized temperature of
700$^\circ$C to avoid the formation of any secondary phases (e.g.,
Cr$_2$Se$_3$). The antiferromagnetic transition appeared at $T_N\sim 126$ K
with a large Curie-Weiss temperature $T_{\rm CW} \sim -371$ via magnetic
susceptibility measurements, indicating weak frustration in CrPSe$_3$ with a
frustration factor $f (|T_{\rm CW}|/T_N) \sim 3$. Evidently, the formation of
long-range incommensurate spin-density wave antiferromagnetic order with the
propagation vector $k = (0, 0.04, 0)$ was revealed by neutron diffraction
measurements at low temperatures (below 120K). The monoclinic crystal structure
of C2/m symmetry is preserved over the studied temperature range down to 20K,
as confirmed by Raman spectroscopy measurements. Our findings on the spin
density wave antiferromagnetic order in two-dimensional (2D) magnetic
materials, not previously observed in the MPX$_3$ family, are expected to
enrich the physics of magnetism at the 2D limit, thereby opening opportunities
for their practical applications in spintronics and quantum devices.
