Incommensurate Antiferromagnetic Order in Weakly Frustrated Two-Dimensional van der Waals Insulator CrPSe 3 .

Although magnetic order is suppressed by a strong frustration, it appears in complex forms such as a cycloid or spin density wave in weakly frustrated systems. Herein, we report a weakly magnetically frustrated two-dimensional (2D) van der Waals material CrPSe 3 . Polycrystalline CrPSe 3 was synthesized at an optimized temperature of 700 °C to avoid the formation of any secondary phases (e.g., Cr 2 Se 3 ). The antiferromagnetic transition appeared at T N ≈ 127 K with a large Curie-Weiss temperature θ CW ≈ -301 K via magnetic susceptibility measurements, indicating weak frustration in CrPSe 3 with a frustration factor of f (|θ CW |/ T N ) ≈ 2.4. Evidently, the formation of a long-range incommensurate antiferromagnetic order was revealed by neutron diffraction measurements at low temperatures (below 120 K). The monoclinic crystal structure of the C 2/ m symmetry is preserved over the studied temperature range down to 20 K, as confirmed by Raman spectroscopy measurements. Our findings on the incommensurate antiferromagnetic order in 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.