The realization of magnetic frustration in a metallic van der Waals (vdW) coupled material has been sought as a promising platform to explore novel phenomena, both in bulk matter and in exfoliated devices. However, a suitable material realization has been lacking so far. Here, we demonstrate that the vdW compound CeSiI hosts itinerant electrons coexisting with exotic magnetism. In CeSiI, the magnetic cerium atoms form a triangular bilayer structure sandwiched by vdW stacked iodine layers. From resistivity and magnetometry measurements, we confirm the coexistence of itinerant electrons with magnetism with dominant antiferromagnetic exchange between the strongly Ising-like Ce moments below 7 K. Neutron diffraction confirms magnetic order with an incommensurate propagation vector $k\sim \left(0.28,0,0.19\right)$ at 1.6 K, which points to the importance of further-neighbor magnetic interactions in this system. The presence of a two-step magnetic-field-induced phase transition along the $c$ axis further suggests magnetic frustration in the ground state. Our findings provide a material platform which hosts the coexistence of an itinerant electron and frustrated magnetism in a vdW system, where exotic phenomena arising from the rich interplay between the spin, charge, and lattice in low dimension can be explored.

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• Antiferromagnetism
• Frustrated magnetism
• Magnetic phase transitions
• Magnetic susceptibility
• Metamagnetism
• 2-dimensional systems
• Antiferromagnets
• Magnetic systems
• Polycrystalline materials
• Single crystal materials
• Strongly correlated systems
• Crystal growth
• Magnetization measurements
• Neutron diffraction
• Neutron techniques
• Sample preparation