Pressure-controlled interlayer magnetism in atomically thin CrI₃

Stacking order can influence the physical properties of two-dimensional van der Waals materials^1,2. Here we applied hydrostatic pressure up to 2 GPa to modify the stacking order in the van der Waals magnetic insulator CrI₃. We observed an irreversible interlayer antiferromagnetic-to-ferromagnetic transition in atomically thin CrI₃ by magnetic circular dichroism and electron tunnelling measurements. The effect was accompanied by a monoclinic-to-rhombohedral stacking-order change characterized by polarized Raman spectroscopy. Before the structural change, the interlayer antiferromagnetic coupling energy can be tuned up by nearly 100% with pressure. Our experiment reveals the interlayer ferromagnetic ground state, which is established in bulk CrI₃ but not observed in native exfoliated thin films. The observed correlation between the magnetic ground state and the stacking order is in good agreement with first principles calculations^3-8 and suggests a route towards nanoscale magnetic textures by moiré engineering^3,9.