Stacking tunable interlayer magnetism in bilayer CrI₃

Bulk CrI3 exhibits a vdW structure and possesses a rhombohedral structure with the R3 space group symmetry at low temperature (the LT phase). When temperature increases to 210–220 K, it undergoes a structural phase transition to a monoclinic lattice with the C2/m space group symmetry (the HT phase) It is generally believed that the experimental low temperature measurement is a low temperature stable structure. However, the CrI3 bilayer prepared at room temperature may not undergo structural transformation during the rapid cooling process and maintain the high-temperature phase structure. The researchers speculate that different stacking in the CrI3 system may correspond to different interlayer magnetic ground states. Based on the above prediction, we found that the interlayer AFM coupling results from a different stacking order with the C2/m space group symmetry, rather than the graphene-like one with R3 as previously believed^[2].

This work solves the problem of the source of the AFM coupling between the two layers of CrI3 in a series of recent experiments and proposes a magnetic coupling mechanism in the weak non-covalent coupling limit. In the past, it was generally assumed that the van der Waals (vdW) interaction dominated the interlayer interaction of two-dimensional materials, and the effect of interlayer coupling on magnetic regulation was significantly underestimated. This work realizes from a new perspective that after the p orbitals of I are polarized by Cr into pxy and pz orbitals with opposite magnetization directions, different interlayer stacking orders leads to different direct exchange between p orbitals of interlayer I atoms, which ultimately determines the magnetic coupling between layers.

This work has been cited positively by Professor Cheung Xiang and Professor Novoselov. The theoretical predictions of this work have been verified and cited by many different experimental groups published in Nature, Science, Nature sub-journals and PRL. Further, in collaboration with experimental collaborators, the theoretical image of orbital splitting of I-pxy and pz was verified by combining scanning tunneling microscopy (STM) and density functional theory (DFT) calculations^[3].