Phonons regulate covalency: a new way to modulate the magnetism and Curie temperature of CrI3

Abstract

Recently, diverse strategies have been exploited to engineer the magnetism of two-dimensional (2D) magnets to widen their application in spintronics. Here, we demonstrate the effect of manipulating optical phonons on the magnetism and Curie temperature (T_C) of ferromagnetic CrI₃. The first-principles calculations reveal that the anisotropy governed by dipole–dipole coupling exhibits negligible variations under 14 optical phonons, while the E⁴_g (∼18.65%) and A¹_1g (∼8.75%) modes can increase the magneto-crystalline anisotropic parameter (K_ij) remarkably. Based on mean-field theory, we theoretically predict the Curie temperature of a CrI₃ monolayer both in the absence and presence of phonon vibrations. Intriguingly, despite the E⁴_g and A¹_1g modes increasing K_ij, they decrease T_C by 3.76% and 16.10%, respectively. Conversely, the E³_u and A²_2g modes enhance T_C by 6.63% and 4.77%. The underlying mechanism is that E⁴_g and A¹_1g weaken the covalency and super-exchange coupling between Cr and I atoms, whereas the E³_u and A²_2g modes strengthen them. Our work offers valuable theoretical insights for the application of 2D magnetic materials in spintronics.