Magnetic field effect on topological spin excitations in CrI$_3$

TL;DR

This study uses inelastic neutron scattering to identify Dzyaloshinskii-Moriya interactions as the origin of topological spin excitations in CrI$_3$, clarifying the microscopic spin interactions and the nature of the Dirac gap in this 2D magnetic material.

Contribution

It provides conclusive experimental evidence that DM interactions, not Kitaev or electron correlation effects, cause the Dirac gap in CrI$_3$, advancing understanding of topological spin excitations in 2D magnets.

Findings

DM interactions are the microscopic origin of the Dirac gap.

Nearest neighbor interactions are antiferromagnetic along the c-axis.

Next nearest neighbor interactions are ferromagnetic.

Abstract

The search for topological spin excitations in recently discovered two-dimensional (2D) van der Waals (vdW) magnetic materials is important because of their potential applications in dissipation-less spintronics. In the 2D vdW ferromagnetic (FM) honeycomb lattice CrI$_3$(T$_C$= 61 K), acoustic and optical spin waves were found to be separated by a gap at the Dirac points. The presence of such a gap is a signature of topological spin excitations if it arises from the next nearest neighbor(NNN) Dzyaloshinskii-Moriya (DM) or bond-angle dependent Kitaev interactions within the Cr honeycomb lattice. Alternatively, the gap is suggested to arise from an electron correlation effect not associated with topological spin excitations. Here we use inelastic neutron scattering to conclusively demonstrate that the Kitaev interactions and electron correlation effects cannot describe spin waves, Dirac gap and their in-plane magnetic field dependence. Our results support the DM interactions being the microscopic origin of the observed Dirac gap. Moreover, we find that the nearest neighbor (NN) magnetic exchange interactions along the axis are antiferromagnetic (AF)and the NNN interactions are FM. Therefore, our results unveil the origin of the observedcaxisAF order in thin layers of CrI$_3$, firmly determine the microscopic spin interactions in bulk CrI$_3$, and provide a new understanding of topology-driven spin excitations in 2D vdW magnets.