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 dissipationless spintronics. In the 2D vdW ferromagnetic (FM) honeycomb lattice CrI3 (TC=61 K), acoustic and optical spin waves are 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 gaps, and their in-plane magnetic field dependence. Our results support the idea that the DM interactions are the microscopic origin of the observed Dirac gap. Moreover, we find that the nearest-neighbor (NN) magnetic exchange interactions along the c axis are antiferromagnetic (AF), and the NNN interactions are FM. Therefore, our results unveil the origin of the observed c-axis AF order in thin layers of CrI3, firmly determine the microscopic spin interactions in bulk CrI3, and provide a new understanding of topology-driven spin excitations in 2D vdW magnets.
Bibliographical noteFunding Information:
We are grateful to Franz G. Utermohlen, Nandini Trivedi, Adam Tsen, Zurab Guguchia, Liuyan Zhao, and Rui He for helpful discussions. The neutron scattering and sample growth work at Rice is supported by U.S. NSF-DMR-1700081 and by the Robert A. Welch Foundation under Grant No. C-1839 (P. D.). The work of J. H. C. was supported by the National Research Foundation (NRF) of Korea (Grants No. 2020R1A5A1016518 and No. 2020K1A3A7A09077712). E. J. G. S. acknowledges computational resources through the CIRRUS Tier-2 HPC Service (ec131 CirrusProject) at EPCC funded by the University of Edinburgh and EPSRC (EP/P020267/1); and ARCHER UK National Supercomputing Service via Project d429. E. J. G. S. acknowledges the Spanish Ministry of Science’s grant program “Europa-Excelencia” under Grant No. EUR2020-112238, EPSRC Early Career Fellowship (EP/T021578/1), and the University of Edinburgh for funding support. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory.
© 2021 authors. Published by the American Physical Society.
ASJC Scopus subject areas
- Physics and Astronomy(all)