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 dissipationless spintronics. In the 2D vdW ferromagnetic (FM) honeycomb lattice (), 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 axis are antiferromagnetic (AF), and the NNN interactions are FM. Therefore, our results unveil the origin of the observed -axis AF order in thin layers of , firmly determine the microscopic spin interactions in bulk , and provide a new understanding of topology-driven spin excitations in 2D vdW magnets.
- Received 18 February 2021
- Revised 6 May 2021
- Accepted 10 June 2021
DOI:https://doi.org/10.1103/PhysRevX.11.031047
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
A van der Waals material is made of a stack of weakly bonded planes of atoms. These 2D materials are powerful platforms for exploring several electronic and magnetic behaviors. Recently, the discovery of robust topological spin excitations in the 2D magnet has spurred huge interest in their potential applications such as in the field of dissipationless spintronics, where electron spins are used to transmit and store information. Here, we use neutron-scattering experiments to explore the microscopic origin of these spin excitations and an accompanying intriguing magnetic phenomenon in this material: a stacking-dependent magnetic order. That is, while a single layer of is ferromagnetic, two stacked layers are antiferromagnetic, which, counterintuitively, is different from that in the ferromagnetic bulk.
In our experiments, we find that spin-orbit coupling (a relativistic interaction of an electron’s spin with its motion) induces asymmetric interactions between the spins. This induces the spins to feel the magnetic field differently, affecting their topological excitations. In addition, our measurements show that the nearest magnetic exchange interaction along the weakly bonded planes is indeed antiferromagnetic.
Our results unveil the origin of the observed antiferromagnetic order in thin layers of and provide a new understanding of topology-driven spin excitations in 2D van der Waals magnets.