Abstract
In two-dimensional honeycomb ferromagnets, bosonic magnon quasiparticles (spin waves) may either behave as massless Dirac fermions or form topologically protected edge states. The key ingredient defining their nature is the next-nearest-neighbor Dzyaloshinskii-Moriya interaction that breaks the inversion symmetry of the lattice and discriminates chirality of the associated spin-wave excitations. Using inelastic neutron scattering, we find that spin waves of the insulating honeycomb ferromagnet () have two distinctive bands of ferromagnetic excitations separated by a gap at the Dirac points. These results can only be understood by considering a Heisenberg Hamiltonian with Dzyaloshinskii-Moriya interaction, thus providing experimental evidence that spin waves in can have robust topological properties potentially useful for dissipationless spintronic applications.
- Received 30 July 2018
- Revised 30 September 2018
DOI:https://doi.org/10.1103/PhysRevX.8.041028
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
When quantum particles such as electrons are confined to move in two dimensions, novel system behavior can arise. Honeycomb ferromagnets, in which moments of magnetic ions coalign to form a honeycomb pattern, are of particular interest because their magnons—quanta of magnetic spin waves—might behave like photons, much as electrons become massless in two-dimensional graphene. In the presence of a magnetic field, electrons in graphene may flow without dissipation along the material edges. Photonlike magnons can flow similarly even in the absence of an external magnetic field if the honeycomb lattice possesses a sufficiently strong internal magnetic field. Here, we experimentally show that spin waves in a honeycomb ferromagnet have two distinct excitation bands that reflect the presence of such a desired internal magnetic field.
We use inelastic neutron scattering to map out the spin-wave energies in a honeycomb ferromagnet as a function of wavelength and direction of wave propagation. We find that the spin waves have two distinctive bands of ferromagnetic excitations, showing that the photonlike magnons are split into higher- and lower-energy modes. Theoretical calculations confirm that this result can be understood only by considering a significant internal magnetic field due to coupling between the spin and orbital motions of electrons.
This work provides experimental evidence that spin waves in can have robust topological properties that are useful for dissipationless spintronic applications.