Topological spin excitations in honeycomb ferromagnet CrI$_3$ (1807.11452v3)

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 (DM) 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 CrI$_3$ ($T_C=61$ K) have two distinctive bands of ferromagnetic excitations separated by a $\sim$4 meV gap at the Dirac points. These results can only be understood by considering a Heisenberg Hamiltonian with DM interaction, thus providing experimental evidence that spin waves in CrI$_3$ can have robust topological properties useful for dissipationless spintronic applications.

• The paper demonstrates that a 4 meV gap in CrI3’s magnon spectrum arises from next-nearest neighbor DM interactions, indicating nontrivial topology.
• It employs inelastic neutron scattering to clearly resolve distinct acoustic and optical magnon modes in the honeycomb ferromagnet.
• The study implies that enhanced spin-orbit coupling in CrI3 may facilitate edge states, offering promising routes for low-loss spintronic devices.

Overview of Topological Spin Excitations in Honeycomb Ferromagnet CrI$_3$

This paper presents an investigation into the spin wave behavior of the honeycomb ferromagnet CrI$_3$ using inelastic neutron scattering. The central focus is on understanding whether the spin waves in CrI$_3$ can exhibit topological characteristics, particularly concerning the gap observed at Dirac points in its magnon spectrum. This paper's results could be pivotal for advancing dissipationless spintronic applications by utilizing topological magnons.

Key Findings

The research identifies two distinctive bands of ferromagnetic excitations in CrI$_3$, attributed to acoustic and optical magnon modes, which are separated by a gap of approximately 4 meV at the Dirac points. This observed gap is a significant deviation from the expected massless Dirac magnons at these points in the absence of additional interactions. Theoretical modeling suggests that this gap arises from the inclusion of a next-nearest neighbor Dzyaloshinskii-Moriya (DM) interaction term in the Heisenberg Hamiltonian. The DM interaction, which breaks the inversion symmetry of the lattice, is found to be crucial for the emergence of nontrivial topological properties in the magnon spectrum.

Implications

The presence of such a notable magnon gap suggests that CrI$_3$ could support nontrivial topological magnons with ensuing edge states, analogous to edge states found in electronic topological insulators. Theoretical predictions and experimental confirmations, as delineated in the paper, indicate that the DM interaction in CrI$_3$ is not only present but also substantial enough to effectively separate the magnon bands, thereby introducing potential avenues for realizing high-efficiency spintronic devices that minimize power loss due to reduced Ohmic heating.

This paper posits that the spin-orbit coupling inherent to the heavier iodine atoms in CrI$_3$ may robustly enhance the DM interaction, thus promoting the topological characteristics of magnons. The findings are supported by well-fitted models, which incorporate measured magnon dispersions and compare them against varying next-nearest neighbor interactions.

Conclusion and Future Directions

The paper advances our understanding of topological properties in two-dimensional honeycomb ferromagnets, with CrI$_3$ acting as a promising candidate for future experiments and potential devices. Moving forward, extended research into manipulating these topological magnons, perhaps by varying external conditions such as applied magnetic fields or by synthesizing heterostructures with other two-dimensional materials, could facilitate the practical realization of spintronic technologies. Moreover, probing similar topological features in other honeycomb lattices or expanding the paper to encompass other magnetic interactions could unravel further insights into topological magnonic behaviors. These developments might not only enhance spintronic functionalities but also enrich the fundamental understanding of quasiparticle dynamics in low-dimensional magnetic systems.