Topological Spin Excitations in Honeycomb Ferromagnet CrI3

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 spinwave excitations. Using inelastic neutron scattering, we find that spin waves of the insulating honeycomb ferromagnet CrI3 (TC = 61 K) have two distinctive bands of ferromagnetic excitations separated by a ∼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 CrI3 can have robust topological properties potentially useful for dissipationless spintronic applications.

Orbitally-resolved ferromagnetism of monolayer CrI3

“…the spin stiffness, being one of the basic characteristics of low-energy magnetic excitations [62,63]. Assuming quadratic form of the spin-wave spectrum at q → 0: (q) ≈ ∆ + D S q 2 , the spin stiffness constant can be estimated as D V =0 s = 17 meV · Å 2 and D V =1 s = 26 meV · Å 2 , which is in reasonable agreement with previous assessments [64]. One can see that although there is almost no effect of bias voltage on T C , the spin stiffness constant can be increased significantly in the presence of vertical electric field.…”

Non-Hermitian topology of spontaneous magnon decay

“…A few possible examples of magnetic materials with linear touching points in the magnon spectrum that may also exhibit significant magnon interaction effects include the quasi-2D honeycomb materials CrBr 3 [62,66,67] and CrI 3 [68], the 3D antiferromagnet Cu 3 TeO 6 [69] and the possible Weyl magnon system Lu 2 V 2 O 7 [70,71]. Another potentially interesting case is the kagome ferromagnet Cu(1,3-bdc) [41] that is thought to have significant anti-symmetric exchange couplings.…”

“…Thus by tuning the field the van-Hove singularity can be pushed closer to the Dirac touching, further enhancing the effect of magnon decay. Given the DM interaction is typically subdominant in transition metal magnets [72], such as Lu 2 V 2 O 7 [71] or the CrX 3 family [66][67][68], a protocol such as this potentially presents a practical experimental route to controlling the effects of magnon decay, allowing full exploration of the non-Hermitian physics.…”

Dirac Magnons in a Honeycomb Lattice Quantum XY Magnet CoTiO3

“…Remarkably, the concept of Dirac particles is not limited to electrons or other fermionic quasiparticles, prompting a search for analogues in photonic crystals [7,8], acoustic metamaterials [9], and quantum magnets [10][11][12][13]. In particular, Dirac magnons, or more broadly defined topological magnons [14][15][16][17][18][19], have attracted much attention as platforms to investigate the effect of inter-particle interaction or external perturbations on Dirac bosons, and are proposed to be of potential interest in spintronic applications.In contrast to light and sound, the symmetry broken states and emergent bosonic excitations of quantum magnets depend crucially on dimensionality and spin symmetry, which provides a fertile playground for examining the physics of topological bosons. To date, gapped topological magnons in Ising-like ferromagnets have been reported in a kagome lattice material Cu(1,3bdc) [16] and in a layered honeycomb magnet CrI 3 [17].…”