Two-dimensional ferromagnetic spin-orbital excitations in honeycomb VI₃

VI₃ is a ferromagnet with planar honeycomb sheets of bonded V³⁺ ions held together by van der Waals forces. We apply neutron spectroscopy to measure the two-dimensional (J/J_c ≈ 17) magnetic excitations in the ferromagnetic phase, finding two energetically gapped ( ≈ k_BT_c ≈ 55 K) and dispersive excitations. We apply a multilevel spin-wave formalism to describe the spectra in terms of two coexisting domains hosting differing V³⁺ orbital ground states built from contrasting distorted octahedral environments. This analysis fits a common nearest-neighbor in-plane exchange coupling (J = −4.4 ± 0.2 meV) between V³⁺ sites. The distorted local crystalline electric field combined with spin-orbit coupling provides the needed magnetic anisotropy for spatially long-ranged two-dimensional ferromagnetism in VI₃.