${\mathrm{VI}}_3$ is a ferromagnet with planar honeycomb sheets of bonded ${\mathrm{V}}^{3+}$ ions held together by van der Waals forces. We apply neutron spectroscopy to measure the two-dimensional ($J/J_c\approx 17$) magnetic excitations in the ferromagnetic phase, finding two energetically gapped ($\mathrm{\Delta }\approx k_B{T}_c\approx 55\mathrm{K}$) and dispersive excitations. We apply a multilevel spin-wave formalism to describe the spectra in terms of two coexisting domains hosting differing ${\mathrm{V}}^{3+}$ orbital ground states built from contrasting distorted octahedral environments. This analysis fits a common nearest-neighbor in-plane exchange coupling ($J=-8.6\pm 0.3\mathrm{meV}$) between ${\mathrm{V}}^{3+}$ 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 ${\mathrm{VI}}_3$.

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• Ferromagnetism
• Magnetic interactions
• Magnetism
• Spin dynamics
• 2-dimensional systems
• Ferromagnets
• van der Waals systems
• Time-of-flight neutron spectroscopy