Two-dimensional ferromagnetic spin-orbital excitations in the honeycomb VI$_{3}$

TL;DR

This study investigates the magnetic excitations in the 2D ferromagnet VI$_{3}$ using neutron spectroscopy, revealing gapped, dispersive modes and elucidating the role of orbital states and spin-orbit coupling in its magnetic properties.

Contribution

The paper provides the first detailed neutron spectroscopy analysis of VI$_{3}$, demonstrating the coexistence of orbital ground states and quantifying the exchange interactions and anisotropy mechanisms.

Findings

Identified two gapped, dispersive magnetic excitations in VI$_{3}$.

Quantified the in-plane exchange coupling as approximately -8.6 meV.

Showed that spin-orbit coupling and crystal field effects enable 2D ferromagnetism.

Abstract

VI$_{3}$ is a ferromagnet with planar honeycomb sheets of bonded 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 ($\Delta \approx k_{B} T_{c} \approx$ 55 K) and dispersive excitations. We apply a multi-level spin wave formalism to describe the spectra in terms of two coexisting domains hosting differing 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 meV) between 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 VI$_{3}$.