Two-dimensional ferromagnetic spin-orbital excitations in honeycomb <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>VI</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math>
Harry Lane, Elise Pachoud, J. A. Rodriguez‐Rivera, M. Songvilay, Guangyong Xu, P. M. Gehring, J. Paul Attfield, R. A. Ewings, C. Stock
Abstract
${\mathrm{VI}}_{3}$ 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 ($J/{J}_{c}\ensuremath{\approx}17$) magnetic excitations in the ferromagnetic phase, finding two energetically gapped ($\mathrm{\ensuremath{\Delta}}\ensuremath{\approx}{k}_{B}{T}_{c}\ensuremath{\approx}55\phantom{\rule{0.28em}{0ex}}\mathrm{K}$) 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 ($J=\ensuremath{-}8.6\ifmmode\pm\else\textpm\fi{}0.3\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$) 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 ${\mathrm{VI}}_{3}$.