Noncollinear two-dimensional triangular lattice antiferromagnets (2D TLAFs) are currently an area of very active research due to their unique magnetic properties, which lead to nontrivial quantum effects that experimentally manifest themselves in the spin excitation spectra. Recent examples of such insulating 2D TLAFs include $(\mathrm{Y},\mathrm{Lu}){\mathrm{MnO}}_3, {\mathrm{LiCrO}}_2$, and ${\mathrm{CuCrO}}_2$. Hexagonal ${\mathrm{LuFeO}}_3$ is a recently synthesized 2D TLAF which exhibits properties of an ideal multiferroic material, partially because of the high spin ($S=5/2$) and strong magnetic superexchange interactions. We report the full range of spin dynamics in a bulk single crystal of $\left({\mathrm{Lu}}_{0.6}{\mathrm{Sc}}_{0.4}\right){\mathrm{FeO}}_3$ (Sc doping to stabilize the hexagonal structure) measured via time-of-flight inelastic neutron scattering. Modeling with linear spin-wave theory yields a nearest-neighbor exchange coupling of $J=4.0\left(2\right)\mathrm{meV}$ (density functional theory calculations for $h-{\mathrm{LuFeO}}_3$ predicted a value of 6.31 meV) and anisotropy values of $K_D=0.17\left(1\right)\mathrm{meV}$ (easy plane) and $K_A=-0.05\left(1\right)\mathrm{meV}$ (local easy axis). It is observed that the magnon bandwidth of the spin-wave spectra is twice as large for $h-(\text{Lu},\mathrm{Sc}){\mathrm{FeO}}_3$ as it is for $h-{\mathrm{LuMnO}}_3$.

• Received 11 May 2018
• Revised 13 August 2018

DOI:https://doi.org/10.1103/PhysRevB.98.134412