Magnetic excitations in the bulk multiferroic two-dimensional triangular lattice antiferromagnet (Lu,Sc)FeO3

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 (Y,Lu)MnO3, LiCrO2, and CuCrO2. Hexagonal LuFeO3 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 (Lu0.6Sc0.4)FeO3 (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(2)meV (density functional theory calculations for h-LuFeO3 predicted a value of 6.31 meV) and anisotropy values of KD=0.17(1)meV (easy plane) and KA=-0.05(1)meV (local easy axis). It is observed that the magnon bandwidth of the spin-wave spectra is twice as large for h-(Lu,Sc)FeO3 as it is for h-LuMnO3.