Fig. 3. Magnetic field dependence of propagation character for various spin excitation modes.

a–c Experimentally obtained magnetic field dependence of (a) magnetic resonance frequency ν₀, (b) frequency shift Δν between ±k^SW, and (c) magnitude of group velocity ∣v_g∣ at 25 K. It is noteworthy that the data for the conical spin state is taken in the H-increasing run after the zero field cooling (Supplementary Fig. 1). d–f The corresponding data calculated from the theoretical dispersion relation. In d, the k^SW → 0 values in the bulk and magnetostatic limit are plotted as dashed and solid lines, respectively, whose difference δν_d provides an estimate of the group velocity v_g ≈ 2πb(δν_d) shown in f with b being the sample thickness. In e, the non-reciprocal frequency shift Δν is shown computed for ∣k^SW∣ = 0.5 μm⁻¹.