Damped spin-wave excitations in the itinerant antiferromagnet $gamma$-Fe$_{0.7}$Mn$_{0.3}$

The collective spin-wave excitations in the antiferromagnetic state of $gamma$-Fe$_{0.7}$Mn$_{0.3}$ were investigated using the inelastic neutron scattering technique. The spin excitations remain isotropic up to the high excitation energy, ${hbaromega}= 78$ meV. The excitations gradually become broad and damped above 40 meV. The damping parameter ${gamma}$ reaches 110(16) meV at ${hbaromega} = 78$ meV, which is much larger than that for other metallic compounds, e.g., CaFe$_2$As$_2$ (24 meV), La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ ($52-72$ meV), and Mn$_{90}$Cu$_{10}$ (88 meV). In addition, the spin-wave dispersion shows a deviation from the relation $({hbaromega})^2 = c^2q^2 + {Delta}^2$ above 40 meV. The group velocity above this energy increases to 470(40) meV{AA}, which is higher than that at the low energies, $c = 226(5)$ meV{AA}. These results could suggest that the spin-wave excitations merge with the continuum of the individual particle-hole excitations at 40 meV.