Damped spin-wave excitations in the itinerant antiferromagnet γ−Fe0.7Mn0.3

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