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

The collective spin-wave excitations in the antiferromagnetic state of gamma-Fe0.7Mn0.3 were investigated using the inelastic neutron-scattering technique. The spin excitations remain isotropic up to high excitation energy, h omega = 78 meV. The excitations gradually become broad and damped above 40 meV. The damping parameter gamma reaches 110(16) meV at h omega = 78 meV, which is much larger than that for other metallic compounds, e.g., CaFe2As2 (24 meV), La2-2x Sr1+2x Mn2O7 (52-72 meV), and Mn90Cu10 (88 meV). In addition, the spin-wave dispersion shows a deviation from the relation (h omega)(2) = c(2)q(2) + Delta(2) above 40 meV. The group velocity above this energy increases to 470(40) meV angstrom, which is higher than that at the low energies, c = 226(5) meV angstrom. These results could suggest that the spin-wave excitations mergewith the continuum of the individual particle-hole excitations at 40 meV.