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

Abstract

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, $\hslash \omega =78$ 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., 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 $(\hslash \omega {)}^2=c^2{q}^2+{\Delta }^2$ above 40 meV. The group velocity above this energy increases to 470(40) meVÅ, which is higher than that at the low energies, $c=226(5)$ meVÅ. These results could suggest that the spin-wave excitations merge with the continuum of the individual particle-hole excitations at 40 meV.

7 pages, 6 figures