Spin-wave gap and spin dynamics of γ-Mn alloys

The magnetic phase diagram of {γ}-Mn alloys contains both collinear and noncollinear magnetic phases in fct and fcc crystal structures. Using a two-band model which incorporates the magnetoelastic coupling, we find that the gap Δsw(T) in the spin-wave dispersion is proportional to the 3/2 power of the sublattice magnetization $M(T),$ in agreement with experiments on both the collinear and noncollinear magnetic phases. For the noncollinear magnetic phases observed in MnNi and FeMn alloys, high-frequency excitations are predicted with ω(→q=0)~Δ, where 2{Δ} is the energy gap in the quasiparticle spectrum.