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 ${\Delta }_{\mathrm{sw}}\mathrm{}\left(T\right)\mathrm{}$ in the spin-wave dispersion is proportional to the 3/2 power of the sublattice magnetization $M\left(T\right),$ 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 $\omega (\overrightarrow{q}=0)\sim \Delta ,$ where 2Δ is the energy gap in the quasiparticle spectrum.