Magnetoelastic effects and spin excitations in γ - Mn alloys

A two-band model which includes the magnetoelastic interaction is used to study the magnetization and spin dynamics of γ-Mn alloys. As previously believed, single (S), double (D), and triple (T) spin-density wave (SDW) states are found in fct (c < a and c > a) and fcc (c = a) lattices, respectively. When the magnetoelastic coupling constant k exceeds the critical value k_c, both the structural and magnetic phase transitions become first order. This critical value drops to zero at the triple point, where the commensurate and incommensurate SDW phase boundaries meet. In agreement with experiments on fct MnNi and fcc FeMn alloys, we find that the gap Δ_sw(T) in the spin-wave dispersion is proportional to the 3/2 power of the sublattice magnetization M(T). For the noncollinear D and T SDW magnetic phases observed in MnNi and FeMn alloys, we find an additional class of collective modes. This class includes a Goldstone mode which is produced by the modified dispersion of holes not directly involved in the SDW. We also find high-frequency excitations with energies of order Δ, where 2Δ≈2 eV is the energy gap in the quasiparticle spectrum. Although these incoherent excitations have the same frequencies in the D and T SDW phases, their neutron-scattering cross sections should be 33% larger in the TSDW phase.