A two-band model which includes the magnetoelastic interaction is used to s
tudy the magnetization and spin dynamics of gamma-Mn alloys. As previously
believed, single (S), double (D), and triple (T) spin-density wave (SDW) st
ates are found in fct (c < a and c > a) and fee (c = a) lattices, respectiv
ely. When the magnetoelastic coupling constant kappa exceeds the critical v
alue kappa(c), both the structural and magnetic phase transitions become fi
rst order. This critical value drops to zero at the triple point, where the
commensurate and incommensurate SDW phase boundaries meet. In agreement wi
th experiments on fct MnNi and fee FeMn alloys, we find that the gap Delta(
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 ph
ases observed in MnNi and FeMn alloys, we find an additional class of colle
ctive 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 Delta, where 2 Delta app
roximate to 2 eV is the energy gap in the quasiparticle spectrum. Although
these incoherent excitations have the same frequencies in the D and T SDW p
hases, their neutron-scattering cross sections should be 33% larger in the
TSDW phase. [S0163-1829(99)05413-2].