Structural and magnetic phase transitions in Mn-Ni alloys

When the Ni concentration exceeds about (Formula presented) Mn-Ni alloys were expected to support two different noncollinear spin-density wave (SDW) phases. A triple-(Formula presented) SDW with moments along the crystal diagonals was believed to appear in the fcc phase between (Formula presented) and (Formula presented) Below (Formula presented) the fct phase with (Formula presented) was believed to contain a double-(Formula presented) SDW with moments in the (Formula presented) plane and at (Formula presented) angles from the crystal axes. Based on resistivity, neutron-scattering, and susceptibility measurements, we show that the structural and magnetic phase transitions in a (Formula presented) alloy with (Formula presented) are actually distinct, with the structural phase transition at (Formula presented) lying far above the magnetic transition at (Formula presented) A Hamiltonian which includes elastic, magnetoelastic, and noncollinearity energies is used to describe these two transitions. In the tetragonal phase between (Formula presented) and (Formula presented) our model predicts a new SDW phase with moments tilted away from the crystal diagonals toward the (Formula presented) plane. The energy gap in the spin-wave spectrum is predicted to change discontinuously at (Formula presented).