Structural and magnetic phase transitions in Mn-Ni alloys

When the Ni concentration exceeds about $18%,$ Mn-Ni alloys were expected to support two different noncollinear spin-density wave (SDW) phases. A triple-Q SDW with moments along the crystal diagonals was believed to appear in the fcc phase between TN and Tₜ. Below Tₜ, the fct phase with $c>a$ was believed to contain a double-Q SDW with moments in the ab plane and at 45^∘ angles from the crystal axes. Based on resistivity, neutron-scattering, and susceptibility measurements, we show that the structural and magnetic phase transitions in a Mn_1-xNiₓ alloy with x≈0.20 are actually distinct, with the structural phase transition at Tₜ≈250 K lying far above the magnetic transition at Tₘ≈125 K. A Hamiltonian which includes elastic, magnetoelastic, and noncollinearity energies is used to describe these two transitions. In the tetragonal phase between Tₜ and Tₘ, our model predicts a new SDW phase with moments tilted away from the crystal diagonals toward the ab plane. The energy gap in the spin-wave spectrum is predicted to change discontinuously at Tₘ.