Structural and magnetic phase transitions in Mn-Ni alloys

Abstract

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 ${T}_{N}$ and ${T}_{t}.$ Below ${T}_{t},$ the fct phase with $c>a$ was believed to contain a double-$Q$ SDW with moments in the $\mathrm{ab}$ plane and at $45\ifmmode^\circ\else\textdegree\fi{}$ angles from the crystal axes. Based on resistivity, neutron-scattering, and susceptibility measurements, we show that the structural and magnetic phase transitions in a ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}$ alloy with $x\ensuremath{\approx}0.20$ are actually distinct, with the structural phase transition at ${T}_{t}\ensuremath{\approx}250 \mathrm{K}$ lying far above the magnetic transition at ${T}_{m}\ensuremath{\approx}125 \mathrm{K}.$ A Hamiltonian which includes elastic, magnetoelastic, and noncollinearity energies is used to describe these two transitions. In the tetragonal phase between ${T}_{t}$ and ${T}_{m},$ our model predicts a new SDW phase with moments tilted away from the crystal diagonals toward the $\mathrm{ab}$ plane. The energy gap in the spin-wave spectrum is predicted to change discontinuously at ${T}_{m}.$