Composition dependence on the martensitic structures of the Mn-rich NiMnGa alloys

The Mn-rich Ni₅₀Mn_25+xGa_25-x (x=0-5) alloys were developed to investigate the structural transitions and magnetic properties. Structural transitions from austenite to 5M, 7M, and non-modulated martensite were observed with the increase of Mn content. The lattice parameter a elongates, as where b and c contract, and the unit cell volume reduces with increasing Mn content. The martensitic transformation start temperatures M _s increase monotonically from 10.7°C for x=2 to 102.7°C for x=5. The saturation magnetization was measured at 5 K, where all the samples exhibit a martensitic structure. The average magnetic moments per Mn atom vary from 4.38 μ_B to 2.93 μ_B for x=0 to x=5. The negative effect of excess Mn atoms changes from -3.00 μ_B for x=2 to -7.25 μ_B for x=5. The excess Mn atoms modify the electronic structures of the unsubstituted Mn atoms, resulting in the sharp decrease of the magnetic moments of the unsubstituted Mn atoms with increasing Mn content. Structural incommensurability was observed with 7M for powder and non-modulated for bulk samper in a specific range of compositions and proved to be reversible when performing martensitic transformation. The 7M and non-modulated martensites Ni₅₀Mn₃₀Ga₂₀ possess similar saturation magnetizations and Curie temperatures. The non-modulated martensite was estimated to have a lower free energy than 7M, and should be more stable for a reverse martensitic transformation, leading to a higher austenite start temperature A_s, which is consistent with the experimental result.