Crystal structures and magnetization distributions in the field dependent ferromagnetic shape memory alloy Ni₅₄Fe₁₉Ga₂₇

The mesoscopic and microscopic mechanisms giving rise to shape memory behaviour in single crystals of the ferromagnetic alloy Ni₅₄Fe ₁₉Ga₂₇ have been investigated using polarized and unpolarized neutron diffraction. The measurements confirm that the Curie temperature T_C coincides with the martensitic phase transition at T_M ≤ 296K. At room temperature the crystal, as grown, had the tetragonal L1₀ structure with c/a≈1.20. It transformed to the cubic Heusler L2₁ structure at ≈330K. In subsequent heating and cooling cycles the transition took place at T_M≈295K and it was found that applying a magnetic field raised T_M by ≈0.3KT ^-1, making the material attractive for applications. The tetragonal structure has space group I4/mmm and is related to the cubic parent phase by a Bain transformation. The change in cell volume at the transition is only ≈1%, suggesting that the atomic moments are unchanged, although the magnetization drops significantly. The polarized neutron results show that in the cubic phase the magnetic electrons at the iron-rich sites have predominantly e_g symmetry (60(3)%), a distribution similar to that observed in Fe₃Al and Fe₃Si. A small transfer of magnetization from Fe to Ni is associated with the martensitic transition, but no significant redistribution of magnetic electrons between orbitals whose degeneracy is lifted, such as that predicated by the band Jahn-Teller mechanism, was observed.