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

Abstract

The mesoscopic and microscopic mechanisms giving rise to shape memory behaviour in single crystals of the ferromagnetic alloy Ni 54 Fe 19 Ga 27 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 = 296 K. At room temperature the crystal, as grown, had the tetragonal L1 0 structure with c/a ≈ 1.20. It transformed to the cubic Heusler L2 1 structure at ≈330 K. In subsequent heating and cooling cycles the transition took place at T M ≈ 295 K and it was found that applying a magnetic field raised T M by ≈0.3 K T −1 , making the material attractive for applications. The tetragonal structure has space group I 4/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 Fe 3 Al and Fe 3 Si. 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.

Thermodynamic, kinetic, and magnetic properties of aNi54Fe19Ga27magnetic shape-memory single crystal

“…17 In the nonstoichiometric compound examined here, the positions not occupied by Fe are expected to be mostly filled by Ni. In a simplified local picture, however, it is not clear if the Ni moments are coupled parallel or antiparallel to the Fe moments.…”

Magnetic shape memory behaviour

“…Under applied stress [35], the sevenfold martensitic structure transforms to a tetragonal L1 0 phase with a ¼ 3.81 and c ¼ 3.27 Å . Neutron diffraction measurements [36] on a single crystal with residual stress have shown that the alloy transforms from the Heusler structure to tetragonal structure with space group I4/mmm at 298 K; the distortion of the unit cell c/a$1.2 is one of the largest yet found in a ferromagnetic shape memory alloy. Furthermore, neutron single-crystal diffraction measurements in a magnetic field show that T M increases by 0.3 K/T as shown in Fig.…”

“…In the cubic phase of Ni 54 Fe 19 Ga 27 [36], the fraction g of magnetic electrons in orbitals with e g symmetry is 0.60(3) at the Fe (A) sites and 0.55(10) at the Ni (C) sites. The A site value is significantly greater than that found for Ni 2 MnGa in which Mn occupies the A sites.…”

Elastocaloric effect in Ni50Fe19Ga27Co4 single crystals

“…Ni-Fe-Ga-based SMAs are new type of Heusler alloys whose mechanical behaviors are greatly improved through replacing Mn element in the classical Ni-Mn-Ga ferromagnetic SMA [9]. The thermal-induced MTs in these SMAs have been systematically investigated, which undergo the typical first-order MTs from the body-centered cubic B2 or Heusler L2 1 structure (parent phase) to the non-modulated L1 0 or modulated five-, six-, seven-layered structures (martensite phase) [10,11]. The transformation temperatures can be controlled in a wide temperature range as wide as 160-400 K by substituting Co for Fe [12,13].…”