Giant negative thermal expansion in Fe-Mn-Ga magnetic shape memory alloys

Fe-Mn-Ga magnetic shape memory alloys can undergo martensitic transformation (MT) from a paramagnetic cubic phase to a ferromagnetic tetragonal phase. The MT is accompanied by a large volume change; yet, these alloys have never been explored for technological applications as negative thermal expansion (NTE) materials. Here, by careful chemical modification, tunable NTE characteristics including wide operating temperature windows (ΔT) and large negative linear coefficients of thermal expansion (α_l) have been achieved in Fe_{44- x}Mn₂₈Ga_{28+ x} (x = 1, 2, and 2.5) alloys. Typically, a giant NTE ΔT of 81 K and α_l = -50.2 × 10^-6K^-1 were realized in the Fe₄₃Mn₂₈Ga₂₉ alloy upon cooling from 290 K. The relationships between the NTE features, the MT, and the substitution of Ga for Fe were discussed. Furthermore, the Fe-Mn-Ga alloys possess excellent mechanical properties, high electrical conductivity and high thermal conductivity. With these advantages, the Fe-Mn-Ga magnetic shape memory alloys show promising prospects for use as advanced NTE materials.