Hysteretic structural changes within five-layered modulated 10M martensite of Ni-Mn-Ga(-Fe)

Modulated structure of Ni-Mn-Ga-based alloys is decisive in their magnetic shape memory (MSM) functionality. However, the precise nature of their five-layered modulated 10M martensite is still an open question. We used x-ray and neutron diffraction experiments on single crystals to investigate structural changes within 10M-modulated martensite of the Ni₅₀Mn₂₇Ga₂₂Fe₁MSM alloy. The modulation vector gradually increases upon cooling from commensurateq= (2/5)g₁₁₀, whereg₁₁₀is the reciprocal lattice vector, to incommensurate withqup to pseudo-commensurateq= (3/7)g₁₁₀. Upon heating, reverse changes are observed with a thermal hysteresis of ≈60 K. The same hysteretic behaviour was detected in the electrical resistivity and the effective elastic modulus. Scanning electron microscopy showed that the changes are accompanied by the refinement of thea/blaminate. These observations indicate that the commensurate state is a metastable form of 10M martensite. Upon cooling, this phase evolves through nanotwinning into a more irregular and more stable incommensurate structure.