Phenomenology of giant magnetic-field-induced strain in ferromagnetic shape-memory materials (invited)

Abstract

Ferromagnetic shape-memory alloys have recently emerged as a new class of active materials showing very large magnetic-field-induced extensional strains. Recently, a single crystal of a tetragonally distorted Heusler alloy in the NiMnGa system has shown a 5% shear strain at room temperature in a field of 4 kOe. The magnetic and crystallographic aspects of the twin-boundary motion responsible for this effect are described. Ferromagnetic shape-memory alloys strain by virtue of the motion of the boundaries separating adjacent twin variants. The twin-boundary motion is driven by the Zeeman energy difference between the adjacent twins due to their nearly orthogonal magnetic easy axes and large magnetocrystalline anisotropy. The twin boundary constitutes a nearly 90° domain wall. Essentially, twin-boundary motion shorts out the more difficult magnetization rotation process. The field and stress dependence of the strain are reasonably well accounted for by minimization of a simple free energy expression including Zeeman energy, magnetic anisotropy energy, internal elastic energy, and external stress. Models indicate the limits to the magnitude of the field-induced strain and point to the material parameters that make the effect possible. The field-induced strain in ferromagnetic shape-memory alloys is contrasted with the more familiar phenomenon of magnetostriction.

Martensitic transitions and the nature of ferromagnetism in the austenitic and martensitic states ofNi−Mn−Snalloys

“…5 The maximum achievable strain ⑀ max depends on the tetragonality c / a of the martensitic phase. 6 Depending on the composition of the Ni-Mn-Ga alloy, the martensitic phase occurs as various modulated phases such as 10M, 14M, or L1 0 . 7 Although Ni-Mn-Ga alloys offer large magnetic field-induced strains, they are brittle which makes it difficult for applications.…”

Magnetically Anisotropic Ni₂MnGa Thin Films: Coating Glass and Si Micro-Cantilevers Substrates

“…Such materials include NiMnGa [2] and other alloys [3][4][5][6][7][8] and are produced by a large variety of techniques, such as arc melting [9], melt-spinning [10,11], sputtering [12][13][14] and Pulsed Laser Deposition (PLD) [15][16][17].…”

Constitutive modelling of magnetic shape memory alloys with discrete and continuous symmetries

“…The macroscopically observable large deformations in magnetic shape memory alloys (MSMAs) are caused by the microstructural reorientation of martensitic variants [1], field-induced phase transformation (FIPT) [2] or by the combination of the two mechanisms. In the variant reorientation mechanism, the variants have different preferred directions of magnetization, and the magnetic field is applied to select specific variants, which results in the macroscopic shape change.…”