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

TLDR

Ferromagnetic shape‑memory alloys exhibit large magnetic‑field‑induced strains through twin‑boundary motion, acting as nearly 90° domain walls, and differ from conventional magnetostriction. The study describes the magnetic and crystallographic mechanisms of twin‑boundary motion underlying the giant strain. Twin‑boundary motion is driven by Zeeman energy differences between nearly orthogonal easy axes, shorting magnetization rotation, and the strain’s field and stress dependence is captured by a free‑energy model incorporating Zeeman, anisotropy, elastic, and stress terms. A NiMnGa Heusler single crystal achieved 5 % shear strain at room temperature under 4 kOe, and modeling identifies strain limits and key material parameters.

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.