Nonuniform twin-boundary motion in Ni–Mn–Ga single crystals

The twin-boundary motion in Ni–Mn–Ga single crystals has been studied with 620 μs magnetic-field pulses on samples of $5 mm×5 mm$ cross section and 10 mm in length. A sequence of pulses applied on a single variant induces an extension. It is close to linear with the pulse’s peak generated driving forces, for peaks in excess of $0.68×Ku,$ where $Ku≃1.9×105 J/m3$ is the magnetocrystalline anisotropy. The extension is caused by the motion of a few distinct twin boundaries, which exhibit differing driving-force thresholds for motion between 1.3 and $1.8×105 J/m3.$ Pulses applied without restoring the single-variant initial condition, cause discrete twin boundaries to expand in a stochastic manner consistent with the existence of obstacles to their motion. The number of obstacles is broadly distributed in pinning strengths, except for a peak near $0.56×Ku≃1.06×105 J/m3.$