The isothermal time-dependent deformation of the ferromagnetic Ni–Mn–Ga martensite in the steady magnetic field or under the stationary mechanical loading is studied in both experimental and theoretical way. A concomitant time-evolution of twin structure is analyzed in order to clarify the physical mechanisms of time-dependent phenomena in the Ni–Mn–Ga martensites exhibiting a magnetic shape-memory. It was shown very recently (V. L’vov et al., Phys. Rev., B, 71, 024421 (2005)), that the time dependent magnetically-induced deformation is originated by the fluctuating mechanical stress. As the next step, the role of fluctuating strains in the wide scope of time-dependent effects is disclosed in the present report. The thermal phonons, which induce the fluctuating strain are considered. A simple relationship between the magnitude of fluctuating strain, temperature-dependent elastic modulus and twin width is derived. The temperature dependence of the fluctuating strain in vicinity of martensitic transformation is derived. A strong interdependence between the twin width and the effectiveness of the affect of thermal phonons on the twin structure is predicted theoretically and supported by the experimental data. The problem of reversibility of magnetically/stress-induced deformation is considered on the base of obtained experimental and theoretical results.

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1. Temperature dependence of magnetically induced deformation of Ni–Mn–Ga martensite
2. Deformation of Ni–Mn–Ga martensite under the conditions of ultrasonic resonance
3. DEPENDENCE OF THE MAGNETIC ANISOTROPY ON THE THICKNESS AND ANNEALING OF SPUTTER-DEPOSITED Ni-Mn-Ga THIN FILMS