Origin of the time-dependent magnetoplasticity in the Ni–Mn–Ga magnetic shape memory martensites

The present paper is a review of published and new original results concerning the phenomenon of time-dependent magnetoplasticity under constant magnetic field at fixed temperature in Ni–Mn–Ga magnetic shape memory martensites. The kinetics and origin of magnetic field-induced strain and the evolution of the martensite structure under stationary conditions is studied in Ni–Mn–Ga. The main external factors affecting the time-dependent response of the martensite structure and the corresponding strain to the fixed magnetic field are presented in relation to magnetic field magnitude, temperature, rate of field application, and crystallographic direction of the external field. The mechanisms of the time-evolution of the martensitic structure responsible for the change in strain are disclosed, revealing the responsible physical processes related to twin boundary obstacles, activation and movement of new twinning systems, and elastic deformation caused by the magnetic field. Experimental results are discussed in relation to the theoretical model of the time-dependent fluctuation of internal microstress relaxation.