Time-dependent effects caused by the magnetic field in the Ni-Mn-Ga magnetic shape memory martensites

Abstract

Present paper is a review of published and new original results concerned on study the phenomenon of the time-dependent evolution of the magnetic-fled-induced strain under constant magnetic field at fixed temperature founded recently. The time dependent evolution of martensitic structure in the constant magnetic field, stability of martensite under the magnetic field, time-dependence of the magnetic field induced strain and magnetization in the Ni—Mn—Ga magnetic shape memory martensites are studied in single and polycrystals. An analogy in the time evolution of the magnetically and mechanically induced deformations in the stationary conditions is shown. Effects of temperature, the magnetic field magnitude and its direction are analyzed. Physical processes responsible for the time dependent evolution of martensitic structure and correspondently the magnetic field induced strain under stationary conditions are analyzed. Mechanisms of observed time dependent evolution of martensite structure and strain in a constant magnetic field are proposed. It is shown basing on the theory of random processes that thermal fluctuations of microstresses caused by the constant field is responsible for the time-dependent magneto-mechanical behavior of the Ni—Mn—Ga magnetic shape memory martensites. Activation of the new twinning systems and their evolution determines the long-time evolution of martensite in the stationary conditions in the case of application of the magnetic field to the hard or some arbitrary direction of magnetization.