Magnetic shape memory effect and highly mobile twin boundaries

Abstract

The magnetic shape memory effect can be classified as an example of the multiferroic effect combining ferro-elasticity and (ferro)magnetism. After short overview of all known effects the focus is on magnetic field induced structure reorientation (MIR) in the martensite of Ni–Mn–Ga. In this material giant deformations of up to 12 have been observed in moderate magnetic fields. The phenomenology of the effect is first discussed and a model presented. The properties of Ni–Mn–Ga relevant to MIR are then considered. One necessary condition for MIR is a highly mobile twin boundary or interface between two differently oriented martensite variants (ferroelastic domains). In 10M modulated martensite, two types of mobile twin boundary (type I and type II) are observed with complex layered microstructures consisting of a hierarchy of twinning systems. The boundaries strongly differ according to the magnitude and temperature dependence of the twinning stress. Finally, the nature of these boundaries and their different behaviour is reviewed.

Polar nature of stress-induced twin walls in ferroelastic CaTiO3

“…Twin walls (TWs) in ferroelastic materials have attracted special attention for several reasons. First, TWs are mobile 15,16 and controllable under stress, which can avoid the serious difficulties connected with the electric conductivity on manipulating ferroelectric domains. The mobile TWs could have large responses in physical properties.…”

Ab initiostudy of Ni2MnGa under shear deformation

“…Different twinning modes can arise in the martensite such as as a-c, a-b, or b-c twinning, as well as monoclinic twinning [24,25] forming the hierarchical twin microstructure [26]. Only reorientation due to motion of the a-c twin boundaries is relevant for the MFIS [27]. In addition two types of a-c twin boundaries with different mobility have been observed in 10M martensite.…”

Equivalence of Mechanical and Magnetic Force in Magnetic Shape Memory Effect

“…Magnetic shape memory (MSM) eect is a general name for a plethora of the multiferroic eects based on interplay between ferromagnetic order and ferroelasticity [1]. One of the MSM eects is magnetically induced structure reorientation (MIR) that leads to giant deformation (512%) in single phase, low-symmetry martensite [2,3].…”

“…These alloys are also the most investigated [1,5,6] as they are the most promising for applications [79]. In NiMnGa alloy, parental high-temperature cubic phase transforms to various martensites [1,10].…”