The initial state of the experiment is depicted on fig. 12–14 However, the twinning or division of the crystal to ferroelastic domains increases the complexity of the magnetic domain pattern. This results in well-defined magnetic domain pattern in single variant state. 5,10,11 The Ni-Mn-Ga martensite, which is usually modulated, exhibits MIR and possesses large uniaxial magnetocrystalline anisotropy along crystallographic c-axis. The field for Type II can be less than 8 kA/m for Ni-Mn-Ga single crystal. Consequently the magnetic field needed for twin boundary motion is higher for Type I than for Type II. The level of stress in which twin boundary starts to move is called twinning stress and it is the order of magnitude higher for Type I boundary. The consideration of monoclinic lattice leads to complex twinning systems 5,7,8 with two types of mobile twin boundary called Type I and II differing by their symmetry 4,9 and more importantly by the mobility in magnetic field and under mechanical stress. 5,6 The modulated structure of martensite can be approximated by monoclinic lattice. Twinned microstructure occurs due to reversible martensitic transformation from high symmetry parent phase (austenite) to low-temperature low-symmetry phase (martensite), 4 however the origin of extreme high mobility of twin boundaries is not fully known.
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