Magnetic domains in martensite of the Ni-Mn-Ga alloy

Temperature dependence of magnetic susceptibility in the vicinity of martensitic transformation in ferromagnetic shape memory alloys

“…At the MT the character of magnetic anisotropy typically changes from cubic (austenite) to a uniaxial (martensite) one and this manifests itself in an abrupt magnetic susceptibility (χ ) decrease. For different martensitic materials the observations of such jumps on temperature dependences of the magnetic susceptibility have been reported in numerous papers [5][6][7][8][9][10][11][12][13][14][15]. Qualitatively, the susceptibility decreases due to a sharp increase of the anisotropy constant at the transition.…”

“…It is well known that low-field quasistatic (low frequency) magnetic susceptibility is caused by domain walls' displacements under an applied ac magnetic field. In general, the susceptibility is strongly affected by magnetic domain wall (DW) pinning by different types of sample defects: grains [30], twin and antiphase boundaries [11,23,31,32], nano/microinclusions [30,33] and atomic lattice defects [34]. The strongest pinning, and therefore the smallest susceptibility, are caused by defects whose mean size is comparable with the domain wall width [30,33,35,36].…”

“…It is important to notice that (11) is valid regardless of which temperature dependence of the saturation magnetization is used. Let us now discuss (11) and its consequences. In the limiting case of a drastic anisotropy increase at the transition, i.e.…”

“…In the limiting case of a drastic anisotropy increase at the transition, i.e. k → ∞, (11) gives the maximal possible value of the jump: χ/χ a = 1 for any values of L m /L a and D a /D m . According to (11) the two ratios-L m /L a and D a /D m -also determine the actual jump of susceptibility, the first one being related to the variants' appearance at the MT.…”

“…k → ∞, (11) gives the maximal possible value of the jump: χ/χ a = 1 for any values of L m /L a and D a /D m . According to (11) the two ratios-L m /L a and D a /D m -also determine the actual jump of susceptibility, the first one being related to the variants' appearance at the MT. Indeed, the twin boundaries which appeared serve as pinning sources restricting the length of a domain wall which could be elastically shifted by an applied ac magnetic field, see figure 2.…”