Magnetic Domain Structure and Magnetically-Induced Reorientation in Ni-Mn-Ga Magnetic Shape Memory Alloy

Abstract

Magnetization process during magnetically induced reorientation and related magnetic domains of cuboid Ni50Mn28.5Ga21.5 single crystal with {100} faces was investigated. Magnetic domains were visualized using magneto-optical indicator. The domains pattern is determined by strong uniaxial magnetic anisotropy of NiMn-Ga martensite. Thanks to magnetically induced reorientation the domains arrangements for all three crystal orientations could be obtained and we showed that the size of domains scales with square root of the thickness and penetrates through whole crystal. Visualization of magnetic domains on all faces of cuboid provides the 3D model of magnetic domains.

Microstructural Model of Magnetic and Deformation Behavior of Single Crystals and Polycrystals of Ferromagnetic Shape Memory Alloy

“…The obtained magnetic structure is the initial structure to which the magnetic field is applied. Then, applying an external magnetic field in the direction of sector c 1 (see Figure 3) in accordance with the experiments described in the previous section [32][33][34][35], we determine the critical value of mass magnetic moment L cr mag at which the detwinning process is realized (Problem 2). By solving this problem we will be able to describe the movement of the domain walls, the rotation of the magnetization vector, and the detwinning process that will allow us to construct the magnetization curves and define the deformation states under the action of external magnetic field H 0 , which is applied to the computational domain shown in Figure 3 at different angles to the y axis in the (x, y) plane (Problem 3).…”

“…The disappearance of the twin structure, which complements the processes of movement and interaction of the magnetic domain walls and rotation of the magnetization vectors, takes place only in a shape memory ferromagnetic alloy as opposed to the last two processes. To define this average critical value L cr mag , we used data from experimental works [32][33][34][35], in which the Ni-Mn-Ga alloys close to stoichiometric Ni 2 MnGa are studied. A single-crystal prismatic sample was cut in the martensitic state from a material that experienced a direct phase transition of the first kind caused by cooling in the absence of the magnetic and force fields.…”

“…In the next section, following any of these algorithms, we will find a specific value of |L| cr mag corresponding to the above-mentioned experiments [32][33][34][35]. This critical value is used to determine the beginning of the detwinning process in the same material, but when the external magnetic field is applied in different directions relative to the c axis of each variant of martensite forming the twin.…”

“…The dependence of Lmag on H0 , shown in Figure 3 for angle φ = 45.84 • , allows us to solve Problem 2 formulated in Section 4.2 and determine the critical value of mass magnetic moment Lcr mag , at which the detwinning process is realized. Here, the vector of external magnetic field H 0 acts along the axis of easy magnetization c 1 of one of the elements of the twin, located in the central region of Figure 3, the same as in experiments [32][33][34][35], in which the magnetization curve is constructed taking into account the detwinning process.…”

“…However, due to a very small value of Lmag in the central region, detwinning will occur in accordance with the first case, when the mass magnetic moment in the peripheral area reaches a critical value | L| cr mag . As follows from the experiments presented in works [32][33][34][35], detwinning begins when the external magnetic field reaches value µ 0 |H 0 | = 0.3 ÷ 0.5 T. For the smallest value µ 0 |H 0 | = 0.3, to which Hcr 0 = 0.41 corresponds, from Figure 4 we find that for angle φ = 45.84 4, and the obtained critical value of the mass magnetic moment allow us to conclude that there is no detwinning until φ is less than ≈ 40 • . In the area of (≈ 40 • ) ≤ φ ≤ (≈ 89 • ), detwinning occurs in accordance with the first case.…”