MSM Mechanism
Magnetic shape memory (MSM) material is a new way to produce motion and force. The quantitative model of the MSM mechanism and experimental results of large magnetic-field-induced strains of Ni-Mn-Ga were presented in several articles.
MSM material consists of internal areas, twin variants (see Fig. 1). These variants have different magnetic and crystallographic orientations. When the actuating element made of MSM material is subjected to a magnetic field the proportions of the variants change resulting in the shape change of the element.
Figure 1 shows a schematic presentation of the MSM effect in Ni-Mn-Ga. Applying the magnetic field H to the single variant material causes the other twin variants to appear and grow. When magnetic field strength increases the boundaries between twins move, as amount of preferentially oriented twin variants grow at the expense of the other twin variants. Unit cells of the martensite phase in which MSM effect occurs in Ni-Mn-Ga alloys are tetragonal.
The inset on the left in Fig. 1 presents a unit cell of one twin variant. The c axis, i.e., the shorter lattice axis of the twin variant is aligned along the long axis of the actuating element. In the other twin variant, the c axis is perpendicular to the previous variant's c axis (right inset). C axis is the easy direction of magnetization of the material. MSM effect is possible if magnetocrystalline anisotropy energy of the material is high enough so as to "turn" the c-orientation of the unit cell along the magnetic field direction. As a result of the interaction between the magnetic field and twins, the length of the sample increases by the amount of the ratio a/c.
Measured magnetic-induced-strains of the MSM material already reach 10 %. This is significantly more than in any other fast responding actuator material. MSM materials research and development is going on in many research laboratories around the world. AdaptaMat is the first company that develops and manufacturers MSM materials and actuators and it owns patent rights to MSM technology.
Figure 1. The left bar represents a sample in zero field consisting entirely of single martensite twin variant with magnetization axis along the yellow short line. These twin variants are described by grey colour. As a transverse magnetic field H1 is applied to the sample (middle), orthogonal twin variants with a magnetization axis in a preferential direction to H appear, described by blue colour. When the field is further increased to H2 (right), the preferentially oriented twins (blue) continue to grow on the expense of other twin variants (grey), resulting to expansion in the vertical direction.
An advantage of MSM mechanism is that depending of its microstructure, a single MSM element can produce various modes of deformation, such as linear strain or bending (Fig. 2).
Figure 2. Different forms of MSM shape change.