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Ferromagnetic Shape-Memory Alloys

Active materials are capable of developing mechanical work in response to a driving force. Piezoelectric ceramics and electrostrictors, for example, can strain in response to an applied electric field. Likewise, magnetostrictive materials will strain in response to an applied magnetic field and conventional shape-memory alloys will strain as a heat flow changes their temperature.

While piezo- and magnetostrictive materials can be driven at frequencies in excess of several kHz, the strains they produce rarely exceed 0.1%-0.3%. Conversely, conventional shape memory alloys can produce strains of the order of 1%, but to the extent that heat removal is a slow process typical actuation frequencies are limited to few Hz.

Fig. 1: Structure of austenitic Ni2MnGa (high temperature phase). Blue atoms represent Nickel, red atoms Manganese, and green atoms Gallium. The (202) planes are indicated. In the low temperature (martensitic) phase they become the twin planes.

Unlike these materials, ferromagnetic shape-memory alloys (FSMAs) are capable of producing strains exceeding 6% at frequencies in excess of 1 kHz. The potential for applications found in the combination of these unique characteristics prompted the extensive research of FSMAs, the best known of which is Ni—Mn—Ga (see Fig. 1), first shown to produce significant strains in 1996 by Ullakko and others at MIT. Recently, single-crystalline samples of this material with orthorhombic martensite structure have been shown to produce close to 10% field-induced strain. Fig. 2 shows a typical extension curve.

Fig. 2: Typical extension curve for a single crystal of Ni—Mn—Ga near stoichiometric Ni2MnGa. Typical features of this kind of measurement are apparent: threshold behaviour (fields in excess of 0.4T are required for actuation), saturation of the strain (limit given by the tetragonality of the crystal lattice) and jump-like motion (following twin boundary de-pinning).

The strain in Ni2MnGa is induced by twin boundary motion in the low-temperature, tetragonal (or orthorhombic) martensitic phase. It  can be accomplished by applying a magnetic field along fixed perpendicular crystal axes, as can be seen from the figure 3.

Fig. 3: (a) Austenite (high-temperature) phase, cubic. The material is assumed to be ferromagnetic already in this phase. (b) Martensite (low temperature phase), modulated tetragonal or orthorhombic. (c) A stress applied in a typical metal which deforms by dislocation motion. (d) The same stress applied on an FSMA induces twinning. (e) A magnetic field applied on a metal with little or no magnetocrystalline anisotropy. (f) In an FSMA the anisotropy is high, and the magnetic moments do not easily align with a field applied perpendicularly to the easy axis. Instead twinning is induced to create an magnetic easy axis (better) aligned with the field.

At EMPA we are striving to tap the application potential of Ni—Mn—Ga. The case at hand is a compact positioning device for the cantilever of an AFM. The concept developed is based on the device in the figure 4:

Fig. 4: Cross section of a crystal of NiMnGa placed inside two perpendicular coils. Depending on how these coils are connected in series, the same current produces a mainly horizontal or a mainly vertical field, providing the means to drive an FSMA.

Depending on how a set of two perpendicular coils is connected in series, fields in the horizontal or vertical direction can be created, which are able to extend and compress the crystal in the vertical direction respectively.

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