Magnetic Shape Memory Materials Research Articles

Crystal structures and magnetic anisotropy properties of Ni-Mn-Ga martensitic phases with giant magnetic-field-induced strain

Summary form only given. Magnetic shape memory materials are expected to have potential for a variety of actuating devices and sensors. Magnetic-field-induced rearrangement of the crystallographic domains (twin variants) can produce a large strain similar to a stress-induced one. We have found a giant magnetic field-induced strain approximately 10% at ambient temperature in a magnetic field less then 1 T in NiMnGa seven-layered martensitic phase. The strain is contributed by twin boundary motion which was confirmed by different experimental methods. From the analysis of X-ray diffraction data it was found that crystal structure of this phase is nearly orthorhombic having lattice parameters at ambient temperature a=0.619 nm, b=0.580 nm and c=0.553 nm (in cubic parent phase coordinates). The magnetic anisotropy properties of this phase were determined on the single-variant constrained samples using the magnetization curves M(H) recorded along [100], [010] and [001] directions. We demonstrate that low twinning stresses and a high level of magnetic anisotropy energy are the critical factors for the observation of a giant magnetic field induced strain.

Magnetic shape memory effect progress from idea to first actuators and sensors

The report will review the recent progress in studies of Magnetic Shape Memory (MSM) materials and latest developments of their applications. Magnetically controlled shape memory (MSM) materials are a new class of actuators and sensory materials. MSM materials can change their shape in a magnetic field applied due to the rearrangement of different martensite variant fractions. Inversely, magnetic field around the materials alters when proportions of the variants are changed by mechanical loading. The first effect can be utilized in actuators and the last one in sensors. Effect of high magnetic field on the ferrous-based shape memory alloys has been observed by Kakeshita et al. (1985) and Sadovsky (1986). A new method based on redistribution of twin martensite variants by low magnetic field as well as early models of magnetic-field-induced strains were suggested by Ullakko (1995, 1996) and James and Wuttig (1996). Magnetic-field-induced strains of about 0.2% in non-stoichiometric NiMnGa alloy was measured by Ullakko et al. (1996). NiMnGa was selected for that research, because it is a ferromagnetic shape memory alloy and low energy is required to drive twin boundaries by stress (Martynov and Kokorin, 1992). Magnetic-field-induced motion of a twinning dislocation that results in a shear strain was modeled by Muellner and Ullakko (1998). MSM effect was modeled by James and Wuttig (1998), O'Handley (1998) and Likhachev and Ullakko (1999, 2000). Field-induced strains of 4.5% after mechanical pre-loading were measured by James et al. (1999). Shear strains of 5% were observed by Murrey et al. (1999). The reversible elongation and contraction effect of over 5% in the rotating magnetic field was found by Ullakko, Sozinov and Yakovenko (1999). In this report recent experimental results including reversible linear and bending strains, magnetization measurements, twin band microstructure change, fatigue and frequency response of non-stoichiometric Ni-Mn-Ga alloys as well as a new MSM model will be discussed. Presentation of functioning MSM actuators and sensors with extension-compression and also bending actuating modes will be made.

Field-induced strain under load in Ni–Mn–Ga magnetic shape memory materials

Single-crystal Ni2MnGa shows a nearly 0.2% strain under a magnetic field of 8 kOe at −8 °C. Polycrystalline samples have been prepared near stoichiometry to study the composition dependence of the magnetic and elastic properties. A narrow band of compositions has been found having a range of Curie and martensitic transformation temperatures, Tc and T0, extending to above room temperature. The compressive stress–strain characteristics in variable transverse field were studied in samples selected to have T0 just below room temperature. Stress-induced martensite was observed as expected and the magnetic field was applied under fixed load for various stresses. A transverse field of 3200 Oe caused the sample to strain under load doing work that increased up to 1.3 J/kg with increasing volume fraction of stress-induced martensite.