Simple rules for the understanding of Heusler compounds

Large magnetic-field-induced strains1 have been observed in Heusler alloys with a body-centred cubic ordered structure and have been explained by the rearrangement of martensite structural variants due to an external magnetic field1,2,3. These materials have attracted considerable attention as potential magnetic actuator materials. Here we report the magnetic-field-induced shape recovery of a compressively deformed NiCoMnIn alloy. Stresses of over 100 MPa are generated in the material on the application of a magnetic field of 70 kOe; such stress levels are approximately 50 times larger than that generated in a previous ferromagnetic shape-memory alloy4. We observed 3 per cent deformation and almost full recovery of the original shape of the alloy. We attribute this deformation behaviour to a reverse transformation from the antiferromagnetic (or paramagnetic) martensitic to the ferromagnetic parent phase at 298 K in the Ni45Co5Mn36.7In13.3 single crystal.

Magnetic-field-induced shape recovery by reverse phase transformation.

Giant magnetic-field-induced strain of about 9.5% was observed at ambient temperature in a magnetic field of less than 1 T in NiMnGa orthorhombic seven-layered martensitic phase. The strain proved to be caused by magnetic-field-controlled twin boundary motion. According to an analysis of x-ray diffraction data, the crystal structure of this phase is nearly orthorhombic, having lattice parameters a=0.619 nm, b=0.580 nm, and c=0.553 nm (in cubic parent phase coordinates) at ambient temperature. Seven-layer shuffling-type modulation along the (110)[110]p system was recorded. The results of mechanical tests and magnetic anisotropy property measurements are also reported.

Giant magnetic-field-induced strain in NiMnGa seven-layered martensitic phase

Field-induced strains of 6% are reported in ferromagnetic Ni–Mn–Ga martensites at room temperature. The strains are the result of twin boundary motion driven largely by the Zeeman energy difference across the twin boundary. The strain measured parallel to the applied magnetic field is negative in the sample/field geometry used here. The strain saturates in fields of order 400 kA/m and is blocked by a compressive stress of order 2 MPa applied orthogonal to the magnetic field. The strain versus field curves exhibit appreciable hysteresis associated with the motion of the twin boundaries. A simple model accounts quantitatively for the dependence of strain on magnetic field and external stress using as input parameters only measured quantities.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1132&context=ameslab_pubs

6% magnetic-field-induced strain by twin-boundary motion in ferromagnetic Ni–Mn–Ga

The increasing understanding of the advantages offered by fish and insect-like locomotion is creating a demand for muscle-like materials capable of mimicking nature's mechanisms. Actuator materials that employ voltage, field, light, or temperature driven dimensional changes to produce forces and displacements are suggesting new approaches to propulsion and maneuverability. Fundamental properties of these new materials are presented, and examples of potential undersea applications are examined in order to assist those involved in device design and in actuator research to evaluate the current status and the developing potential of these artificial muscle technologies. Technologies described are based on newly explored materials developed over the past decade, and also on older materials whose properties are not widely known. The materials are dielectric elastomers, ferroelectric polymers, liquid crystal elastomers, thermal and ferroelectric shape memory alloys, ionic polymer/metal composites, conducting polymers, and carbon nanotubes. Relative merits and challenges associated with the artificial muscle technologies are elucidated in two case studies. A summary table provides a quick guide to all technologies that are discussed.

Artificial muscle technology: physical principles and naval prospects

Magnetic-field-controlled twin boundaries motion and giant magneto-mechanical effects in Ni–Mn–Ga shape memory alloy

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.

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

Starting from the realistic experimental picture of twin and magnetic domain microstructure we represent a consistent model of magnetic shape memory taking into account all the most important energy contributions. We also perform a detailed analysis of some other models and discuss their quantitative applicability to some experimental results obtained for Ni–Mn–Ga. Performing mechanical testing experiments in all the different martensitic phases of Ni–Mn–Ga under the constant magnetic field applied perpendicular to the load direction we show that such a magnetic field can dramatically modify standard zero field strain stress relationships of magnetic shape memory alloys (MSMAs) like Ni–Mn–Ga. In a particular case of 5 and 7M martensites we observe a so-called pseudo-elastic or rubber-like behavior during the standard compression–decompression cycling under the field at about 1 T. This effect is finally discussed from the point of our modeling concept.

Different modeling concepts of magnetic shape memory and their comparison with some experimental results obtained in Ni-Mn-Ga

A quantitative model describing large magnetostrain effect observed in several ferromagnetic shape memory alloys such as Ni2MnGa is briefly reported.The paper contains an exact thermodynamic consideration of the mechanical and magnetic properties for a similar type materials. As a result, the basic mechanical state equation including magnetic field effect is directly derived from a general Poisson's rule. It is shown that the magnetic field induced deformation effect is directly connected with the strain dependence of magnetization. A simple model of magnetization and its dependence on the strain is considered and applied to explain the results of experimental study of large magnetostrain effects in Ni2MnGa.

A.A. Likhachev

Papers

Giant magnetic-field-induced strain in NiMnGa seven-layered martensitic phase

Magnetic-field-controlled twin boundaries motion and giant magneto-mechanical effects in Ni–Mn–Ga shape memory alloy

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

Different modeling concepts of magnetic shape memory and their comparison with some experimental results obtained in Ni-Mn-Ga

Quantitative Model of Large Magnetostrain Effect in Ferromagnetic Shape Memory Alloys