Jorge Feuchtwanger

ac field-induced actuation of single crystal Ni–Mn–Ga

Single crystals of Ni–Mn–Ga have shown 6% field induced strain in quasistatic actuation against an applied load. Here we report the ac actuation response and a magnetostrictivity constant of an off-stoichiometry single crystal of Ni–Mn–Ga measuring approximately 14×5.6×1 mm. This sample shows a maximum 3% strain under a bias stress of 1.1–1.5 MPa. The deficit between the low-frequency ac field-induced strain and the 6% dc results are discussed. Additionally, the e–H indicate a peak differential value for d13 of 1%/kOe (13×10−8 m/A) which is about two times that of Terfenol-D.

Energy absorption in Ni-Mn-Ga/ polymer composites

Martensitic Ni-Mn-Ga single crystals are easily twinned and the twin boundaries can be displaced by applying a mechanical stress or magnetic field. Twin boundary motion is a highly dissipative process. Composites of aligned Ni-Mn-Ga particles in a polymer matrix show evidence of stress-induced twin boundary motion that results in stress-strain curves with significantly greater hysteresis than for Fe-polymer composites or polymer samples.

Large energy absorption in Ni–Mn–Ga/polymer composites

Ferromagnetic shape memory alloys can respond to a magnetic field or applied stress by the motion of twin boundaries and hence they show large hysteresis or energy loss. Ni–Mn–Ga particles made by spark erosion have been dispersed and oriented in a polymer matrix to form pseudo 3:1 composites which are studied under applied stress. Loss ratios have been determined from the stress-strain data. The loss ratios of the composites range from 63% to 67% compared to only about 17% for the pure, unfilled polymer samples.

Chemical order in off-stoichiometric Ni–Mn–Ga ferromagnetic shape-memory alloys studied with neutron diffraction

The chemical order of three off-stoichiometry Ni–Mn–Ga compositions has been measured in the austenitic phase using powder and single-crystal neutron diffraction. The compositions studied, 48–52 at.% nickel, having excess manganese and deficient in gallium, are of technical interest due to the observed large room-temperature, magnetic-field-induced strain. It has been determined that compositions with less than 50% nickel have the excess Mn atoms occupying Ni and Ga sites. Compositions enriched in nickel are best fit with Ni atoms in excess of 50% occupying Mn sites while the excess and displaced Mn occupy Ga sites. The saturation magnetic moments calculated from the site occupations determined here and using Ni and Mn moments reported for Ni2MnGa, agree within 4% with the low-temperature measured moments.

Acoustic assisted, field-induced strain in ferromagnetic shape memory alloys

A technique has been developed that uses acoustic energy to assist a magnetic field in driving twin boundary motion in a NiMnGa single crystal. Acoustic assisted magnetic-field-induced strain has been observed to increase the magnetic-field-induced strain response by up to one order of magnitude. This effect is most pronounced for magnetic field drives near the twin boundary threshold field. Increasing frequency of the acoustic wave input is shown to increase strain up to about 4 kHz after which there is a small decline in FSMA strain for higher frequencies.

AC performance and modeling of ferromagnetic shape memory actuators

Very large DC field-induced strains ((epsilon) approximately equals 6%) have been reported for Ni-Mn-Ga single-crystal ferromagnetic shape memory alloys (FSMAs) at room temperature. Described here is an AC test system that provides a dynamic bias stress to an FSMA sample. The low- frequency (epsilon) -H curves show a stress dependence consistent with the DC results, i.e. the maximum output strain peaks for a bias stress of order 1.4 Mpa. The AC (epsilon) -H hysteresis at sub-optimal bias stress can be considerably smaller than that for DC actuation. A thermodynamic model of field-induced twin-boundary motion is expanded to include external stress, threshold field and hysteresis in the twin boundary motion. Twin-boundary motion is driven by the Zeeman energy difference across the domain wall, 2MsH, in the high anisotropy limit and is suppressed by domain magnetization rotation in the weak anisotropy limit. The magnitude of the threshold field and hysteresis can be obtained from features on mechanical stress-versus-strain curves. The field dependence and stress dependence of the AC strain are reasonably well accounted for by the model.

Relaxation in Ni–Mn–Ga ferromagnetic shape memory alloys

Evidence of relaxation has been observed in ferromagnetic Ni–Mn–Ga single crystals. The relaxation may be explained by a change in symmetry-conforming short-range ordering according to Ren and Otsuka in this off-stoichimetric ordered alloy. Martensite stabilization has also been found after martensite ageing.

Vibration damping in Ni-Mn-Ga composites

Ferromagentic shape memory alloy composites exhibit good qualities as vibration absorbers. Loss ratios in excess of 25% have been measured in polymer samples containing 20 vol% Ni-Mn-Ga. The ability to dissipate large amounts of energy is due to the same mechanism that is also responsible for the large strains observed in single crystals used as actuators, namely twin-boundary motion. The loss ratios of the FSMA-loaded composites are compared to those for pure polymer samples and polymer loaded with inert filler. The effects of the pre-processing of the filler material on its performance are also shown.

Microscopic considerations for models of ferromagnetic shape memory actuation

Micro magnetic and analytic models have been sued to describe the equilibrium twin structure and quasistatic actuation behavior of ferromagnetic shape memory alloys. However, these models do not incorporate microscopic aspects of the twin-boundary strain field, interactions with defects or non-equilibrium behavior. A model is described that accounts for the interaction of a 90 degree domain wall with such a twin boundary. Application of a magnetic field can displace the domain wall from a pinned twin boundary with the Zeeman energy being stored elastically in the domain- wall anisotorpy energy. Finally, the departure of the magnetization and twin structure from equilibrium configurations can be incorporated in thermodynamic models to describe AC behavior and hysteresis.

Ferromagnetic shape memory alloys: where might they be useful?

In this paper, ferromagnetic shape memory alloys, most notably NiMnGa have received considerable attention because of the remarkable 6% to 10% field induced strains they have exhibited at room temperature in the magnetic fields of order 0.5 T. In order to address this question, the important technical properties that these materials exhibit will be reviewed and compared with those of competing active materials.