•Production of Ni2FeGa Heusler micro-wire with high shape memory effect is reported.•The microwires show 2% reversible strain in the axis of microwire.•The 2% strain is accompanied by a 1600% ...variation of initial permeability.•Therefore they are ideal material for SMART actuators.
We report on the production and characterization of Heusler-based Ni2FeGa microwires exhibiting two – way shape memory effect. The microwires are characterized by a monocrystalline structure with a strong preferred crystallographic orientation that shows 1 1 1 axis parallel to the wire’s axis for high-temperature L21 austenite phase, while the 0 1 7 axis is preferred for low-temperature monoclinic phase. Variation of crystallographic axis (and corresponding easy magnetization axis) leads to 1600% variation of magnetic permeability due to a 2% strain in axial direction. Such straining is reversible immediately after production without the necessity of further thermal treatment. These properties give the microwire function of very sensitive SMART actuators that can be easily produced in a large amount.
In the given contribution, the production of shape-memory glass-coated microwires based on Ni2FeZ (Z = Ga, Sn, Sb) and Co2Cr(GaSi) alloys is shown, focusing to their repeatable production. Such wires ...are characterized by monocrystalline structure along entire length. This leads to 1.5% reversible temperature shape memory effect for Ni2FeGa microwire in the as-cast state without necessity of additional thermal treatment. Moreover, well defined anisotropy results in the variation of permeability up to 550% during the phase transition. On the other hand, Co2Cr(GaSi) microwires show reversible superelastic straining up to 1.1% with a very small irreversible strain (<0.1%).
•Ni2FeZ and Co2Cr(GaSi) glass-coated magnetic microwires were produced.•They are characterized by monocrystalline structure along entire length.•Ni2FeGa microwire exhibit 1.5% reversible temperature shape memory effect without necessity of thermal treatments.•Co2Cr(GaSi) microwire shows reversible superelastic straining up to 1.1%.
We have studied the effect of thermal treatment on the sensitivity and stability of the switching field in bistable glass-coated Fe-Ni-Si-B microwire. We have found that annealing at 300°C/1 hour ...leads to the increase in the sensitivity of the switching field to the applied external stress. Moreover, the switching field fluctuation decreases after such treatment as a result of domain structure stabilization through the structural relaxation.
Despite the intensive studies for decades, it is still not well understood how qualitatively different magnetic behaviors can occur in a narrow composition range for the Fe-rich Fe-transition metal ...(TM) amorphous alloys. In this study of amorphous Fe sub(100-x) Zr sub(x)(x=7, 9, 12) metallic glasses, normal ferromagnetism (FM) is found at 12 % Zr where only the FM-paramagnetic (PM) transition is observed at the Curie temperature, T sub(C). In contrast, spin-glass (SG)-PM transition at a temperature, T sub(g), called SG temperature, is only observed at 7% Zr, while in the transient re-entrant composition range (x=8-11), an SG-FM transition at a temperature, T sub(f), called spin-freezing temperature, is also observed at low temperature besides the normal FM-PM transition at T sub(C). In order to understand this unusual behavior, a detailed characterization of pressure (atomic volume), composition, and temperature dependence of the magnetic properties is coupled with high pressure synchrotron x-ray diffraction determination of the pressure dependence of the atomic volume. The results on Fe sub(100-x) Zr sub(x)(x=7, 9, 12) are compared to those obtained for the FM Co sub(91) Zr sub(9) metallic glass not showing any kind of anomalous magnetic properties. It is confirmed that the unusual behavior is caused by a granularlike magnetic structure where weakly coupled magnetic clusters are embedded into a FM bulk matrix. Since the mechanism of the magnetization reversal was found to be of the curling type rather than homogeneous rotation, the energy barrier determining the blocking temperature of the clusters is calculated as AR, where A is the exchange constant and R is the cluster size, in contrast to the usual characterization of the energy barrier by KV where K is the anisotropy energy and V is the cluster volume. The volume fraction of the FM part is a fast changing function of the bulk composition: Almost 100% FM fraction is found at 12% of Zr while no trace of real FM is observed at 7 at % Zr. The driving force of this surprising magnetic character is the atomic volume: The lower the Zr content, the higher is the fraction of Fe atoms with compressed atomic volume having low magnetic moment. The percolation of their network separates the clusters from the FM bulk. The complex magnetic behavior of the Fe-rich Fe-Zr amorphous system at low temperatures can thus be interpreted with the only assumption of a cluster-size distribution and a weak coupling of the clusters to the FM matrix. The introduction of this coupling is able to explain the opposite pressure dependence of T sub(g) and T sub(f). The threshold atomic volume in the low magnetic moment regions is found to be comparable to the atomic volume characteristic to the low-spin limit of the face-centered-cubic Fe alloys. The extensive literature results on the anomalous magnetism for various Fe-rich Fe-TM amorphous alloys and especially for the Fe-rich Fe-Zr glassy system are also found to be in agreement with this granular magnetic behavior.
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