Evaluation of Magnetic-Field Induced Shape Memory Effect in Ferromagnetic Shape Memory Alloy Films and its Mechanism

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Ferromagnetic shape memory alloy is a multifunctional material with ferromagnetism and shape memory effect (SME), and is expected as a magnetic-field (MF) induced actuator material. The MF induced SME will enable the quick response and remote control. In the present study, the relation between the composition and SME was investigated for the Ni_2MnGa films. The films with various compositions were constraint-aged to make the two-way SME. The martensitic transformation (MT) temperature was higher than RT and lower than the Curie temperature (Tc) for the films. The mechanical properties are very important in the practical use. The effects of temperature and MF on mechanical properties of the films have been also investigated. Furthermore, the Fe addition as a ferromagnetic metal in the alloy will be effective for the improvement of magnetic properties and MF induced SME. The structure and SME of Fe-added films were investigated under the MF.The reversible two-way SME by the thermal change was confirmed for the all constraint-aged films through the MT and its reversion. The two-way SME by the MF up to 5 T was observed around the MT temperature on cooling for the constraint-aged film with the large gradient and the small thermal hysteresis in the strain-temperature curve. The MT temperatures decrease with increasing Fe content, while the Tc increases. The two-way SME by thermal change was also observed for the Fe-added films. The MF induced MT for the Fe-added film was evaluated by an XRD apparatus in high MF up to 5 T. It was confirmed the martensitic phase was stabilized by the MF. Furthermore, the MF induced SME for the Fe-added films was observed by the inducting the MT with MF around MT temperature on cooling. The strain change by the low MF of 1 T became large for the Fe added films.