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

铁磁形状记忆合金是一种具有铁磁性和形状记忆效应的多功能材料，有望成为一种磁场感应致动器材料。由MF诱导的SME将实现快速响应和远程控制。在本研究中，研究了Ni_2MnGa薄膜的组成与SME之间的关系。对不同成分的薄膜进行约束老化，制备双向SME。马氏体相变（MT）温度高于RT温度，低于居里温度（Tc）。在实际应用中，力学性能是非常重要的。研究了温度和磁场对薄膜力学性能的影响。此外，在合金中加入铁作为铁磁性金属，可以有效地改善合金的磁性能和中磁诱导的SME。在MF下研究了添加铁薄膜的结构和SME。通过MT及其还原，证实了所有约束老化膜的可逆双向热变化SME。应变-温度曲线上梯度大、热滞后小的约束老化膜，在MT温度附近存在高达5 T的MF双向SME。MT温度随Fe含量的增加而降低，Tc含量增加。对添加铁的薄膜也观察到双向热变化的SME。用XRD分析了在高达5 t的高磁场条件下，铁掺杂膜的磁场诱导MT，证实了磁场对马氏体相的稳定作用。此外，在冷却时，在MT温度附近用MF诱导MT，观察了添加铁薄膜的MF诱导SME。加入铁的薄膜在1 T的低MF下应变变化变大。

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.