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，有望成为磁场诱导的致动器材料。MF诱导的SME将实现快速响应和远程控制。本文研究了Ni_2MnGa薄膜的成分与SME的关系。对不同成分的薄膜进行强制时效，形成双向SME。薄膜的马氏体相变温度(MT)高于RT，低于居里温度(TC)。其力学性能在实际使用中非常重要。研究了温度和磁场对薄膜力学性能的影响。此外，Fe作为铁磁性金属的加入将有效地改善合金的磁性能和磁致磁致伸缩效应。在MF.下对添加Fe的薄膜的结构和SME进行了研究，通过MT及其反转证实了所有约束时效薄膜的可逆双向SME。对于应变-温度曲线具有较大梯度和较小热滞的约束时效薄膜，在冷却时的MT温度附近观察到了磁场高达5T的双向SME现象。随着Fe含量的增加，Mt温度降低，而Tc值增加。对于添加Fe的薄膜，还观察到了热变的双向SME。用X射线衍射仪测定了添加Fe的薄膜在5℃以下的磁场诱发MT，证实了磁场对马氏体相的稳定作用。此外，通过在冷却时在MT温度附近用MF诱导MT，观察到了MF对加Fe薄膜的SME的诱导。对于添加了Fe的薄膜，1T的低MF引起的应变变化较大。

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.