| Martensitic thin films have attracted much attention because of their potential applications as microactuators in micro-electro-mechanical-system (MEMS). In this thesis, single crystal ferromagnetic shape memory Ni-Mn-Ga films have been grown on GaAs (001) by molecular beam epitaxy (MBE), in order to investigate the effects of composition on the structural and magnetic properties of the single crystal films. Upon substrate removal, martensitic transformation was allowed to occur. Magnetization versus temperature curves were measured for all films in order to study the martensitic transformation behaviors. Using variable temperature transmission electron microscopy (TEM), the phase transformation and microstructure of the Ni-Mn-Ga films can also be realized. The martensitic transformation temperatures of the Ni-Mn-Ga films increased monotonically with increasing valence electron concentration, and can be tuned to above room temperature. Although the martensitic transformation temperatures have been successfully controlled by varying the compositions, the Curie temperatures did not show a clear trend. Moreover, the highest Curie temperature was only about 380 K. In order to increase the Curie temperature so that to increase magnetocrystalline anisotropy and to utilize ferromagnetic shape memory effect in a wider temperature range, new material systems including Ni2FeGa and Co2NiGa single crystal films have also been developed.; Single crystal ferromagnetic shape memory Ni2FeGa and Co 2NiGa films have been grown using similar growth strategies as of Ni-Mn-Ga films on GaAs (001) substrates by MBE. Magnetic measurements and plan-view TEM studies of the free-standing films suggested that both Ni2FeGa and Co2NiGa systems have room temperature martensites and high Curie temperatures (630 and 670 K, respectively). The demonstrated shape memory effect in free-standing membranes and actuator prototypes have been reported for the first time in the Ni2FeGa and Co2NiGa single crystal films. Although Ni-Mn-Ga thin films showed moderate Curie temperatures, the development of the Co2NiGa and Ni2FeGa single crystal films have achieved the goal of developing high Curie temperature ferromagnetic shape memory thin films, with room temperature martensites. |
