Synthesis of NiCoMnX (X = In, Al) Heusler-type Magnetic Shape Memory Alloy Thin Films

Magnetic shape memory alloys are a class of shape memory alloys, and therefore exhibit a thermoelastic martensite phase transformation between symmetric and asymmetric crystalline states induced by appropriate temperature and/or stress changes. Shape memory alloys are able to recover strain when stress is applied, which can generate higher actuation forces and displacements compared to piezoelectrics and magnetostrictive materials when the material is constrained. While shape memory alloys have found applications in biomedical and aerospace industries, actuator applications are limited to relatively low frequencies compared to piezoelectric materials. The slow response of shape memory alloys is associated with heating or cooling the material from an external source. Compared to traditional shape memory alloys, the coupling of structural and magnetic ordering result in magnetic and structural transformations that increase the functional properties in magnetic shape memory alloys, such as magnetic field-induced rapid martensite transformation (forward and reversed), giant magnetoresistance, and the magnetocaloric effect. While bulk MSMAs can be used for structural components, in many cases MSMA thin films are preferred for device applications, such as miniaturized actuators, small scale propulsion devices, and micro-electro-mechanical systems (MEMS). This thesis focuses on the synthesis of NiCoMnX (X=In, Al) Heusler-type magnetic shape memory alloy thin films via physical vapor deposition, and details the challenges associated with controlling film composition, precipitation, microstructure, residual stress, and mechanical properties. As-deposited films were found to contain a mixture of amorphous and nanocrystalline microstructure, and thus, did not exhibit a martensitic transformation. Appropriate post-deposition heat treatments were required to crystallize the films, tailor the grain size, and reduce the formation of precipitates. Crystallized films exhibited martensitic transformations that showed a grain size dependence. An analytical model that uses a thermodynamic framework was developed to explain the suppression of the martensitic transformations for films with submicron-sized grains. Hence, in addition to chemical composition, sub-micron grain size can be used to tailor the martensitic transformation temperature of NiCoMnX (X=In, Al) thin films for device applications. Additionally, the analytical model may reduce the uncertainty associated with a direct scale-up of thin film compositions used for combinatorial investigations of magnetic shape memory alloys