Magnetic and Structural Properties of Ni-Mn-Ga Films Produced Via Physical Vapor Tri-Deposition

The structural, thermal, magnetic, and mechanical properties of Ni-Mn-Ga magnetic shape-memory alloys depend strongly on composition. Compositions of sputter deposited films differ from the composition of the single alloy targets from which they originate, where the composition change depends on sputter parameters. Films produced via physical vapor deposition from single alloy targets have sound film-substrate adhesion and film uniformity, however, accurate control of composition is difficult. Tri-sputter deposition from multiple targets allows the flexibility of varying deposition rates and film compositions. A robust procedure with three targets (nickel, nickel-gallium, and manganese) was developed to deposit Ni-Mn-Ga films on silicon with defined composition, structural and magnetic properties. The sputter power was controlled and varied independently and systematically for each target. A film with a targeted composition of Ni50Mn28.6Ga21.4 at. % Energy-Dispersive X-ray Spectroscopy (EDS) yielded the composition Ni50.5Mn29.2Ga20.3 at.-%. X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) revealed the 14M modulated martensite structure, a fiber texture, an average grain size of approximately 100 nm. The martensitic start and finish temperatures, measured with multi-beam optical sensor wafer curvature deflectometry, were 122 °C and 81 °C respectively, indicating stress-induced martensite formation at high temperature. Tri-deposition using three targets provides a method to control composition of Ni-Mn-Ga films and adjust film properties such as martensite structure and transformation temperature via target power adjustment