Phase Transformation Behavior And Properties Of Magnetron Sputtered Ni-Mn-Ga Alloy Thin Films

Ni-Mn-Ga magnetic shape memory alloy thin films have been fabricated by usig magnetron sputtering technique. The results clarify the effect of sputtering parameters on surface morphology and chemical compositions of Ni-Mn-Ga thin films, revealing the physical mechanism of composition variations. At the same time, martensitic transformation, microstructure, magnetic-field-induced strain and optical reflectivity of the thin films have been systematically investigated by means of DSC, XRD, TEM, magnetic and optical properties measurements, respectively. The martensitic structure of Ni-Mn-Ga thin films with high reflectivity is determined.The experimental results show that process parameters have remarkable influence on surface roughness and chemical compositions of Ni-Mn-Ga thin films. The surface roughness of the thin films increases with increasing Ar working pressure, sputtering power and substrate temperature. It is shown that Ni content in Ni-Mn-Ga thin films decreases with increasing the sputtering power and increases with the substrate negative bias voltage increasing, whereas Mn and Ga content increase with increasing the sputtering power and decrease with the substrate negative bias voltage increasing.According to experimental results, residual compressive stress exists in Ni-Mn-Ga thin films. The residual compressive stress dramatically depends on film thickness and the substrate negative bias voltage, decreasing with increasing film thickness and increasing with the substrate negative bias voltage increasing. Based on XRD and DSC analysis, the as-deposited Ni-Mn-Ga thin films are in partially crystallized state when substrates are not heated. During crystallization the residual compressive stress is beneficial to reducing the volume of Ni-Mn-Ga thin films, consequently facilitating crystallization of the thin films. As a result, the crystallization activation energy decreases with increasing residual compressive stress. After annealing at 823K for 1 hour, the as-deposited Ni-Mn-Ga thin films are fully crystallized.It is shown that Ni-Mn-Ga thin films undergo one-step thermoelastic martensitic transformation during the process of cooling and heating. And martensitic transformation temperatures increase with increasing valence electron concentration due to the instability of parent phase with a larger valence electron concentration. It is found that the temperature Ms of Ni56Mn27Ga17 thin film can approach as high as 584K, which causes this thin film to be a potential high temperature shape memory alloy thin film. In addition, superelasticity in Ni-Mn-Ga thin films has been obtained during loading and unloading in Nanoindentation experiments.Based on TEM observations, it is found that the grain size of Ni-Mn-Ga thin films is in the range between 200 and 500nm, two orders of magnitude smaller than that of Ni-Mn-Ga bulk alloys. Ni54Mn25.1Ga20.9 thin film is in martensite state at room temperature, and the film is determined to be seven-layered modulated orthorhombic structure. For Ni56Mn27Ga17 thin film, the body-centered cubicγphase exists in the matrix of non-modulated tetragonal martensite. From TEM and HREM observations, the interfaces of martensite variants in Ni-Mn-Ga thin films are clear and straight. And the substructure of martensite variants in Ni54Mn25.1Ga20.9 and Ni56Mn27Ga17 thin film are type I (011) and (111) twin relationship, respectively.In terms of the analysis of magnetic properties, it is found that the Curie temperatures of Ni-Mn-Ga thin films are insensitive to film compositions, and the saturation magnetization, coercive force and residual magnetization of Ni-Mn-Ga thin films increase with decreasing operation temperature. It is also shown that the saturated magnetic-field-induced strains (MFIS) of Ni-Mn-Ga thin films remarkably depend on operation temperatures. The saturated MFIS of Ni49.34Mn26.96Ga23.4 and Ni50.3Mn27.3Ga22.4 thin films firstly increases and then decreases with increasing operation temperature below Af, and the maximum saturated MFIS can be obtained around Ms temperature. When the operation temperature is lower than Mf,the saturated MFIS of Ni49.33Mn30.1Ga20.57 thin film increases with decreasing operation temperature. It is found that reflectivity of Ni-Mn-Ga thin film through optical properties measurements is closely related to film surface roughness and crystallographic structure. The reflectivities of Ni-Mn-Ga thin films in martensite state have a remarkable dependence on crystal structure and surface roughness. The reflectivities of Ni-Mn-Ga thin films increase with increasing root-mean-square surface roughness of the thin films. Among Ni-Mn-Ga thin films with various martensitic structures (5M, 7M and T), the reflectivity of the non-modulated Ni-Mn-Ga thin film is larger than those of Ni-Mn-Ga thin films with 5M or 7M structure.