| Ni-Mn-Ga alloys with large strain (10%) and high activation frequence have attracted considerable scientific and technological interest in recent years. As the development of the miniaturization and integration of devices, fabrication and investigation of ferromagnetic shape memory film becomes hotspot of this field. Ferromagnetic shape memory Ni-Mn-Ga films were prepared by D. C. Magnetron sputtering technology, including low temperature deposition high temperature annealing and high temperature deposition. Influence of the synthesis procedure, such as substrate, sputtering power, sputtering pressure and depositing temperature, on surface morphology, chemical composition, transformation behaviors and magnetic properties was studied to achieve the film with modulated martensitic structure at room temperature. Meanwhile, two actuation modes, magnetically induced reorientation (MIR) of the twin variants of martensite and magnetically induced martensite transition (MIM), were studied. Moreover, freestanding films were fabricated; transformation behavior, the evolution of the crystal and magnetic structure of freestanding film was investigated.Specifically speaking, the influence of Si, SiO2, Al2O3and MgO substrates on the morphology and magnetic properties of the films were studied. Effect of sputtering power on the composition and morphology of the films is investigated. Martensitic transformation and magnetic properties of the films is remarkably affected by sputtering pressure. Effect of depositing temperature on the microstructure, crystal structure, transformation behaviors and magnetic properties of Ni-Mn-Ga films is investigated. The evolution of magnetic domain structure was controlled by applying magnetic field and temperature field to analyze the process of martensitic transformation. Based on the relationship of microstructure and magnetic domain structure, the evidence of MIR was directly observed.Microstructure and magnetic properties of Ni-Mn-Ga films is significantly affected by the substrate. For the Si substrate, nonmagnetic Ni-Si and Mn-Si compounds were formed when depositing temperature is in excess of873K. While the Al2O3, MgO and SiO2is chemically stable at elevated temperature, which is suitable for the fabrication of ferromagnetic Ni-Mn-Ga film. The difference of thermal expansion coefficient between SiO2substrate and the film is large, thus the film is cracked after heat treated.The composition and morphology of Ni-Mn-Ga films is significantly affected by sputtering power. The root-mean-square roughness of films was increased with the increase of sputtering power. With sputtering power increasing, the content of Ni in film firstly decreased then increased, Mn content changed little, Ga increased.Morphology and magnetic domain observation, magnetic properties measurement were carried out for the Ni-Mn-Ga films deposited at room temperature under0.2and0.5Pa on MgO(001) substrate then annealing at1073K. Chemical composition of the films is closely related to the deposition pressure; moreover, the martensitic transformation temperature is adjusted due to the volatilization of Mn and Ga under different pressure. The as-deposited film grows in the form of columnar mode. After annealing at1073K, the size of columnar grains increases, the film is completely crystallized with saturation magnetization compared with same component bulk. The magnetic domain pattern of austenite displays a maze-like structure; while for the martensite, its domain structure demonstrates macro-lamme, directly related to the morphology. Further, the effect of depositing pressure on Ni-Mn-Ga films deposited at923K on MgO(001) substrate, is investigated. The martensitic transformation starting temperature was310,245and160K for the films deposited under0.3,0.5and0.8Pa, respectively. With the increase of sputtering pressure, Ni content decreased, Mn and Ga increased. Ni-Mn-Ga films deposited at various temperatures, from673to923K, under0.3Pa are fabricated. Martensitic transformation behavior and crystal structure is profoundly affected by depositing temperature. With depositing temperature increasing, surface morphology of film changed from granunar to continous. The film deposited at673K is not completely crystallized. When deposition temperature increases from773via873to923K, crystal structure of martensitic film changed from5M via7M to2M. Saturation magnetization of the films increases with depositing temperature increasing. The magnetization process of the film deposited at873K reveals the occurrence of MIR by providing a typical step-increase. The MIR occurs at the temperature where martensitic transformation starts, the switching field increased with testing temperature, resulting from the increase of energy barrier for the motion of twin boundary. The film deposited at873K was selected for further study.Two actuation modes, MIR and MIM, are realized in the film deposited at873K. The magnetically induced martensitic transformation is attributed to the difference of magnetization between martensite and austenite. The martensitic transformation temperature shifts to higher temperature under large magnetic field. Based on the thermal-magneto curves recorded under magnetic field of0.01,3and6T, a shift of martensite temperature of dT/dH=0.43K/T is observed, in favorable consistent with the theoretical value,0.41K/T.A method is developed to control the magnetic domain structure with magneto-thermal treatment. Then, based on the relationship of microstructure and magnetic domain structure, the microstructure is studied by observing the evolution of magnetic domain structure.Process of martensitic transformation was directly observed. When the martensitic transformation starts, a few stripe domains emerge. Afterwards, the number of stripe domains increases. At last, the stripe domains grow and interconnect till the magnetic moments of the entire film aligned along the direction of external field. It is revealed that martensitic transformation could be divided into two parts, nucleation and growth. Process of martensite reorientation was directly observed. For the magneto-thermal treated film, cooling with magnetic field to a stripe domain state followed by cooling without magnetic field to RT, the stripe domains change into chaining domains due to the demagnetizing field. When the magnetic field was applied parallel to the pre-treated film plane, the chaining domains irreversibly vanished, which is attributed to the reorientation of martensites.Freestanding Ni-Mn-Ga films were prepared with sacrified NaCl substrate. Compared with the constrained film, freestanding film has higher Curie temperature and reduced transformation hysteresis. The occurrence of inter-martensitic transformation was observed. After releasing from the substrate, the twinning structure of martensite can not be directly observed. |
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