Transformation behaviors, structural and magnetic characteristics of Ni-Mn-Ga films on MgO (001)

Ferromagnetic Ni-Mn-Ga films were fabricated by depositing on MgO (001) substrates at temperatures from 673 K to 923 K. Microstructure, crystal structure, martensitic transformation behavior, and magnetic properties of the films were studied. With increasing deposition temperature, the surface morphology of the films transforms from granular to continuous. The martensitic transformation temperature is not dependent on deposition temperature; while transformation behavior is affected substantially by deposition temperature. X-ray analysis reveals that the film deposited at 873 K has a 7M martensite phase, and its magnetization curve provides a typical step-increase, indicating the occurrence of magnetically induced reorientation (MIR). In situ magnetic domain structure observation on the film deposited at 873 K reflects that the martensitic transformation could be divided into two periods: nucleation and growth, in the form of stripe domains. The MIR occurs at the temperature at which martensitic transformation starts, and the switching field increases with the decrease of temperature due to damped thermal activation. The magnetically induced martensitic transformation is related to the difference of magnetization between martensite and austenite. A shift of martensite temperature of dT/dH = 0.43 K/T is observed, consistent with the theoretical value, 0.41 K/T.