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

Abstract

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.

Origin of steps in magnetization loops of martensitic Ni-Mn-Ga films on MgO(001)

“…[10][11][12][13] It is remarkable that the reported values of the switching field are similar despite different growth techniques, sample orientations, and used substrates. In the following, we will show that this behavior can be explained on the basis of the model, which assumes the martensitic twin variant structure as fixed and only considers the magnetostatic interactions and the evolution of magnetic domains.…”

“…[10][11][12][13] In analogy to bulk materials, this effect has also been identified as MIR of twin variants. However, a conclusive MIR model for thin films on a rigid substrate is still missing, owing to the fact that the substrate inhibits any macroscopic length change and thus hampers the magnetically induced change in the martensitic microstructure of an MSM film.…”

“…1(d)) and in published data. 11,12 In conclusion, we have examined the temperature dependent magnetization properties of Ni-Mn-Ga epitaxial films in the martensitic phase. Micromagnetic simulations have demonstrated that the abrupt changes observed in the hysteresis slope originate from the magnetization switching within the martensitic variant pattern.…”