Effect of proton irradiation on microstructural and magnetic properties of ferromagnetic Ni-Mn-Ga thin films

To explore the effect of proton irradiation on ferromagnetic Ni-Mn-Ga alloy, Ni54.5Mn26.3Ga19.2 thin films with the seven-layer modulated (7 M) martensite structure were prepared by magnetron sputtering and irradiated by protons with an energy of 3 MeV. The influence of proton irradiation on the microstructural and magnetic properties was investigated and discussed in detail. It is found that although nanoscale amorphous zones could be introduced by proton irradiation, the crystalline structure of the irradiated films retained the 7 M martensite structure according to X-ray diffraction and transmission electron microscopy results. Moreover, the broad diffraction peak at 2θ = 42.0° for the unirradiated films is split into two peaks at diffraction angles of about 41.5° and 42.2°, and the peak at 47.2° disappeared for the irradiated films. Regarding the magnetic properties, the irradiation-induced nanoscale amorphous defects may hinder the movement of magnetic domain walls, causing increases in the coercivity (Hc) and remanence (Mr). In addition, irradiation-enhanced thermal ordering will play a dominant role in improving the ordering of incompletely annealed films, ultimately resulting in slight increases in saturation magnetization (Ms) and the Curie temperature (Tc).