High spin polarization in epitaxial Fe₄N thin films using Cr and Ag as buffer layers

Fe₄N thin films with (001) texture were prepared by reactive sputtering on MgO substrates, utilizing either a Cr or Ag buffer layer to facilitate the epitaxial growth. X-ray diffraction, atomic force microscopy, and vibrating sample magnetometry measurements show that the Fe₄N thin film grown on the Ag buffer layer is superior to that grown on the Cr buffer layer. The point contact Andreev reflection measurement was then conducted, and the spin polarizations were determined to be 61.1% and 81.3% for Fe₄N thin films with Cr and Ag buffer layers, respectively. The 81.3% spin polarization is significantly higher than the ratio reported previously for Fe₄N and is comparable with that of state-of-the-art Heusler alloys. This result is in agreement with the theoretical prediction on the discrepancy between the two differently defined spin polarizations for Fe₄N. Moreover, our study indicates that an optimized growth process for Fe₄N thin films is crucial for achieving a high spin polarization and that true half-metallicity could potentially be realized with Fe₄N. The high spin polarization of Fe₄N combined with its low fabrication temperature and simple composition makes Fe₄N a competitive candidate to be a half-metallic ferromagnet in spintronic devices.