Formation of ferromagnetic iron nitrides in iron thin films by high-dose nitrogen ion implantation

Abstract

Synthesis of thin films of Fe16N2 has been investigated by use of a high-dose nitrogen ion implantation into thin iron films. Nitrogen molecular ions of N+2 were accelerated to 150 keV and implanted into α-iron films with a thickness of 200 nm. As a result of an implantation with the dose of 4.0×1016 (N2/cm2), 35% of the α-Fe was transformed into an iron nitride with a body-centered-tetragonal (bct) structure. The c axis of the bct lattices was oriented perpendicularly to the film plane. It was found that nitrogen implantation into iron films gives rise to both an increase in the spontaneous magnetization and a perpendicular anisotropy. These facts are explained on the basis of the bct nitride formation and the c-axis orientation. Conversion electron Mössbauer spectroscopy was used to study hyperfine structures and magnetic properties of the implanted films. Three different Fe sites corresponding to the bct nitride were recognized in a spectrum of the implanted film. The maximum hyperfine field was 372.8 kOe at room temperature which is ascribed to the second-nearest-neighbor Fe sites for the nitrogen atoms. It was deduced that the bct iron nitride has an ordered structure analogous to Fe16N2.