Structure and magnetic properties of iron nitride thin films on Cu(001)

The structural and magnetic properties of iron nitride thin films on a Cu(001) surface were investigated by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), x-ray absorption spectroscopy, and x-ray magnetic circular dichroism (XMCD). In the STM and LEED observations of 1 and 2 monolayer (ML) films, the nitride films exhibit p4gm(2×2) reconstructed smooth surfaces without significant amounts of dislocations or islands. The stoichiometry of the 1 ML film was found to be Fe₂N, while that of 2 ML was Fe₄N, consisting of a topmost layer with the Fe₂N composition and a second layer containing Fe atoms only. The XMCD measurements were performed for nitride films up to 4 ML. The angle-dependent XMCD spectra indicate that the films are ferromagnetic, with easy axes along the surface parallel direction. In addition, according to the results of the sum-rule analysis, strong magnetic anisotropy appears in the 1 ML film, where the orbital magnetic moment perpendicular to the surface is almost zero. The total magnetic moment, which is a combination of the spin and orbital magnetic moments, increases with the increase in film thickness up to 3 ML. The magnetic moment for more than 2 ML is ~2.1 μB, which is nearly equal to that of bulk γ^'-Fe₄N of 2.2 μB.