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

Abstract

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 $p4gm(2\ifmmode\times\else\texttimes\fi{}2)$ reconstructed smooth surfaces without significant amounts of dislocations or islands. The stoichiometry of the 1 ML film was found to be ${\text{Fe}}_{2}\text{N}$, while that of 2 ML was ${\text{Fe}}_{4}\text{N}$, consisting of a topmost layer with the ${\text{Fe}}_{2}\text{N}$ 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 $\ensuremath{\sim}2.1\text{ }{\ensuremath{\mu}}_{\text{B}}$, which is nearly equal to that of bulk ${\ensuremath{\gamma}}^{\ensuremath{'}}{\text{-Fe}}_{4}\text{N}$ of $2.2\text{ }{\ensuremath{\mu}}_{\text{B}}$.