Magnetic moment of iron in metallic environments

Rare-earth iron nitrides are emerging as an important class of magnetic materials. In certain rare-earth iron compounds, the insertion of small atoms such as nitrogen and boron has resulted in significant changes in the magnetic properties in the form of higher Curie temperatures, enhanced magnetic moments, and stronger anisotropies. In an attempt to understand some of the above, we have focused on two nitride phases of Fe, namely ${\mathrm{Fe}}_4\mathrm{N}$ (cubic) and ${\mathrm{Fe}}_{16}{\mathrm{N}}_2$ (tetragonal). For the ${\mathrm{Fe}}_{16}{\mathrm{N}}_2$ phase, the average Fe moment reported by different experimental groups varies over a wide range of values, from $2.3{\mu }_B$ to $3.5{\mu }_B.$ We will discuss some of the recent experiments and examine some related theoretical questions with regard to Fe having such an unusually large moment in a metallic environment. Employing a Hubbard-Stoner-like model in addition to local-density results, it is shown that an unusually large on-site Coulomb repulsion is necessary if one is to obtain a moment as large as $3.5{\mu }_B.$