Fabrication of $\hbox{Fe}_{16}\hbox{N}_{2}$ Films by Sputtering Process and Experimental Investigation of Origin of Giant Saturation Magnetization in $\hbox{Fe}_{16}\hbox{N}_{2}$

Abstract

We present a systematic study to address a longstanding mystery in magnetic materials and magnetism, whether there is giant saturation magnetization in Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and why. Experimental results based on sputtered thin film samples are presented. The magnetism of Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is discussed systematically from the aspects of material processing, magnetic characterization and theoretical investigation. It is observed that thin films with Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> +Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> N mixture phases and high degree of N ordering, exhibit a saturation magnetization up to 2.68T at room temperature, which substantially exceeds the ferromagnetism limit based on the traditional band magnetism understanding. From X-ray magnetic circular Dichorism (XMCD) experiment, transport measurement and first-principle calculation based on LDA+U method, it is both experimentally and theoretically justified that the origin of giant saturation magnetization is correlated with the formation of highly localized 3d electron states in this Fe-N system. A large magnetocrystalline anisotropy for such a material is also discussed. Our proposed “cluster+atom” theory provides promising directions on designing novel magnetic materials with unique performances.