Effect of lattice mismatch strain on Fe8N-Fe16N2 phase transformation and aging process.

Abstract

It has been debated for decades whether ”-Fe16N2 has a giant saturation magnetization, which couldn’t be predicted by the traditional band magnetic theory. Because of a lot of previous inconsistent research results in the past 40 years, ”-Fe16N2 has been regarded as a debatable mystery material in magnetic research community. Recently, Wang’s group has successfully rationalized a partially localized 3d electron model based on the first principles calculation and predicted the existence of the giant saturation magnetization in ”-Fe16N2. Furthermore, we have synthesized FeN thin films with partially ordered ”-Fe16N2 phase on GaAs substrate with Fe as the underlayer. Our repeatable experimental results proved the existence of giant saturation magnetization of partially ordered ”-Fe16N2 phase. However, there has been a critical question on this topic for any experimentalist to answer, which has actually bothered magnetic researchers for almost four decades. Why did most experimental research groups not succeed to report the giant saturation magnetization value even with their fabricated FeN films with observed ”-Fe16N2 phase? This critical question has been answered in this MS thesis work. The effect of the initial strain on the phase transformation between Fe8N and Fe16N2, which is caused by the lattice mismatch between the FeN layer and its growth template, was investigated. A model was proposed, based on the Johnson-Mehl-Avrami equation, to describe the phase transformation process between Fe8N and Fe16N2 phases. Aging effect of the partially ordered Fe16N2 phase, e.g. degradation of giant magnetization behavior vs. time, was analyzed semi-quantitatively. Based on this proposed model, we have successfully rationalized the inconsistency of the fabricated ”-Fe16N2 films by different research groups using different growth technologies. An integrated facing-target sputtering system has been set up during my MSEE thesis work, which has three pairs of facing targets and ultra high vacuum capability. Note: Please see PDF abstract for correct chemical symbols that are not properly copied