Saturation magnetization of iron sixteen nitrogen two, a 40- year mystery

Abstract

While the material α”-Fe16N2 and its interesting magnetic behavior were discovered decades ago, there is still no unified answer to the question on whether this phase has a giant saturation magnetization (Ms). There are three important “missing pieces” that emerge upon examination of the magnetization and discussing the magnetism of this system 1) There is lack of direct measure of the saturation magnetization on Fe16N2. Conventional magnetometer-based (VSM or SQUID) methods can only measure the total magnetic moment of the samples. The evaluation of the Ms value involves the challenging estimation of the thin film sample volume and the subtle assessment of magnetic contributions from underlayers, substrates or possible impurity phases, resulting in unpredictable errors. 2) There is no convincing theory/model to rationalize the existence of giant Ms. The conventional band theory based first-principles calculations only predicts an Ms value similar to that of pure Fe. 3) There is lack of experiments to explore the underlying physics of the magnetism in Fe16N2 from an electronic state viewpoint l. The previous investigations only focus on the saturation magnetization measurement. Fundamental physics experiments using advanced tools such as polarized neutron and synchrotron x-rays were seldom reported, which may provide unique and independent information on understanding the magnetism in this material system. On the purpose of addressing these three issues mentioned above. We first picked up a unique facing-target sputtering approach to synthesize Fe16N2 epitaxial thin films. A detailed structure and chemical analysis confirmed the crystallinity and epitaxial quality of fabricated Fe16N2 films. In terms of magnetic characterization, in addition to systematically study the saturation magnetization of the prepared films using a vibrating sample magnetometer (VSM) based method., for the first time,we have discovered the partial localization behavior of 3d electrons in Fe16N2 thin film samples by using polarized synchrotron x-rays. Furthermore, we have used polarized neutron reflectometry (PNR) to directly measure the saturation magnetization in absolute unit, which confirms the presence of giant saturation magnetization. The observed saturation magnetization of Fe16N2 phase is up to 2500 emu/cm3, which is significantly larger than that of the currently known limit (Fe65Co35 with saturation magnetization of 1900 emu/cm3) To understand the origin of the giant magnetization, we proposed a “cluster + atom” model, which pointeded out a possible scenario to develop this unusual magnetism in this Fe-N system. Synchrotron x-ray experiments also provided supporting evidences of the charge transfer from the itinerant iron atom to the Fe6N cluster, which is consistent with the proposed model. We further discussed the perpendicular anisotropy and the relatively large spin polarization ratio of these Fe16N2 films, which will be very useful for future magnetoresistive devices with perpendicular anisotropy and low damping constant.