Giant Saturation Magnetization Fe16N2 Thin Film

Abstract

High saturation magnetization (Ms) material has always been a focus for many applications either in industry or academia. Fe16N2 has been a candidate since its first high Ms discovery in 1972. Does it have high Ms or not? This has been a question for the past five decades. Conventional magnetometer measures the magnetic moment of the iron nitride sample and yield its Ms by dividing the volume of the film, which both contain errors during experiments. A unique facing target sputtering system is used to fabricate epitaxial Fe16N2 thin film. To better measure the magnetic properties of Fe16N2 thin film, I use polarized neutron reflectivity (PNR), which directly detects the magnetization of the film in the depth profile. And the Ms of our film measured by PNR shows high Ms (up to 2500 emu/cm3), which is significantly higher than that of FeCo alloy. I studied the thermal stability of Fe16N2 thin films on MgO and GaAs substrate by elevating the temperature of the films. Despite the strain differences due to the substrate and seed layer, Fe16N2 thin film reserves its crystallinity and magnetic properties under 250 °C. One unaddressed physics related with Fe16N2 is how to understand its Mössbauer Spectroscopy. M. Takahashi and his co-authors, found three sets of hyperfine field splitting were linked to the low Ms of the sample, which was based on the assumption that the hyperfine field of iron is proportional to its magnetic moment. To resolve this “low Ms” issue, I conducted Mössbauer experiment and proposed a model that explains well the high Ms nature of Fe16N2 to understand the unique “high moment but low hyperfine field” predicament. If Fe16N2 is to be used in magnetic writer in hard drive, it needs to have lower anisotropy and lower saturation field. In the present work, Fe16N2 was modified with carbon dopant. With ~ at. 5% carbon addition in the film, I observed the existence of high Ms and low saturation field at the same time upon the appropriate amount of nitrogen in the film.