Magnetic properties and potential applications of Fe16N2

Abstract

Fe16N2 is a magnetic material with giant saturation magnetization that has potential applications in the hard drive and permanent magnet industries. In this thesis, fundamental magnetic properties of Fe16N2 are studied experimentally on thin-film samples and the potential application of Fe16N2 as a rare-earth-free permanent magnet is investigated theoretically. For the experimental part, the sputtering growth of Fe16N2 thin films on nonmagnetic seed layers is reported first, which provides the foundation for determining the magnetic structure of Fe16N2. The magnetic structure of high-magnetization Fe16N2, solved using polarized neutron diffraction, is reported for the first time. The magnetic structure also helps understand the origin of the giant magnetization observed in Fe16N2. Using the techniques of polarized neutron reflectometry, transmission electron microscopy, and vibrating sample magnetometry, we clarified the origin of the interface enhanced magnetization and perpendicularly magnetized components in Fe16N2 thin films. For the theoretical part, Monte Carlo methods were applied to explore the possibilities of antiferromagnet-ferromagnet exchange- coupled composite magnets, using Fe16N2 as the ferromagnet as it has large saturation magnetization and a reasonably high magnetic anisotropy constant. A new Monte Carlo-sampling based algorithm for comparative analysis of coercivity is proposed. It is confirmed that, with proper choice of an antiferromagnetic material and an optimized microstructure, large coercivity can be achieved in antiferromagnet-Fe16N2 composite magnets and that the maximum energy product can be enhanced by up to 10% as compared to pure iron nitride magnets.