A Study of the Oxidation of Fe_1-xCo_x Alloys and their Resulting Magnetic Properties

Iron-cobalt (FeCo) and its various alloys have many applications where soft magnetic materials are needed, especially in high temperature applications. Recent research has looked into the nanocrystallization of amorphous alloys of FeCo and very briefly into the oxidation of FeCo nanoparticles and bulk materials. Attempts will be made to more carefully investigate the oxidation of FeCo and its alloys utilizing nanoparticles, and thin films with (100), (110), and (211) texture to observe the kinetics of oxidation. Thin film epitaxial relationships between the substrate and thin films have been determined, and this will be extended to the oxide and thin film. The role of alloying has been discussed, especially in the context of oxidation of FeCo. The composition of the oxide at different oxidizing temperatures is also proposed.

FeCo-based nanoparticles have been analyzed to understand their change in magnetization and oxide phase as a function of temperature. The oxide thickness has been measured at various temperatures, along with the observation of a voided core. This research has been coupled with thin film work to show that the core gets richer in cobalt as oxidation progresses, with Fe acting as the mobile species. Oxygen may diffuse early in the oxidation, but only until a certain oxide thickness has been established. The oxidation kinetics seen in the nanoparticles is slower than that seen in thin films, and it has currently been analyzed to follow a logarithmic rate law at lower temperatures.

To understand the formation of faceted nanoparticles, nucleation and growth has been modeled for both BCC and FCC systems showing the surface energy ratios necessary to produce different faceting of nanoparticles. It has been shown that the critical nuclei are the same as the growth shapes.

To extend the basic science research into the applications field, thin film work on CoCrPt has been performed to achieve out-of-plane anisotropy in thicker films for use in a portable AGFM. While this has been achieved, further study is necessary to improve the remnant magnetization and make it more comparable to SmCo, which is the current standard. The magnetic properties have been measured as a function of temperature and film thickness to begin understanding the system better to produce the desired thin film properties for a biomedical sensor.