Off-campus UNL users: To download campus access dissertations, please use the following link to log into our proxy server with your NU ID and password. When you are done browsing please remember to return to this page and log out.
Non-UNL users: Please talk to your librarian about requesting this dissertation through interlibrary loan.
Strain and Surface Effects on the Magnetism of Epitaxial Nickel Cobaltate Thin Films
Research into the behavior of NiCo2O4 (NCO) has accelerated in the last two decades, especially due to its promising applications in catalysis and in supercapacitive energy storage. The transport properties of NCO have been shown to be tied to its magnetic behavior, suggesting research into the magnetism of the material, especially in thin film form, may help inform other aspects of its properties. While NCO is known to exhibit perpendicular magnetic anisotropy (PMA), no work has analyzed the size of the effect, nor the mechanism of the anisotropy. This dissertation presents the observations of strong magnetic anisotropy for epitaxial films grown via pulsed laser deposition onto MgAl2O4 substrates. By fabricating a variety of sample geometries, the size of the anisotropy is determined and is close to that of similar oxides & similar thin-film materials. Using both phenomenological and microscopic studies, we then demonstrate how the modest strain induces the tetragonal distortion leading to the PMA. The second part of the dissertation presents the observation of surface magnetization which is not observed in conventional SQUID magnetometry. Through use of surface-sensitive x-ray magnetic circular dichroism (XMCD) measurements, the surface magnetization is observed to be confined to approximately the top one nanometer of the film, with magnetic transition temperature well below that of the bulk film. Evidence for the reduced magnetism coming from vacancies within the spinel structure are also presented.
Physics|Condensed matter physics|Materials science
Mellinger, Corbyn David, "Strain and Surface Effects on the Magnetism of Epitaxial Nickel Cobaltate Thin Films" (2022). ETD collection for University of Nebraska - Lincoln. AAI29165788.