Competing anisotropies in epitaxial exchange biased thin films and patterned nanostructures

Abstract

Magnetization reversal properties in thin-films and patterned nanostructures are essential for various modern magnetic devices. The magnetization reversal is determined by the magnetic anisotropies of the system, which can come from different origins. Specifically, the exchange bias, also known as the exchange anisotropy, brings in a shift of the magnetic hysteresis loop which has been widely applied in magnetic recording technologies and magnetic sensors. In an exchange bias system, other types of magnetic anisotropies, such as the magnetocrystalline anisotropy or the shape anisotropy, can be present and interacts with the exchange anisotropy thus affecting the magnetic reversal cooperatively. In order to study the competing magnetic anisotropies in the exchange bias system, we have developed approaches to grow epitaxial exchange-biased ferromagnetic-antiferromagnetic thin-film bilayers, which bring significant magnetocrystalline anisotropy into the exchange bias system. We studied the magnetization reversal of these bilayers using magneto-optic Kerr effect and anisotropic magnetoresistance, and investigated systematically the dependence of magnetic anisotropies on various sample parameters, including layer thickness, temperature, relative orientation, interfacial spin behavior, and the antiferromagnetic bulk structure. To explain the samples' unique magnetic behaviors, we developed a quantitative model based on magnetic domain-wall nucleation and propagation process, and offered a consistent and integrated interpretation on the magnetic reversal properties. To further incorporate the shape anisotropy into the exchange bias system, we developed a large-area, efficient epitaxial patterning process using nanoimprint lithography, which allows us to pattern epitaxial magnetic thin-film bilayer and multilayers into nanostructures. By combining the bilayer-resist template and Molybdenum liftoff, we are able to create high-quality, patterned structures with different sizes ranging from ~ 70 nano-meter to ~ 5 micro-meter. The magnetization reversals, especially the domain-wall structures during the reversal, are subsequently studied by hysteresis loop measurements and direct imaging techniques. Finally, in addition to fundamental studies on the magnetic reversal, the nanoimprint processes developed for this thesis can also find great potential in relevant industries.