Topological Properties of Magnetic Nanostructures in Amorphous Fe/Gd Multilayers

The dissertation explores in detail the formation of dipole stabilized skyrmions and skyrmion lattices that we discovered in amorphous Fe/Gd multilayers. The dipole skyrmion phase exists in amorphous Fe/Gd films which is commonly associated as a classical magnetic bubble material. However, as we discuss throughout the thesis, there are clear distinctions between both magnetic textures in topology, magnetic energies required to favor their formation and their response under external perturbations. Bubble domains typically form in films where the uniaxial anisotropy Ku exceeds the demagnetization anisotropy Kd or have a Q = Ku/Kd > 1 whereas the dipole skyrmions and skyrmion lattices we report exist in thin films with Q < 1. Over the course of the thesis, we present comprehensive studies as individual chapters describing the magnetic, ferromagnetic resonance and transport properties associated with the dipole skyrmions. For each of these studies, we first identified the dipole skyrmion phase using real and reciprocal space imaging techniques and then quantified the respective properties of these films. We then established a one-to-one correlation between magnetic, ferromagnetic resonance and transport measurements and the domain morphologies observed thus allowing us to assign unique attributes to the dipole skyrmion phase. With aid of numerical simulations, we are able to further validate our experimental observations and investigate additional magnetic information that is not easily accessible in experiments.Overall, dipole skyrmions possess interesting physics that are comparable and complements the physics observed in skyrmions stabilized by the Dzyaloshinskii–Moriya interaction. The sub-100nm features exists over a broad range of temperatures and magnetic fields which can further tuned by modifying the film composition or thickness. The dipole skyrmion phase exists in a material parameter space that had previously not been investigated by neither bubble domain nor skyrmion scientific communities. Last, our results show that a lot of the interesting physics resulting from these textures appears to be a result of topology and not the physical mechanism that forms these chiral cylindrical-like textures