Rohe Salomon Da Rosa Rodrigues, Davi (2018-05). Towards a New Generation of Nonvolatile Memory Devices: Creation and Manipulation of Topological Magnetic Structures by Electric Current. Doctoral Dissertation.
Thesis
In this thesis we propose a novel method to study the dynamics of topological magnetic textures. Based on the stability of these objects, scaling and symmetry arguments, we show that, despite the complexity of the micromagnetic model, the electric and magnetic driven dynamics can be described in terms of a few relevant dynamical parameters. This method reproduces well known behaviors reported in the literature without the assistance of sophisticated micromagnetic numerical calculations. Moreover, it allows for the study of new phenomena relevant for proposing new memory devices based on topological textures. Based on a specific configuration of a nanowire with a strong pinning point, we predict a periodic injection of domain walls by all electrical means. Our analytical results reveal the existence of a critical current. For currents below the critical current, the magnetic configuration is stable and fully defined by a single parameter. For currents slightly above the critical current, this parameter becomes dynamical and is associated to the periodic injection of domain walls into the nanowire. The period is given by a universal exponent T (wavy line) (j - jvc)1/2. The process is very general and independent of microscopic details. A major feature is that the process is independent of "twisting" terms or applied external magnetic field. We also propose a Hamiltonian dynamics formalism for the current and magnetic field driven dynamics of ferromagnetic and antiferromagnetic domain walls in one-dimensional systems. We obtain Hamiltonian equations for pairs of the dynamical parameters that describe the low energy excitations of domain walls. This model independent formalism includes both the undamped and damped dynamics. We use it to study current induced domain wall motion in ferromagnetic and antiferromagnetic materials. In the second material, we include also the influence of magnetic fields and predict an orientation switch mechanism for antiferromagnetic domain walls which can be tested experimentally. Moreover, we extend the formalism from nanowires to thin-films and study extended domain walls as string objects. The description includes the dynamics of vortices and curvatures along the domain wall as well as boundary effects. We provide an effective action that describes the dynamics of domain walls with periodic boundary conditions. By considering closed domain walls, we included the dynamics of smoothly deformed skyrmions in the large radius limit. Our theory provides an analytical description of the excitation modes of magnetic skyrmions in a natural way. The method developed along the thesis proves to be rich and powerful, being crucial for the development of a new generation of memory devices based on magnetic topological textures.
