Magnetic domain wall dynamics in patterned nanowires

Abstract

This thesis describes the dynamics of magnetic domain walls in two very different ferromagnetic systems. In one system, the magnetization prefers to lie in the plane of the film, which leads to relatively large and complex domain wall structures. In the other system, in which the magnetization is oriented out of the film plane, the domain walls are much smaller and simpler but are more susceptible to the influence of interface effects. In both cases, the dynamics of the domain walls are strongly influenced by the presence of defects, e.g. the inevitable edge and surface roughness of a patterned nanowire. In this dissertation, I explore the resonance dynamics of isolated and coupled transverse domain walls. I show that an intrinsic domain wall mode arises as a result of the pinning effects of the wire edge roughness, by comparing experimental results with micromagnetic simulations. Because of the strong pinning effects, the dynamics of coupled transverse domain walls are also influenced by the edge roughness, leading to the presence of two distinct modes. Using a simple onedimensional model, the domain wall separation dependence of the two resonant frequencies is explained. As edge roughness is unavoidable, I argue that stochastic pinning effects will be present in other DW resonance experiments. I also explore the propagation of a domain wall through a wire, driven under the influence of magnetic fields and electrical currents. The velocity of the domain wall is shown to follow a model where the domain wall moves via a succession of thermally-activated jumps between pinning sites in the wire. A current-to-field equivalence is established, and the nature of this equivalence is explored through the application of static magnetic fields in the plane of the film. These fields influence the internal structure of the domain wall and thus the torques generated by the current. From the symmetry of these torques, it is revealed that the application of a current leads to a spin-Hall effect in an adjacent non-magnetic layer in the film structure, which influences the domain wall dynamics. A nontrivial suppression of the current-to-field efficiency is observed under large in-plane fields, which is potentially linked to how the domain wall moves through a defect landscape.