Magnetic domain wall dynamics and spin transport in confined geometries

In this work we study the controlled propagation of magnetic domain walls in ferromagnetic nanowires made of Permalloy (Ni80Fe20), including curved geometries, with varying width (asymmetric rings). Two types of motion were studied, firstly field driven domain wall motion via fast rotating magnetic field pulses, and secondly the automotive domain wall propagation in nanoscale spintronic devices. In the first experimental approach, we directly observed domain wall spin structure transformations during motion and quantitatively determined the contribution of the spatially varying potential landscape to its propagation. An angular dependence of the domain wall velocity has been observed and explained by the interplay between the domain wall spin structure and relevant forces that act on the vortex wall. However, in contrast to symmetric ring systems, the interplay between these forces leads to distortion-free domain wall motion. Therefore, using this varying domain wall potential landscape, we are able to control spatially the internal domain wall spin structure transformation and synchronization of the domain wall velocities in ring geometries, even above the Walker breakdown. For the second experimental approach, we report a direct dynamic experimental visualization of spontaneous domain wall propagation in asymmetric ferromagnetic rings, with different widths in the narrowest part. Surprisingly, we observed domain wall automotion with an average velocity of about ~ 60 m/s, which is a significant speed for spintronics devices. We show that the domain wall inertia and the stored energy allow the walls to overcome both the local extrinsic pinning and the topological repulsion between domain walls. Our observation can be explained based on the minimization of the magnetostatic and exchange energies. In order to provide more device functionality we went beyond the propagation of one or two walls and managed to achieve a major breakthrough in the development of methods of information processing in spintronics, by demonstrating a scheme to induce synchronous motion of multiple in-plane domain walls in ferromagnetic nanowires using perpendicular field pulses. This paradigm shifting achievement provides the required functionality for nonvolatile domain wall-based shift register devices. The direct visualization of the domain wall spin structure in all experiments was performed employing time resolved scanning transmission X-ray microscopy, which combines the requisite temporal and lateral resolution needed in our measurements. Finally in order to investigate the influence of miniaturization for ultra-small devices we studied magnetic nanocontacts in order to understand the interaction between spin polarized charge carriers and magnetization on the nanoscale. In particular we studied the evolution of the domain wall magneto-resistance in electromigrated ferromagnetic nanocontact fabricated in ultra-high vacuum conditions. We find that the domain wall pinning strength increases on decreasing the contact cross section. Moreover, we measured the depinning field’s angular dependence and symmetry in order to determine the complete domain wall pinning potential in a device with a narrow constriction. The work presented here paves the way for the development of a new generation of non-volatile spintronic components, which could be implemented in a wide range of applications for logic, sensing as well as data storage devices based on the reliable manipulation of domain walls