Domain walls in ferromagnetic nanowires for atom trapping applications

In this thesis I demonstrate that is possible to transport DWs in ferromagnetic nanowires at arbitrarily low velocities. The approach used here is to confine the DWs into geometrically defined energy minima. DWs are transported by applying a rotating field. The velocity at which the DWs travel depends on the ring radius and the frequency of the applied field.

Several rings of different widths were analysed under different applied fields and a range of frequencies of the applied field. The results show that the walls are pinned by defects as they travel in the ring. The defects cause the walls to lag behind the vector field. The lag depends strongly on the field strength, so that at higher fields the walls move smoother. On the other hand, a high field would cant the magnetic moments in the ring and widen the DW. Also the frequency of the applied field influences the local velocity when DWs depin from defects and a method of calculating this local velocity was described.

The analysis is extended further to a structure which allows linear transport of DWs still at low velocities. The structure is based on the analysis presented for rings, where rotating fields drive the DWs in the structure. By applying successive clockwise and anticlockwise fields, the walls travel through half-rings over large distances.

Finally, multi-ring structures are analysed to study the influence of large periodic pinning of DWs across a correlated system. This is relevant in the light of interacting magnetic systems as well as to DW atom trapping as a means of stimulating collisional atomic interactions. Neighbour rings in this kind of structures can introduce DWs into other rings through junctions. Also, due to pinning, DWs might also annihilate. In low fields, each field cycle the number of DWs can be different.