Domain walls in spin-valve nanotracks : characterisation and applications

Abstract

Magnetic systems based on the manipulation of domain walls (DWs) in nano-tracks have been shown to store data at high density, perform complex logic operations, and mechanically manipulate magnetic beads. The magnetic track has been a model system to study magnetic and magneto-electronic phenomena, such as field induced DW propagation and spin-transfer torque. This thesis focuses on DW manipulation and DW-based devices in spin-valve (SV) tracks. In comparison to monolayer tracks, the SV track enables more sensitive and versatile measurements, as well as an electronic output of DW-based devices, of crucial interest to applications. However, these multi-layered tracks add new, potentially disruptive magnetic interactions, as well as fabrication challenges. In this thesis, the DW propagation in SV tracks of different compositions was studied, and a system with DW propagation properties comparable to the state-of-the-art in monolayer tracks was demonstrated, down to an unprecedented lateral size of 33nm. Several logic devices were demonstrated and studied, namely a turn-counting DW spiral, a DW gate, DW logic NOT gates, and a DW-DW interactor. It was found that the magnetic behaviour of these devices was analogous to that of monolayer structures, and the device performance, as defined by the range of field wherein they function desirably, was found to be comparable to that of monolayer systems. The interaction between DWs in adjacent tracks was studied and new phenomena were characterised, such as DW depinning induced by a static or travelling adjacent DW. The contribution of different mechanisms to electrical current induced depinning were quantified, and it was found that the Oersted field, negligible in monolayer tracks, was responsible for large variations in depinning field in SV tracks, and that the strength of spin-transfer effect was similar to that reported in monolayer tracks.