Domain wall behaviour in ferromagnetic nanowires with interfacial and geometrical structuring

Abstract

The magnetic behaviour in nanoscale structures is of great interest for the fundamental understanding of magnetisation processes and also has importance for wide ranging technological applications. This thesis examines mechanisms for the enhanced control of domain walls in these structures via focussed ion beam modifications to magnetic nanowires and through the inclusion of periodic geometrical modifications to the nanowires geometry.

A detailed investigation into the effect of focussed ion beam irradiation on the structure of NiFe/Au bilayers was performed through x-ray reflectivity and fluorescence techniques. This analysis revealed the development of interfacial intermixing with low dose irradiation. This is associated with complex changes of the magnetic behaviour including a rapid decrease, followed by a recovery of the saturation magnetisation with low dose irradiation. This behaviour is attributed to changes in the local environment of the atoms at the interface; resulting in modifications to the magnetic moment on Ni and Fe. The development of an induced moment on Au and a change in the spin-orbit interaction is also suggested.

Localised control of the magnetic properties in nanowires demonstrates the ability to manipulate domain walls in these structures. Here, irradiated regions provide pinning sites where the width and dose of the irradiated region give control over the pinning potential.

The inclusion edge modulation to nanowires geometry provides additional control over their magnetic behaviour. The direct magnetisation reversal field of these structures is explained by an analytical model based on the torque on the spins following the modulated wire geometry. This model is scalable for different modulation parameters and combines with the effect of localised regions of orthogonal anisotropy along the wire; explaining the reversal behaviour over the entire parameter space.

Domain wall mediated reversal in modulated wires was also investigated in these structures. The inclusion of modulation shows an improvement in dynamic properties by the suppression of Walker breakdown. This is due to the relationship between geometrical modulations and the periodicity of micromagnetic domain wall structural changes during the Walker breakdown process. The combination of this work shows a route to the optimisation of the dynamic properties whilst minimising the detrimental increase in the de-pinning field from the modulation.