The chemical and magnetic structural characterisation of magneto-resistive devices using x-ray techniques

Abstract

The academic and industrial interest in magnetic data storage has been fuelled by the information age. The number of applications for magnetic thin-films has increased rapidly, along with the popularity of using X-ray techniques as a tool by which to characterise them. Structural characterisation is a key process to their development and understanding, correlating chemical and magnetic structure to magneto-transport. In this thesis a number of magneto-resistive devices are studied, including spin-valves, magnetic tunnel junctions and magnetic multilayers. The credence of using grazing incidence X-ray techniques for characterisation is initially demonstrated by accounting for variations in MR from nominally identical spin-valves, through observing subtle fluctuations in the pinning NiO layer thickness. The study of magnetic tunnel junctions has revealed discrepancies in barrier thickness as measured by X-ray reflectivity and through fits to the 1-V profile using Simmons’ model. This demonstrates localised tunnelling and the inaccuracy of 1-V modelling in determining average barrier thickness. Specular reflectivity and diffuse analysis have also determined a far larger than expected level of intermixing across Al bilayer interfaces with Al on X and X on Al, where X are transition metals from groups 3,4 and 5 of the periodic table. AF coupled multilayers are studied using polarised neutron reflectivity, the results of which have been compared directly with the relatively new technique: resonant soft X- ray magnetic scattering (SoXMaS). Specular and diffuse scattering studies reveal striking discrepancies between the two techniques, due to differences between the relative magnetic to chemical cross sections. Simulations further demonstrate SoXMaS and its sensitivity to magnetic structure. Finally, reflectivity data from Co/Cu Bragg peaks are used to calculate the refractive index of Co across the Co Lm and Ln edge under different magnetisation orientations.