Magnetization Dynamics in Exchange Coupled Multilayers

Abstract

In this thesis, the static and dynamic properties of magnetic multilayer samples were studied using a variety of experimental techniques, based both at Exeter and the Diamond Light Source (DLS) and Advanced Light Source (ALS) synchrotron facilities. The exchange interaction, which acts to align spins, is a fundamental part in these magnetic multilayer samples. First, the glancing angle deposition (GLAD) technique was investigated as a tool for creating magnetic multilayers with exciting new exchange interactions. For this, Co thin films were grown by DC magnetron sputtering to tailor the magnetic anisotropy of the samples. These Co samples were structurally characterized using x-ray diffraction (XRD) and transmission electron microscopy (TEM). Vibrating sample magnetometry (VSM) was then performed to investigate the magnetic properties of the thin films as a result of the GLAD technique. From this, the necessary conditions for effective anisotropy control using the GLAD technique were identified. Synchrotron x-ray measurements, such as x-ray magnetic circular / linear dichroism (XMCD/XMLD) for static measurements, are vital for investigating magnetic multilayer samples with elemental resolution. To add depth-sensitivity to the synchrotron measurements, the idea of an ultra-thin Mn “spy layer” was investigated by inserting different thicknesses (tMn) of Mn into the NiFe layer in a FePt / NiFe bilayer. The effect on the static magnetic properties was studied using VSM and XMCD hysteresis loops before structural information was obtained using scanning transmission electron microscopy (STEM). The magnetization dynamics were probed using vector network analyzer ferromagnetic resonance (VNA-FMR) and element resolved x-ray ferromagnetic resonance (XFMR) measurements. From this, the ideal “spy layer” thickness of Mn was found to lie in the region 0Å < tMn < 5Å . Spin currents are a dynamic process found in magnetic multilayers and are driven by the exchange interaction. The measurement of a transverse charge current generated via the inverse spin Hall effect (ISHE) has become the principal technique for observing spin currents. During ISHE measurements, parasitic microwave effects were observed and a method to separate out the inverse spin Hall effect was identified. This method was then tested for a reference YIG / Pt bilayer. A more complex NiFe / NiO / Pd / FeCo sample was then studied using this procedure and the ISHE voltage was identified, despite the presence of additional parasitic effects. In addition to the DC spin current component, there is an AC spin current contribution. The AC spin current component was also investigated for the NiFe / NiO / Pd / FeCo sample series using XMCD, XMLD and XFMR measurements. The XMCD and XMLD data revealed the Ni and (Fe)Co spins possess perpendicular in-plane coupling relative to the magnetic moments within the NiO layer. To understand the magnetization dynamics in these samples, an evanescent spin wave model was invoked. This provides crucial insights for interpreting spin current propagation through NiO. Through the combination of work described above, new avenues for the fabrication of magnetic multilayers and the measurement of the magnetization dynamics in such systems are presented to yield a more complete understanding of the crucial role of the exchange interaction in the magnetization dynamics of magnetic multilayers.