Magnetic Multilayer Structures for Spin Torque Nano Oscillator

The Spin-Torque Nano-Oscillators (STNOs) have been attracting tremendous attention as an innovative nano-scale microwave signal generator for modern electronics systems. The key features include large range of frequency tunability, nanoscale size and compatibility with standard silicon technology. This work explored different magnetic multilayers both from the material perspective and the device aspect with an ultimate goal to lower the critical current density of spin torque switching and to create coherent spin torque oscillation with output power towards the µW range.

The major task of this dissertation was to establish the benchmark for spin-torque nano-oscillators fabricated using the Bias Target Ion Beam Deposition (BTIBD) technique and the photolithography techniques. A Co2FeAl/Cr/Co2FeAl trilayer was evaluated to have a small MR ratio ~0.07% at room temperature. The small MR ratio will severely affect the output voltage and impede its application on spin-transfer oscillation devices. Optimizations were made on CFA/Cu/CFA and nano-oxide layer(NOL)/CFA/Cu/CFA multilayers to improve the MR ratio. Significant enhancement of MR ratio was observed in the NOL/CFA/Cu/CFA pseudo spin valve (PSV) due to the specular reflection from the NOL. The origin of the magnetoresistance in single layer L10 MnAl was studied by linking the resistivity change to the domain wall (DW) scattering of charge carriers. Quantitative analysis on remanent states’ MFM images and the corresponding resistivity confirmed the contribution of DWs to the electric resistivity of MnAl. Magnetic static and magneto transport properties for a magnetic hybrid multilayer configuration CFA/Cr/MnAl have been investigated for the purpose of spin transfer study. Low room temperature MR ratio of ~0.05% was observed on the hybrid PSV possibly due to the small spin-polarization in the thin MnAl films.

From the device perspective, DC current induced magnetization reversal and magnetization oscillation was observed in 500 nm large size Co90Fe10/Cu/Ni80Fe20 pillars. A perpendicular external field enhanced the coercive field separation between the reference layer (Co90Fe10) and free layer (Ni80Fe20) in the pseudo spin valve, allowing a large window of external magnetic field for exploring the free-layer reversal. A magnetic hybrid structure was achieved for the study of spin torque oscillation by applying a perpendicular field > 3kOe. The magnetization precession was manifested in terms of the multiple peaks on the differential resistance curves. Depending on the bias current and applied field, the regions of magnetic switching and magnetization precession on a dynamical stability diagram has been discussed in details. Micromagnetic simulations are shown to be in good agreement with experimental results and provide insight for synchronization of inhomogeneities in large sized device. The ability to manipulate spin-dynamics on large size devices could be proved useful for increasing the output power of the spin-transfer nano-oscillators (STNOs).