Enhanced spin-orbit torque (SOT) and spin accumulation quantification in perpendicularly magnetised systems

Abstract

The ever increasing demand for high-speed data transfer and data processing have pushed the field of electronics toward spintronics, making the utilisation of electron spin in devices a reality. Spin-orbit torque (SOT) is a current-induced phenomenon which manipulates magnetisation via momentum transfer from accumulated spins at the heavy metal (HM)/ferromagnet (FM) interface. SOT strength is related to the anti-damping like (DL) and fieldlike (FL) effective fields. Due to its potential for faster switching and lower energy consumption, the pursuit of SOT physics becomes imperative. In this thesis, systematic investigations of SOT physics were carried out by evaluating the SOT effective fields, i.e. DL term and FL term, through a harmonic Hall measurement technique, in perpendicularly magnetised single FM (CoFeB) and multilayer (Co/Ni) structures. A combination of in-situ Kerr and harmonic measurement enabled the evaluation of the co-existing SOT fields while imaging the magnetisation behaviour during the measurement. Observation of dendritic-like domains indicates the influence of the interfacial Dzyaloshinskii Moriya interaction (DMI) at the CoFeB/Ta interface as affirmed by micromagnetic simulations. In the pursuit of alternative spin Hall materials, rare-earth metal Tb in Pt/[Co/Ni]2/Co/Tb multilayer structures with perpendicular magnetic anisotropy (PMA) have demonstrated enhanced SOT. Due to the large spin-orbit coupling in Tb, the effective damping like efficiency, eff DL  , is determined to be 0.55 for 9-nm Tb as compared to 0.18 eff DL  = for the reference Pt/[Co/Ni]2/Co/Ta stack. Enhanced magnetisation switching efficiency is observed with increasing Tb thickness affirming the sizable anti-damping torque responsible for deterministic switching. Furthermore, due to a more substantial interfacial coupling in Co/Tb interface than in Co/Ta interface, strong angular dependence of the dampinglike and field-like terms is present but diminishes with increasing Tb thickness. These results affirm that Tb plays a significant role in tuning the SOT efficiency in these structures as the increased Tb concentration leads to an enhancement of the effective spin Hall angle and switching efficiency. In a further investigation, an anomalous second harmonic Hall resistance was induced when a biasing current was applied to the structures. The analytical derivations and experimental results reveal an anomalous second Hall resistance originating from the spin accumulation. This measured anomalous second harmonic Hall resistance enables the adiabatic quantification of the spin accumulation in the structures. Spin accumulation ranges from ~3.0% in tTb = 5nm to ~6.6% in tTb = 8 nm, and ~1.3% in the reference stack of tTa = 5nm of the local magnetisation recorded when the total applied current density is 10 11 A/m 2 .