Enhancing the spin-orbit torque efficiency in Pt/CoFeB/Pt based perpendicularly magnetized systems

Abstract

Controlling and switching magnetization by the process of current-induced torque has a considerable advantage in lower power consumption for a memory device and spin logic device application. For a heavy metal (HM) / ferromagnet(FM) / heavy metal (HM) based perpendicular magnetic anisotropy system with broken normal space inversion symmetry, current-induced

magnetization reversal (CIMR) takes place by spin-orbit torque (SOT) with the assistance of an in- plane magnetic field. For a similar HM/FM/HM-based PMA system having broken lateral

symmetry, also known as the quasi-PMA system, the CIMR occurs without the assistance of the in-plane field. Thus the quasi -PMA system has a massive significance for magnetic field-free current-induced magnetization reversal. SOT appears due to the spin-Hall effect and the Rashba effect. Spin Hall effect and Rashba effect evolve due to the space inversion asymmetry (SIA). A field-induced domain wall motion study can help detect the presence of any SIA in a system. The idea about the SIA can help to give an insight view regarding CIMR and qualitative idea about the SOT for any unknown system. In this thesis work, we have performed a comprehensive study on the Pt/CoFeB/Pt-based PMA system to decrease the critical current density for CIMR. In the first work, we studied field-induced domain wall motion in Pt/CoFeB/Pt based quasi-PMA system with varying bottom Pt thickness to understand the influence of the bottom Pt layer in the lateral SIA. This study has helped us optimize the bottom layer Pt thickness for which the lateral SIA is maximum. In the second work, we have chosen Pt/CoFeB/Pt based quasi-PMA heterostructure for CIMR study with the optimal bottom layer Pt thickness for which the effect of lateral SIA is maximum and consists of a ferromagnetic layer having in-plane magnetic anisotropy property at the top layer of the heterostructure. The IMA ferromagnetic layer contributes to the CIMR process by producing the spin-transfer torque (STT) and the stray field. Thus by combing the SOT originated by spin hall effect, STT originated by IMA ferromagnetic layer, and the lateral SIA effect, how the critical current density for CIMR has been reduced for this system has been shown here. Another way to decrease the current density is by enhancing the SOT efficiency using two opposite spin Hall materials. In the following work, we have performed the CIMR study on the Pt/CoFeB/Pt based quasi-PMA heterostructure with the optimal bottom layer Pt (positive spin Hall material) thickness for which the lateral SIA is maximum. The following system has been consists of an IMA ferromagnetic layer at the bottom layer and with varying Ta (negative spin Hall material) thickness at the top layer of the heterostructure to observe the effect of the Ta in the CIMR process. Thus by combing the effect of the enhanced SOT, STT, and the lateral SIA, how the critical current density for CIMR has been reduced further for this system has been shown here.