Tailoring spin dynamics in asymmetric FM1/Pt/FM2 trilayers via Pt spacer thickness

The study of trilayers with a non-magnetic (NM) spacer layer separating two ferromagnetic layers (FM/NM/FM) has been an active area of spintronics research due to their real-world applications, such as in development of giant magnetoresistance (GMR) devices where NM spacer layer enables the control of magnetization orientation in free FM layer with respect to pinned FM layer; with application in hard disk drive read heads, MRAM, GMR magnetic sensors etc. For trilayer systems, the generation of spin current, control of magnetization using spin orbit torques (SOTs), understanding the interlayer indirect exchange coupling between the FM layers, effects of NM/FM interface on static and dynamic spin properties etc. are active areas of research. Asymmetric trilayer system (FM1/NM/FM2) comprising dissimilar FM layers and hence dissimilar NM/FM interfaces have additional level of complexities with non-uniform environment for generation and transport of spin current the complex spin dynamics and SOTs remain relatively underexplored. This makes exploration of asymmetric trilayers system, a very interesting area of research for the development of advanced device design and achieving greater spintronic functionalities. This thesis sheds light on the interplay between Pt spacer layer thickness and the magnetization dynamics of an asymmetric trilayer of Ni80Fe20/Pt/Co, particularly focusing on the role of spin pumping, indirect exchange coupling of FM layers, differential spin transport properties of Co/Pt and Ni80Fe20/Pt interfaces, the relative strengths and significances of damping-like and field-like torques in coupled and decoupled magnetization state of the trilayer system. The key findings of this research include: Spin pumping and Spin Transport in Asymmetric Trilayer: The FMR spectroscopy experiments revealed two distinct resonance peaks in the decoupled regime (thicker Pt), indicating independent precession of the Co and NiFe layers, while in coupled regime (thinner Pt) only one resonance peak was observed due to the in-phase precession of the FM layers' magnetizations. The coupling in the trilayer was attributed to indirect exchange coupling (IEC) mediated by the Pt layer. The role of spin pumping in driving spin currents across the the NiFe/Pt and Co/Pt interfaces was iterated and it was found that the spin-pumping-induced damping (????????????) has a direct relationship with the Pt spacer layer thickness. Interfacial Spin Characteristics in Asymmetric Trilayer: The role and importance of spin-mixing conductance and spin transparency in characterizing the efficiency of spin transfer at the dissimilar FM/NM interfaces of trilayer system was iterated through this study. The Co/Pt interface was found to be more efficient at transferring spin angular momentum as its spin-mixing conductance (4.12 × 10¹⁹ m⁻²) was estimated to be higher than that of NiFe/Pt interface (1.73 × 10¹⁹ m⁻²). Spin transparency was calculated to be higher (~62%) for the Co/Pt interface compared to NiFe/Pt interface (~25%). The spin current densities estimations at two interfaces also showed similar trend, emphasizing the stronger spin transport capability of the Co/Pt interface. Spin Orbit Torques (SOTs) and Their Origin: The spin torque ferromagnetic resonance (ST-FMR) and dc-bias ST-FMR experiments conclusively showed that both damping-like torque (DLT) and field-like torque (FLT) were present in the trilayer and primarily originated due to the spin Hall effect (SHE) and the Rashba effect respectively. The study demonstrated that the FLT plays a crucial role in systems where the Pt spacer thickness is small (coupled regime). Influence of NM Pt Spacer Layer Thickness on SOT Efficiencies: SOT efficiencies were found to be highly sensitive to the Pt spacer thickness. For thinner Pt layers (coupled regime), the Rashba effect was found play a more dominant role in generating FLT, whereas in the decoupled regime (thicker Pt), the spin Hall effect (SHE) dominated, leading to more efficient spin current absorption and higher DLT. These results provide valuable insights into the complex interplay between spin pumping, indirect exchange coupling of FM layers and spin current generation and absorption at the two FM/NM interfaces. It is expected that these insights could be essential for developing more efficient spintronic devices such as SOT-MRAM and spin-torque nano-oscillators (STNOs).