Investigation of Spin Pumping in Yttrium Iron Garnet/Tungsten-Titanium (YIG/W-Ti) Bilayers

Abstract

The development of energy-efficient spintronic technologies hinges on the ability to generate and manipulate pure spin currents with minimal energy loss. Among the mechanisms to achieve this, spin pumping—wherein a precessing ferromagnet injects spin current into an adjacent non-magnetic material—has emerged as a powerful approach. The efficiency of this process is governed by the spin mixing conductance at the ferromagnet/non-magnet (FM/NM) interface and is strongly influenced by the choice of materials, their structural phases, and interfacial quality. In this context, Yttrium Iron Garnet (YIG) stands out as a magnetic insulator with ultra-low Gilbert damping, making it ideal for studying spin dynamics. However, optimizing the non-magnetic layer for efficient spin current absorption remains an ongoing challenge. This thesis investigates the spin pumping behavior in bilayers composed of YIG and a Tungsten-Titanium (W–Ti) alloy, a material system that has received limited attention despite the potential of tungsten-based alloys in spintronic applications due to their strong spin-orbit coupling. YIG thin films were grown using pulsed laser deposition and subsequently capped with W–Ti layers of varying thicknesses (1–10 nm). Structural and compositional characterizations were performed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and profilometry. Magnetodynamic properties such as the Gilbert damping and effective magnetization were extracted via broadband ferromagnetic resonance (FMR) measurements. Our results reveal a pronounced enhancement in magnetic damping in YIG/W-Ti bilayers compared to bare YIG films, confirming the observation of spin pumping. By systematically varying both YIG and W–Ti thicknesses, we quantified the spin mixing conductance and uncovered two distinct regimes of spin transport. At lower W–Ti thicknesses, a higher spin mixing conductance is observed, attributed to the formation of the β-phase of W–Ti, known for its high spin-orbit coupling. In contrast, thicker W–Ti layers exhibit a transition to the α-phase, associated with reduced spin current transmission. This work sheds light on the structural-phase-dependent nature of spin pumping in W–Ti alloys and demonstrates the potential of engineering FM/NM interfaces to optimize spin current generation and transfer. These insights provide valuable guidance for the design of next-generation spintronic devices based on magnetic insulator/heavy metal heterostructures.