Topological Insulator-based Spintronics: Study of Magnetization Dynamics and Spin-pumping Phenomenon

Pal, Sayani (2024) Topological Insulator-based Spintronics: Study of Magnetization Dynamics and Spin-pumping Phenomenon. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Sayani Pal (16RS037)) 16RS037.pdf - Submitted Version Restricted to Repository staff only Download (9MB)

Abstract

With the advent of spintronics, a great deal of research has been done to achieve effective spin injection, transport, manipulation, and detection. Scientists are focusing on finding novel materials whose properties are governed by spin-orbit coupling (SOC) because high SOC devices are capable of efficiently control and detect spin. One interesting group of materials that has presented new possibilities for spintronics, is the three-dimensional topological insulators (3D TIs). 3D TIs are defined by an insulating bulk state and topologically protected surface states. The conducting surface states of 3D TI connect bulk conduction and valence bands and forms spin-helical Dirac cone surface states that span the bulk band gap. The surface states preserve the timereversal symmetry and are resistant to both minor perturbations and nonmagnetic disorder. Because of the time-reversal symmetry and the generally high spin-orbit coupling in these materials, the spin orientation of the surface-state charge carriers is directly correlated with the momentum, a phenomenon called "spin-momentum locking." This characteristic makes 3D TIs an exciting area of research because it opens up possibilities for efficiently converting spin into charge, producing large spin-orbit torque and exploring how the topological surface states interact with magnetic systems. Understanding these interactions could lead to the discovery of new phenomena at TI surfaces and interfaces in the TI-based heterostructure. The current thesis seeks to comprehend key elements of spin-based devices, including spin current generation, spin transport, and effective spin-charge interconversion in TI-based heterostructure. Through the use of a custom-built cryo-FMR (ferromagnetic resonance) spectroscopy technique, we investigated the spin-pumping phenomenon in various multilayers of ferromagnetic metal (FM) and topological insulators (TI). We explored topics such as the spin-to-charge conversion at the TI/FM system interface, the generation of efficient spin current in TI/FM systems, and the impact of spin pumping on the magnetization dynamics of FM. In the first experimental part, we have accomplished the complete instrumentation of the low-temperature Vector network analyzer Ferromagnetic Resonance spectroscopy (VNA-FMR) set up in the laboratory to measure the properties of magnetic thin films. We have designed and fabricated an one-end shorted coplanar waveguide (CPW) as an integral part of the home-built cryo-FMR spectroscopy setup. The simple structure, potential applications and easy installation of the CPW inside the cryostat space made it advantageous especially for the low-temperature measurements. It provides a wide band of frequencies in the gigahertz range essential for FMR measurements. Our spectroscopy setup with short-circuited CPW has been used to extract Gilbert damping coefficient(α) and effective magnetization(4πMeff ) values for standard ferromagnetic thin films like Py and Co. The thickness and temperature dependent studies of those magnetic parameters have also been done for the afore mentioned magnetic samples. In the second experimental part, we have discussed the spin-pumping phenomenon in FM (Ni₈₀Fe₂₀)/TI (BiSbTe₁.₅Se₁.₅) bilayer system to understand the magnetization dynamics of ferromagnetic metal (FM) in contact with a topological insulator (TI).We evaluated the parameters like effective damping coefficient, spin-mixing conductance and spin current density to demonstrate an efficient spin transfer in Ni₈₀Fe₂₀/TI (BiSbTe₁.₅Se₁.₅) heterostructure. To probe the effect of the topological surface state, a systematic lowtemperature study of effective damping coefficient, effective magnetization and magnetic anisotropy field have also been accomplished. We have witnessed a spin chemical potential bias-induced spin current injection into the surface states of TI that gets enhanced at low temperatures. Combining the results of our low-temperature measurements we found the existence of an exchange coupling between the TI surface state and FM which does not create any long-range ferromagnetic order in the TI and is unable to alter the overall spin texture of the TI surface state at the interface. However, the presence of topological surface state affects the magnetization dynamics of the ferromagnetic metal quite significantly. These added features of enhancing the damping coefficients enable another fast control of magnetization dynamics in the FM layer. In the third experimental part, we have reported efficient spin-to-charge conversion (SCC) in the topological insulator (TI) based heterostructure (BiSbTe₁.₅Se₁.₅/Cu/Ni₈₀Fe₂₀) by using the spin-pumping technique where BiSbTe₁.₅Se₁.₅ is the TI and Ni₈₀Fe₂₀ is the ferromagnetic layer. We applied external magnetic field (Hdc) in the plane of the sample and microwave field (hr f ), perpendicular to Hdc. The magnetization precession inside the FM layer at ferromagnetic resonance conditions pumps spin angular momentum into the TI layer and results spin accumulation at the TI surface state, which in turn develops into a transverse voltage across the TI in the microvoltage range. We measured the transverse voltage signal (V) as a function of the external magnetic field (H) and the typical FMR resonance spectra to determine the SCC efficiency of the system. The SCC efficiency, characterized by inverse Edelstein effect length (λIEE) in the TI material gets altered with an intervening Copper (Cu) layer and it depends on the interlayer thickness. The introduction of the Cu layer at the interface of TI and ferromagnetic metal (FM) provides a new degree of freedom for tuning the SCC efficiency of the topological surface states. The significant enhancement of the measured spin-pumping voltage and the linewidth of ferromagnetic resonance (FMR) absorption spectra due to the insertion of the Cu layer at the interface indicates a reduction in spin memory loss at the interface that resulted from the presence of exchange coupling between the surface states of TI and the local moments of ferromagnetic metal. The temperature dependence (from 8K to 300K) of the evaluated λIEE data for all the trilayer systems, TI/Cu/FM with different Cu thicknesses confirms the effect of exchange coupling between the TI and FM layer on the spin to charge conversion efficiency of the topological surface state.

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