Probing Spin Current in Heavy Metals at the Interface of a Ferromagnet and a Superconductor

Miah, Abu Bakkar (2025) Probing Spin Current in Heavy Metals at the Interface of a Ferromagnet and a Superconductor. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Abu Bakkar Miah (17RS055)) 17RS055.pdf - Submitted Version Restricted to Repository staff only Download (42MB)

Abstract

Today’s society generates, processes, and stores an ever-growing amount of digital data. Our capacity for processing and storing data needs to grow at the same speed to sustain this trend. We constantly need better-performing computers, both in terms of computing power and energy efficiency. Regarding data storage, one energy-efficient approach involves using magnetic media as non-volatile computer memory, exploiting electronic spin to store information. This field, known as spintronics, adds the spin degree of freedom to conventional electronics. One of the most promising spintronic applications is magnetic computer memories based on spin-orbit torque (SOT) technology. SOTs were discovered in heavy metal(HM)/ferromagnet(FM) bilayers and are current-induced magnetic torques that enable deterministic manipulation of the ferromagnet’s magnetization. Since the first SOT studies, the continuous quest for more energy-efficient SOTs (related to the charge-to-spin conversion efficiency of the bilayer) has led to exploring new materials as possible SOT generators. Among the all promising metals, β-W is the best candidate to generate high charge to spin conversion efficiency. This feature makes them excellent candidates for generating efficient SOTs in ferromagnetic heterostructures. This thesis primarily focuses on probing spin current in heavy metals at the interface of a ferromagnet and a superconductor. First, we focus on the ferromagnet (FM)/heavy metal (HM) interface, where the strong spin-orbit coupling (SOC) in the HM results in spin-polarized currents via the spin Hall effect and the Rashba-Edelstein effect. We explore the emergence of spin-orbit torques (SOTs), which enable the manipulation of magnetization in the ferromagnet. This interface offers promising prospects for spintronics applications. In this study, we use tungsten (W) and platinum (Pt) as the HMs and permalloy (Py, NiFe) as the FM. To explore the applications of β-W in spintronic research, we vary the resistivity of W from 100-1000 μΩ-cm. Generally, resistivities of 100-300 μΩ-cm in W correspond to the (α + β)-W phase, whereas resistivities from 300 to 1000 μΩ-cm correspond to the pure β-W phase. We observe the spin-orbit torque efficiency (ξ) as a function of resistivity, confirming that ξSL increases with resistivity, while ξFL shows resistivity-independent behavior. Secondly, we study the voltage pick-up line or the aspect ratio-dependent behavior of ξSL and ξFL in β-W/Py bilayers. We have conducted simulations of the bilayer Hall bars that support our experimental results. We scaled the ξSL data with different aspect ratios, and it matches well. Additionally, we vary the W thickness in the W/Py bilayer to calculate the spin diffusion length of β-W. Next, we delve into the superconductor interface, where the combination of superconductivity and SOC gives rise to exotic phenomena such as currentinduced spin-triplet supercurrents and unconventional spin-polarized Andreev reflections. We investigate these effects in the context of superconducting spintronics and potential implications for quantum computing. In this study, we fabricate several cross-junction devices consisting of a superconducting electrode (Nb) and a non-superconducting electrode with high SOC. We measure local, non-local, and 2-terminal I-V characteristics at different magnetic fields while keeping the sample temperature below the transition temperature of Nb. Differential resistance vs. I characteristics are obtained by differentiating the V-I data. We observe unconventional superconducting behaviors in the dV/dI vs. I characteristics at the interface of Nb and spin-orbit metals. Additionally, we perform basic simulations of local measurements using the BTK model to calculate interface transparency, polarization, and superconducting gaps. Lastly, we study the interface between 2D materials and ferromagnetic systems. The presence of SOC in 2D materials leads to interesting spinvalley coupling and spin Hall effects, enabling efficient spin manipulation and detection in these structures. We employ advanced experimental techniques to probe the underlying mechanisms at these interfaces, such as harmonic Hall measurements and magnetotransport studies. Additionally, we utilize theoretical models to provide deeper insights into the observed phenomena. Our combined experimental and theoretical investigations comprehensively understand spin-orbit coupled effects at these interfaces. The findings presented in this study provide valuable insights into the emerging field of spintronics and the potential for harnessing spin-related phenomena in various electronic devices.

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