Material designs and fabrications for various spintronics applications
Date
2018
Authors
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Publisher
University of Delaware
Abstract
Spintronics which deploys the spin in addition to or sometimes in place of the charge of the electron has exhibited rich physics and industrial potential in the past two decades. Intensive research has been carried out probing the interaction between spin and magnetic field, electric current, electromagnetic waves and more recently temperature gradient. Work presented in this dissertation focuses on utilizing spin dynamics in contemporary applications. ☐ Generation of pure spin current in a magnetic material subjected to a temperature gradient is called the spin Seebeck effect (SSE). SSE measured for metallic ferromagnetic thin films in commonly used longitudinal configuration is contaminated by the anomalous Nernst effect (ANE), considered to arise from the bulk of the ferromagnet (FM) and the proximity-induced FM boundary layer. Thus, isolation of pure SSE from other parasitic effects in FM metals has been a matter of significant interest and a point of hot debate. We optimized an FM alloy with zero Nernst coefficient to mitigate the ANE contamination of SSE and inserted a thin layer of Cu to separate the heavy metal (HM) from the FM to avoid the influence of the proximity effect. These modifications to the experiment should permit the complete isolation of SSE from ANE. However, further thickness dependence studies and careful analysis of the results revealed, ANE of the isolated FM alloy is twofold, surface and bulk. The existence of a surface contribution to ANE in FM metals was first uncovered in our research. Magnitudes of surface and bulk contributions are comparable to that of the SSE and can be modified by the neighboring layer. Hence, the surface contribution is an important effect that needs to be considered. ☐ Nanomedicine, the amalgamation of nanotechnology with medical diagnostics has gained a lot of traction in recent years. Especially point of care (POC) assay systems using magnetic nanoparticles (MNPs), which provide improved sensitivity and specificity along with better reproducibility and reliability. Profound efforts have been made to fabricate a bio-detection system incorporating magnetic nanoparticles in a magnetoresistive (MR) sensor. Details of fabrication and optimization of a magnetic tunnel junction (MTJ) based sensor for bio-detection applications are discussed in this work. The goal of this project is to develop an assay for early risk assessment of genetically pre-disposed children with Type-I diabetes. We combine biotechnology with MNPs to enable enrichment of the diabetes autoantibodies in a test sample. Then, detect and measure MNPs with enhanced sensitivity on an MTJ sensor. ☐ Local electrical manipulation of magnetization is a promising candidate for information storage and processing. Electrical control of magnetization can be achieved by leveraging the spin-orbit coupling (SOC), through which an electrical current exerts a spin-orbit torque (SOT) on the magnetization. While there are still debates over the dominant SOC mechanism, one thing is quite clear, the interfacial electronic structures and spin textures play a critical role in determining SOC phenomena. The interfacial electronic structures and spin textures are intimately related to materials and fabrication. They are very difficult to characterize directly. However, their characteristics are manifested in SOT behaviors. Most recent first principle and semi-classical calculations indicate SOTs will depend on the current distributions in HM and FM layers. To experimentally validate this theory, we compare temperature dependent SOTs in structures with markedly different resistivities. Temperature-dependent Magneto-optical Kerr spectroscopy (MOKE) has been employed to probe the SOTs and the results of CoFeB(t)/Pt and Py(t)/Pt structures were compared. In addition, magnetic structures with perpendicular magnetic anisotropy (PMA) were developed and optimized. Temperature-dependent SOTs for these structures were extracted with DC-MOKE measurement. Moreover, PMA samples were used to perform time-resolved MOKE (TR-MOKE) study of magnetization switching.