Electrical transport in hybrid spintronic structures

Injection and detection of spin polarised current in a metal/semiconductor device and the measurement of the degree of injected spin polarisation are two key issues in the development of hybrid spintronics. This thesis touches on both themes as it details the development of planar Andreev spectroscopy as a tool to measure injected spin and the electrical characterisation of MgO tunnel barriers for efficient spin injection and detection. Point contact Andreev reflection spectroscopy has been widely used tomeasure transport spin polarisation in magnetic materials. Planar Andreev structures have an advantage over point contacts as they offer greater control over interface quality and the possibility of spatially resolved information about the spin polarisation using nanojunction arrays. We find that planar junctions compare favourably to point contacts in that they can yield low interface barriers and minimal nonthermal smearing. We show that a low interface barrier is critical for accurate detection of spin polarisation, particularly in semiconductorswhere large Fermi velocitymismatch contributes to the barrier. Furthermore, the fabrication method strongly affects all parameter values. For Pb/InAs planar junctions we demonstrate that the most feasible way to obtain interfaces suitable for spin detection is an “etch-back” processing strategy. The processing routes are shown to be scalable to nanoarray fabrication to allow measurement of spin accumulation. We also examine the electrical properties of ultrathin MgO barriers grown on InAs epilayers and the dependence of barrier characteristics on InAs surface pretreatment and growth conditions. Chemical pretreatment improves the yield of tunnel junctions and changes the roughness of the interface between the oxide and the semiconductor. Electrical characterisation confirms that tunnel barriers with appropriate values of interface resistance for efficient spin injection/detection have been achieved. Using the Rowell criteria and various tunnelling models we show that single step tunnelling occurs above 150 K and a thermal smearing model suggests that tunnelling is the dominant transport process down to 10 K

Double magnetic proximity in Fe/Fe0.32V0.68 superlattices

Publisher's version (útgefin grein)The conventional magnetic proximity effect and double-proximity effects were studied in a set of fully coherent high-quality Fe/Fe0.32V0.68 superlattices. Applying a simple model to the saturation magnetization, it is seen that the magnetic proximity effect is gigantic in magnitude in the alloy—the magnetization is enhanced by 20–450 % and the ordering temperature is enhanced by a factor of 2. The magnitude of the effect can be explained by the large susceptibility of the alloy above its intrinsic ordering temperature. Additionally, a strong dependence of the ordering temperature of single monolayers of Fe on the interlayer distance is observed. The results give insight into new ways of using alloying and large magnetic susceptibility combined with magnetic proximity effects to enhance the functionality of materials that are of interest for spintronic devices.F.M. acknowledges funding from the Icelandic Centrefor Research (Grant No. 174271-051). H.P. wishes to thankGiuseppe Muscas for fruitful discussions.Peer reviewe

Giant magnetic proximity effect in amorphous layered magnets

Publisher's version (útgefin grein)Here we study the magnetic proximity effect in amorphous layered magnets of alternating high- and low-Tc materials using magnetometry and polarized neutron reflectivity. By altering the thickness of either the high- or low-Tc layer we are able to extract the induced magnetic moment in the low-Tc layer directly and study how it scales with thickness. We observe that the ordering temperature of the low-Tc layer is enhanced and above which a second magnetically ordered state with a very large extension is observed. This induced magnetic state survives to a temperature at least three times that of the ordering temperature of the low-Tc layer and the induced magnetization is approximately constant throughout at least a 10-nm-thick layer. The induced magnetic region within the low-Tc layer does not depend on the thickness of the adjacent high-Tc layer.This work was supported by the Icelandic Centre forResearch, Grant No. 174271-051, the University of IcelandResearch Fund, and the Swedish Research Council (VR).Peer reviewe

Phase Evolution and Microstructure Analysis of CoCrFeNiMo High-Entropy Alloy for Electro-Spark-Deposited Coatings for Geothermal Environment

Publisher's version (útgefin grein)In this work, a CoCrFeNiMo high-entropy alloy (HEA) material was prepared by the vacuum arc melting (VAM) method and used for electro-spark deposition (ESD). The purpose of this study was to investigate the phase evolution and microstructure of the CoCrFeNiMo HEA as as-cast and electro-spark-deposited (ESD) coating to assess its suitability for corrosvie environments encountered in geothermal energy production. The composition, morphology, and structure of the bulk material and the coating were analyzed using scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The hardness of the bulk material was measured to access the mechanical properties when preselecting the composition to be pursued for the ESD coating technique. For the same purpose, electrochemical corrosion tests were performed in a 3.5 wt.% NaCl solution on the bulk material. The results showed the VAM CoCrFeNiMo HEA material had high hardness (593 HV) and low corrosion rates (0.0072 mm/year), which is promising for the high wear and corrosion resistance needed in the harsh geothermal environment. The results from the phase evolution, chemical composition, and microstructural analysis showed an adherent and dense coating with the ESD technique, but with some variance in the distribution of elements in the coating. The crystal structure of the as-cast electrode CoCrFeNiMo material was identified as face centered cubic with XRD, but additional BCC and potentially σ phase was formed for the CoCrFeNiMo coating.This work is part of the H2020 EU project Geo-Coat: Development of novel and cost-effective corrosion resistant coatings for high temperature geothermal applications. Call H2020-LCE-2017-RES-RIA-TwoStage (Project No. 764086).Peer Reviewe

Electrical transport in hybrid spintronic structures

Injection and detection of spin polarised current in a metal/semiconductor deviceand the measurement of the degree of injected spin polarisation are two key issuesin the development of hybrid spintronics. This thesis touches on both themes asit details the development of planar Andreev spectroscopy as a tool to measureinjected spin and the electrical characterisation of MgO tunnel barriers for efficientspin injection and detection. Point contact Andreev reflection spectroscopy has been widely used tomeasuretransport spin polarisation in magnetic materials. Planar Andreev structures havean advantage over point contacts as they offer greater control over interface qualityand the possibility of spatially resolved information about the spin polarisationusing nanojunction arrays. We find that planar junctions compare favourably topoint contacts in that they can yield low interface barriers and minimal nonthermalsmearing. We show that a low interface barrier is critical for accurate detectionof spin polarisation, particularly in semiconductorswhere large Fermi velocitymismatchcontributes to the barrier. Furthermore, the fabrication method strongly affectsall parameter values. For Pb/InAs planar junctions we demonstrate that themost feasible way to obtain interfaces suitable for spin detection is an ?etch-back?processing strategy. The processing routes are shown to be scalable to nanoarrayfabrication to allow measurement of spin accumulation. We also examine the electrical properties of ultrathin MgO barriers grown onInAs epilayers and the dependence of barrier characteristics on InAs surface pretreatmentand growth conditions. Chemical pretreatment improves the yield oftunnel junctions and changes the roughness of the interface between the oxide andthe semiconductor. Electrical characterisation confirms that tunnel barriers withappropriate values of interface resistance for efficient spin injection/detection havebeen achieved. Using the Rowell criteria and various tunnelling models we showthat single step tunnelling occurs above 150 K and a thermal smearing model suggeststhat tunnelling is the dominant transport process down to 10 K.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Magnetostrictive properties of amorphous SmCo thin films with imprinted anisotropy

We examine the magnetostriction in amorphous SmCo thin films with a composition in the range 4-27 at.% Sm. The magnetostriction increases significantly with increasing Sm content but is small compared to terbium-based ferromagnetic compounds, despite the large imprinted anisotropy. The magnetostriction and anisotropy both increase approximately linearly as the temperature is reduced. The magnetoelastic energy is found to be far smaller than the anisotropy energy so the magnetoelastic atomic displacements during growth cannot be the origin of the imprinted anisotropy. The anisotropy is only slightly altered by the application of large tensile stresses, indicating that the local strain fields involved in magnetostriction are not equivalent to the global strain produced by mechanical bending

Tailoring anisotropy and domain structure in amorphous TbCo thin films through combinatorial methods

We apply an in-plane external magnetic field during growth of amorphous TbCo thin films and examine the effects on the magnetic anisotropy and domain structure. A combinatorial approach is employed throughout the deposition and analysis to study a continuous range of compositions between 7â\u80\u9395 at.% Tb. Magnetometry measurements show that all samples have a strong out-of-plane anisotropy, much larger than any in-plane components, regardless of the presence of a growth field. However, magnetic force microscopy demonstrates that the growth field does indeed have a large effect on the magnetic domain structure, resulting in elongated domains aligned along the imprinting field direction. The results show that the anisotropy can be tuned in intricate ways in amorphous TbCo films giving rise to unusual domain structures. Furthermore the results reveal that a combinatorial approach is highly effective for mapping out these material properties