The discovery of high-temperature superconductivity in 1986 in copper-oxide materials have opened up new avenues to investigate new families of quantum materials that were previously not known. Understanding the mechanism of superconductivity in high-T[sub]c superconductors has been an important research theme in condensed matter physics, as it is believed to be essential to realize the next generation engineered materials that become superconducting at room temperature. Discovered in 2006, iron based superconductors are a new addition to the family of high-T[sub]c superconductors, these materials exhibit several interesting properties and show some vivid similarities with cuprates and other families of high-temperature superconductors. In this thesis, I will present the spin-polarized scanning tunneling microscopy (SPSTM) study carried out on the parent compound of iron chalcogenide high temperature superconductor Fe[sub](1+y)Te to investigate the bi-collinear antiferromagnetic order. Magnetic tips in this work are prepared using a novel preparation technique by picking up excess iron atoms and clusters of FeTe from the surface of the sample. Next, I will present the SP-STM results obtained in the spin glass phase of Fe[sub](1+y)SeₓTe₁₋ₓ visualizing the interplay between the short ranged bi-directional bi-collinear antiferromagnetic order and superconductivity at the atomic scale. In this thesis, I will also present the scanning tunneling microscopy and spectroscopy (STM/STS) study of the native and engineered defect bound states in the iron-pnictide superconductor LiFeAs. This study addresses the pairing symmetry of the superconducting order parameter and understanding of dip-hump features seen in STM spectra outside the superconducting gap in iron pnictide superconductor LiFeAs.

This thesis presents investigations on hydrogenated graphene by scanning tunneling microscopy and spectroscopy (STM/STS) as well as the implementation of spin-polarized STM. Preparation processes for a magnetic standard sample and spin-sensitive chromium tips are developed. The measurements on graphene reveal specific hydrogen adsorption sites in low coverage and the formation of a pattern at higher coverage. Both is found to be in agreement with previous predictions and calculations. Upon hydrogenation, an impurity midgap state emerges in the density of states which is measured directly for the first time. Complementing angle resolved photoemission experiments confirm that this state is dispersionless over the whole Brillouin zone. A routine is developed to prepare the standard sample system of ultra-thin iron films on tungsten (Fe/W(110)). Investigations on this system confirm the magnetic properties known from literature, including the presence of a spin spiral, and prove that it is well suited for the characterization of spin-polarized tips. Different approaches for the preparation of tips from the antiferromagnetic material chromium are tested. Among these, a promising new method is presented: The coating of crystalline chromium tips with fresh chromium material suggests reproducibility of the tip characteristics. The performance of the produced tips in STM measurements is excellent in regard to a fixed spin-polarization, high resolution and stability. Especially, a recovery of the tip magnetization direction proposed in this thesis makes this new preparation method superior to all processes yielding antiferromagnetic tips reported so far.:1 Introduction 2 Basics 2.1 Scanning tunneling microscopy 2.2 Spin-polarized STM – access to magnetic information 2.3 Measurement setup 3 Probing local hydrogen impurities in quasi-free-standing graphene 3.1 Functionalization of graphene 3.2 In-situ fabrication of quasi-free-standing graphene and its functionalization 3.3 Interpretation of the results 3.4 Short summary 4 Chromium tips for spin-polarized tunneling experiments 4.1 Magnetism at the nanoscale 4.2 Growth and properties of Fe/W(110) 4.3 Preparation of tips with outstanding properties 4.4 Short summary 5 Summary and outlook / Inhalt der vorliegenden Arbeit sind Untersuchungen von hydogeniertem Graphen mittels Rastertunnelmikroskopie und -spektroskopie (RTM/RTS) sowie die Einführung spin-polarisierter RTM. Im Rahmen dessen wurden Präparationsprozesse für magnetische Standardproben und spin-sensitive Chrom-Spitzen entwickelt. Die Messungen an Graphen zeigen spezifische Wasserstoff-Adsorptionsstellen bei geringer Bedeckung und die Ausbildung eines Musters bei höherer Bedeckung, jeweils in Übereinstimmung mit Vorhersagen und Berechnungen. Der durch Hydrogenierung entstehende Störstellenzustand in der Bandlücke der Zustandsdichte wurde zum ersten Mal direkt gemessen. Ergänzende winkelaufgelöste Photoelektronenspektroskopieexperimente bestätigen, dass dieser Zustand in der gesamten Brillouinzone dispersionsfrei ist. Ein Verfahren zur Herstellung magnetischer Standardproben aus ultradünnen Eisenfilmen auf Wolfram (Fe/W(110)) wurde entwickelt. RTM-Untersuchungen an diesem System bestätigen die bereits aus der Literatur bekannten magnetischen Eigenschaften, insbesondere das Vorhandensein einer Spinspirale. Damit ist Fe/W(110) hervorragend geeignet für die Charakterisierung spin-polarisierter Spitzen. Verschiedene Ansätze, die zur Herstellung von Spitzen aus dem antiferromagnetischen Material Chrom verfolgt wurden, werden präsentiert, darunter auch eine vielversprechende neue Methode: Das Aufwachsen eines frischen Chromfilms auf kristalline Spitzen desselben Materials verspricht eine Reproduzierbarkeit von Spitzeneigenschaften. Der Einsatz von so hergestellten Spitzen in RTMMessungen ist geprägt von einer festgelegten Spin-Polarisation, hohem Auflösungsvermögen und Stabilität. Insbesondere die mögliche Reproduzierbarkeit der Magnetisierungsrichtung, die in dieser Arbeit diskutiert wird, macht diese Methode allen bisher berichteten Herstellungprozessen antiferromagnetischer Spitzen überlegen.:1 Introduction 2 Basics 2.1 Scanning tunneling microscopy 2.2 Spin-polarized STM – access to magnetic information 2.3 Measurement setup 3 Probing local hydrogen impurities in quasi-free-standing graphene 3.1 Functionalization of graphene 3.2 In-situ fabrication of quasi-free-standing graphene and its functionalization 3.3 Interpretation of the results 3.4 Short summary 4 Chromium tips for spin-polarized tunneling experiments 4.1 Magnetism at the nanoscale 4.2 Growth and properties of Fe/W(110) 4.3 Preparation of tips with outstanding properties 4.4 Short summary 5 Summary and outlook

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No description available.

Spin-polarized electron beam has not yet been produced from an unpolarized electron beam using Stern-Gerlach type spin filter, because of the Lorentz force and Heisenberg uncertainty principle. At present, electron spin detectors and filters work on the basis of spin-dependent scattering of an electron beam from crystal surfaces. Single channel efficiencies of all the spin detectors for electrons are orders of magnitudes lower than the ideal one. Specular reflection type spin-polarized low energy electron diffraction (SPLEED)-spin detectors are having higher single channel efficiencies compared to the conventional Mott detectors. Moreover, multichannel detection can be realized from specular reflection type SPLEED-spin detectors. They have higher effective efficiency than the ideal spin detector. In order to develop specular reflection type spin filter, it is important to develop a spin-polarized low energy electron diffraction system. In addition, SPLEED system allows us to study the spin-orbit and exchange scattering at crystal surfaces. The general direction of the thesis has been the development of spin-polarized low energy electron diffraction (SPLEED) system. This system has been used to investigate the spin-orbit interactions on Ir(100) surface and exchange interactions of Fe grown on Ir(100). The thesis is organized into chapters as follows. Chapter 1 introduce the reader to some of the basic concepts of polarized electrons and the evolution of spin-polarized electron sources and detectors. Sources of polarized electrons are discussed with emphasis on photocathodes such as GaAs and strained GaAs. Widely used spin detector is the Mott detector which works in the higher energy range. The working principle of the Mott detector is discussed. Commonly used spin detector in the lower energy range is the LEED detector. The concept of the LEED detector is also discussed. Working principle and recent developments of specular reflection type SPLEED spin filters are introduced. Evolution of electron spin detector is discussed towards the end of the chapter. Chapter 2 discusses about the two instruments designed and developed during the course of the thesis. The first one is a spin-polarized low energy electron diffraction system working in the reflected electron pulse counting mode in UHV. This system is capable of measuring spin asymmetries due to spin-orbit interaction and exchange interaction. This instrument is useful in understanding structure and magnetism at surfaces as well as helps to develop new spin polarimeter based on SPLEED by evaluating spin asymmetries from different surfaces. All instruments connected to SPLEED system, measurement protocol and controlling software are discussed with some details. Along with this, standard characterization tools such as X-ray diffraction and magneto-optic Kerr effect measurements are discussed. The second instrument is a novel quadratic magneto-optic Kerr effect measurement system using permanent magnets, which is simple, compact and cost-effective. We have used rotating field method to extract QMOKE component in saturation. So there is no need for precise real-time measurement of magnitude and direction of the magnetic field as in the case of vector magnet. This instrument can easily quantify QMOKE coefficients for ferrimagnetic and ferromagnetic thin films and single crystals. Chapter 3 discusses SPLEED experiments carried out on Ir(100)-(1×5)-Hex and Ir(100)-(1×2+2×1)-O surfaces. The surface structure and surface preparation techniques are discussed. The stability of the Ir(100)-(1×5)-Hex surface is evaluated by monitoring the spin asymmetry as the function of time. Within 25 hours after the surface preparation, the profile of the spin asymmetry and the reflected electron count for Ir(100)-(1×5)-Hex surface resembles that of hydrogen adsorbed Ir(100)-(1×5)-H surface. The electron energy-angle of incidence landscape of reflectivity, spin asymmetry and figure of merit are recorded for Ir(100)-(1×2+2×1)-O surface. Many wide regions with a large figure of merit are identified in the E- landscape. Chapter 4 reports SPLEED experiments carried out on Ir(100)-(1×5)-H surface. The comparison between asymmetries evaluated for the Ir(100)-(1×5)-Hex surface after 25 hours and Ir(100)-(1×5)-H surface suggests that Ir(100)-(1×5)-Hex surface is transforming to Ir(100)-(1×5)-H surface, in 25 hours. This can be due to the adsorption of more than four Langmuir of residual hydrogen during this time. The energy-angle landscape of reflectivity, asymmetry and figure of merit are recorded for Ir(100)-(1×5)-H surface in an energy range 20 eV to 100 eV and angle range 10 to 60 . Many regions are identified as the working point for specular reflection type spin filter based on SPLEED. The surface structure and surface preparation techniques are discussed. The stability of the surface is also evaluated. Chapter 5 investigates the growth and magnetic properties of Fe(100) film on Ir(100)-(1×1), Ir(100)-(1×5)-Hex and Ir(100)-(1×2+2×1)-O surfaces. LEED, MEED, LMOKE and QMOKE studies were presented. The growth is found to be layer-by-layer at least up to 20 monolayers (ML) at room temperature. At higher deposition temperature, the MEED oscillations disappear around 3-5 ML. Magnetic anisotropy of the Fe(100) film grown on Ir(100)-(1×2+2×1)-O surfaces is evaluated using LMOKE measurement using Kerr microscope. Simultaneous in-situ LMOKE and MEED measurements were carried out during the deposition. Ferromagnetic ordering with an in-plane easy axis starts above 4.5 ML at room temperature. The Kerr rotation normalized by thickness is evaluated in the pseudomorphic regime and strain relaxed regime. The probing depth of the MOKE is found to be around 14 nm in Fe(100)/Ir(100). An antisymmetric component is observed in the re-magnetization loop measured using MOKE. This antisymmetric loop arises due to the quadratic magneto-optic coupling which is separated by symmetrization and antisymmetrization procedure. The observed quadratic magneto-optic coupling suggests that the analysis based on the assumption that the magneto-optic coupling is linear in magnetization has to be modified. In order to quantify the quadratic magneto-optic coupling parameters, a QMOKE measurement system is developed and measurements were carried out. Chapter 6 discusses SPLEED experiments carried out on various thicknesses of Fe(100) film. Fe(100) films grown on Ir(100) substrate with the thickness less than or equal to 4 ML is not ferromagnetic with in-plane easy axis at room temperature. The non-zero exchange asymmetry observed for 5 ML and above indicates the presence of ferromagnetic ordering. A difference in the profile of exchange asymmetry is observed between pseudomorphic and strain relaxed regime. Large spin-orbit asymmetry is observed for 1 ML and 2 ML Fe(100) which is unexpected from a low atomic number (Z) material. The reason for large spin-orbit asymmetry is still unknown. The energy-angle landscape of reflectivity, exchange asymmetry, spin-orbit asymmetry and figure of merit were evaluated for 21 ML of Fe(100). Many working points were identified for different types multichannel spin filter based on exchange interaction Finally, the various results are summarized and a broad outlook is given.

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Two of the principal phenomena observed and exploited in the field of spintronics are giant magnetoresistance (GMR) and spin transfer torque (STT). With GMR, the resistance of a magnetic multilayer is affected by the relative orientation of its magnetic layers due to (electron) spin dependent scattering. For the STT effect, a spin-polarized electric current is used to alter the magnetic state of a ferromagnet. Together, GMR and STT are at the foundation of numerous technologies, and they hold promise for many more applications. To achieve the high current densities (~10¹² A/m²) that are necessary to observe STT effects, point contacts – constricted electrical pathways (~1–100 nm in diameter) between conducting materials – are often used because of their small cross-sectional areas. In this sense, we have explored STT in bilayer magnetic nanopillars, where an electric current was used to induce precession of a ferromagnetic layer. This precessional state was detected as an increase in resistance of the device, akin to GMR. Temperature dependent measurements of the onset of precession shed light on the activation mechanism, but raised further questions about its detailed theory. Point contacts can also be used as local sources or detectors of electrons. In this context, we have observed transverse electron focusing (TEF) in a single crystal of bismuth. TEF is a k-selective technique for studying electron scattering from within materials. Using lithographically fabricated point contacts, we have studied the temperature dependence of the relaxation time for ballistic electrons from 4.2 to 100 K. These measurements indicated a transition between electron-electron dominated scattering at low temperatures and electron-phonon scattering as the Debye temperature was approached. We present preliminary work toward a TEF experiment to measure spin dependent scattering from a non-magnet/magnet interface. We also investigated spin wave propagation in thin, magnetic waveguide structures. At the boundary between the waveguide and continuous magnetic film, spin wave rays were found to radiate into the film, or to reflect and form standing waves in the waveguide. A circular defect in the waveguide was observed to cause diffraction of spin waves, generating an interference pattern of higher modes of oscillation. / text

In dieser Arbeit werden Resultate elektronischer Transportmessungen von supraleitenden Eisenpniktiden und ferromagnetischen Heusler-Verbindungen diskutiert. Die Eisenpniktide sind eine neuartige Klassen von Hochtemperatursupraleitern, deren Eigenschaften sich aus einem Zusammenspiel von Supraleitung und Magnetismus ergeben. Während die sogenannten 122-Pniktide Antiferromagnetismus aufweisen und unter Dotierung in einen supraleitenden Zustand übergehen, konnte in dotiertem LiFeAs Ferromagnetismus beobachtet werden. Undotiert hingegen zeigt dieses Material interessante supraleitende Eigenschaften. Die Heusler-Verbindungen sind u.a. durch ihren Ferromagnetismus bekannt. Das hier untersuchte Co2FeSi ist einer der stärksten Ferromagnete. Der in diesem Material vorhergesagte vollständig spinpolarisierte elektronische Transport, d.h. alle Leitungselektronen besitzen den gleichen Spin, konnte nachgewiesen werden. Die hier genannten Eigenschaften können exzellent mit der Methode der elektronischen Transportmessungen untersucht werden. Deren Ergebnisse aus Messungen an Einkristallen werden in dieser Arbeit diskutiert. / In this work, results of electronic transport measurements are discussed for superconducting iron pnictides as well as for ferromagnetic Heusler compounds. The iron pnictides are a recently discovered class of high temperature superconductors where magnetism might play a crucial role. While the 122-pnictides show antiferromagnetism and migrate to the superconducting state upon doping, ferromagnetism has been observed in doped LiFeAs. On the other hand, in the undoped state this material shows interesting superconducting properties. Among other propierties, Heusler compounds are well known due to their ferromagnetism. Co2FeSi, which was investigated in this work, is one of the strongest ferromagnets. Beside this, one predicts this compound to be a half-metallic ferromagnet with completely spin polarized electronic transport where all conducting electrons have the same spin. The here addressed properties can well be investigated with the method of electronic transport measurements, whose results on single crystals are discussed in this work.

Dans cette thèse, nous proposons d’explorer la relation entre transport électronique et dynamique d’aimantation afin de mieux comprendre certaines propriétés des films minces de métaux ferromagnétiques. Afin d’extraire l’influencede la diffusion des électrons par les surfaces sur les résistivités dépendantes du spin, des séries d’épaisseur de films de permalloy (Ni80Fe20) ont été déposées et étudiées. En plus de mesures électriques et magnétiques conventionnelles,nous avons réalisé une étude détaillée de la propagation des ondes de spin dans ces films. La technique du décalage Doppler d’ondes de spin induit par un courant électrique a été utilisée pour extraire le degré de polarisation en spin du courant électrique. Nous avons observé que ce degré de polarisation décroît lorsque l’épaisseur du film décroît, ce qui suggère que les surfaces contribuent aux résistivités dépendantes du spin et qu’elles ont tendance à dépolariser le courant électrique. / In this thesis, the interplay between electron transport and magnetization dynamics is explored in order to access to fundamental properties of ferromag- netic metal thin films. With the aim of extracting the influence of the electron surface scattering on the spin-dependent resistivities, thickness series of permal-loy (Ni80Fe20) films were grown and studied. In addition to standard electrical and magnetic measurements, a detailed study of the propagation of spin waves along these films was performed. Resorting to the current-induced spin-wave Doppler shift technique, the degree of spin-polarization of the electrical current was extracted. This degree of spin-polarization was found to decrease when the film thickness decreases, which suggests that the film surfaces contribute to the spin dependent resistivities and tend to depolarize the electrical current.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2016-12-22T12:37:32Z No. of bitstreams: 1 josielcarlosdesouzagomes.pdf: 11267373 bytes, checksum: 393a01f57f4f5afaaf46890b84f4a7ac (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2016-12-22T12:42:22Z (GMT) No. of bitstreams: 1 josielcarlosdesouzagomes.pdf: 11267373 bytes, checksum: 393a01f57f4f5afaaf46890b84f4a7ac (MD5) / Made available in DSpace on 2016-12-22T12:42:22Z (GMT). No. of bitstreams: 1 josielcarlosdesouzagomes.pdf: 11267373 bytes, checksum: 393a01f57f4f5afaaf46890b84f4a7ac (MD5) Previous issue date: 2015-02-26 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A nanotecnologia é uma área de estudo promissora e que nos mostra resultados bastante surpreendentes. Amostras magnéticas (Cobalto e liga de Permalloy (Ni81Fe19), por exemplo) em escala nanométrica, têm como aplicabilidade importante a gravação magnética devido à crescente demanda por meios de gravação cada vez mais rápidos e de alta capacidade de armazenamento. Para determinados tamanhos de nanofios, observa-se a presença de domínios magnéticos e paredes de domínios do tipo vórtice ou transversal que podem ser transportadas para diferentes regiões sem deformação. Pode-se usar tais paredes como bit de informação mas, para isso, precisa-se conhecer com detalhes o comportamento dessas paredes em diversas situações. Neste presente trabalho utilizamos simulações numéricas para estudar o comportamento da magnetização em nanofios retangulares (nanofitas) de Permalloy-79, que apresentam parede de domínio transversal entre domínios “head-to-head”. Utilizamos nestas simulações um modelo no qual os momentos magnéticos interagem através da interação de troca e a interação dipolar. Embora a maioria dos trabalhos encontrados utilizem campo magnético para mover a parede, optamos por aplicar corrente de spin-polarizado na direção do nanofio devido ao fato de ser mais prático de ser produzido. A dinâmica do sistema é regida pelas equações de Landau-Lifshitz-Gilbert e a atuação da corrente é introduzida nessas equações. Fizemos uma abordagem teórica na qual pode-se mostrar como esta equação de Landau-Lifshitz-Gilbert para aplicação de corrente foi obtida. A integração da equação de Landau-Lifshitz-Gilbert é feita utilizando o método de Runge-Kutta e de Predição-Correção. Baseado nessas teorias, escrevemos um programa na linguagem Fortran-90 para realizar as simulações. Em nossos resultados observamos o comportamento da velocidade da parede de domínio em função do tempo e da densidade de corrente. Comparamos estes resultados com a bibliografia. / Nanotechnology is a promising field of study and show us pretty amazing results. Magnetic samples (Cobalt and alloy Permalloy (81NiFe19), for example) at the nanometer scale, have as important applicability the magnetic recording due to the growing demand for recording media ever faster and high storage capacity. For certain sizes of nanowires, it is observed the presence of magnetic domains and vortex domain walls or transverse domain wall which can be transported to different regions without deformation. It can use such walls as bit of information, but for that it is necessary to know in detail the behavior of these walls in various situations. In this work we used numerical simulations to study the behavior of the magnetization in rectangular nanowires (nanostrip) of Permalloy-79, which have transverse domain wall between domains "head-to-head."We used in these simulations a model in which the magnetic moments interact through the exchange interaction and the dipolar interaction. Although most studies found use magnetic field to move the wall, we decided to apply spin-polarized current toward the nanowire due the fact that it is more practical to be produced. The dynamics of the system is governed by the equations of Landau-Lifshitz-Gilbert and the current performance is introduced in these equations. We made a theoretical approach in which you can show how this equation of Landau-Lifshitz-Gilbert for applying current was obtained. The integration of the equation of Landau-Lifshitz-Gilbert is done using the Runge-Kutta and Prediction-Correction methods. Based on these theories, we wrote a program in Fortran-90 language to perform the simulations. In our results we observed the behavior of the domain wall velocity as a function of time and current density. We compare these results with the literature.