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Half-metallic ferromagnets (HMFs), with fully spin-polarized conduction electrons, are prime candidates for optimizing spintronic devices. Many Heusler compounds (a class of ternary and quaternary intermetallics) are predicted to be HMFs, in particular Co$_{2}YZ$ (where $Y$ is usually another transition metal, and $Z$ is an s-p element). Crystal structure is controlled by thermodynamics to a large extent. Ideally, one should be able to control and optimize properties which are of interest by appropriately "tuning" the structure (e.g. annealing), but first one must understand the structure and its relation to observed physical properties. A local structural probe technique such as nuclear magnetic resonance (NMR) is an essential tool for identifying and quantifying the various atomic-scale orderings. Different Heusler structure types and antisite disorders affect the material's physical properties. In this thesis, order-disorder phenomena in both bulk and thin film samples of Co$_2$Mn$_{1-x}$Si$_x$ and Co$_2$Mn$_{1-x}$Fe$_x$Si have been systematically studied using NMR. Though it is the films which are directly implemented in actual devices, studying bulk samples as model systems provides invaluable information regarding the material properties. The evolution of local atomic structure in numerous thin films has been shown to depend greatly on preparation parameters, including post-deposition annealing temperature, and specific stoichiometry. For Co$_2$MnSi films, the ideal post-annealing temperature for promoting the $L2_1$ atomic structure was found; the threshold temperature above which structure continues to become higher-ordered in the bulk, but where too much interdiffusion at the buffer interface occurs, degrading the smooth interfaces necessary for high magnetoresistance ratios. NMR also adds evidence that Co$_2$Mn$_x$Si$_{0.88}$ ($x>$1) electrodes in magnetic tunnel junctions have highest tunneling magneto-resistance because the excess Mn suppresses the formation of detrimental Co$_{Mn}$ antisites. A systematic investigation of several thermal and magnetic properties, including Sommerfeld coefficients, Debye temperatures, saturation magnetic moments, spin-wave stiffness, and magnon specific heat coefficient, were measured for selected Co$_2$-based ternary and quaternary Heusler compounds. Obtained values were compared with theoretical ones calculated using electronic band structure methods. It has been systematically shown that adding a magnon term to the specific heat has a negligible effect on the electronic contribution in all cases.

Magnetic ordering of dopants in diamond holds the prospect of exploiting diamond’s unique properties in the emerging field of spintronics. Several transition metal defects have been reported to order ferromagnetically in various semiconductors, however, low Curie temperatures and lack of other fundamental material properties have hindered practical implementation in room temperature spintronic applications. In this Thesis, we consider the energetic stability of 3d transition metal doped-diamond and its magnetic ordering properties at various lattice sites and charge states using ab initio Density Functional Theory methods. We find the majority of 3d transition metal impurities in diamond at any charge state to be energetically most stable at the divacancy site compared to substitutional or interstitial lattice sites, with the interstitial site being highly unstable (by ~8 - 10 eV compared to the divacancy site). At each lattice site and charge state, we find the formation energies of transition metals in the middle of the 3d series (Cr, Mn, Fe, Co, Ni) to be considerably lower compared to those early or late in the series. The energetic stability of transition metal impurities across the 3d series is shown to be strongly dependent on the position of the Fermi level in the diamond band gap, with the formation energies at any lattice site being lower in p-type or ntype diamond compared to intrinsic diamond. Further, we show that incorporation of isolated transition metal impurities into diamond introduces spin polarised impurity bands into the diamond band gap, while maintaining its semiconducting nature, with band gaps in both the spin-up and spin-down channels. These impurity bands are shown to originate mainly from s, p-d hybridization between carbon sp 3 orbitals with the 3d orbitals of the transition metal. In addition, the 4p orbitals contribute significantly to hybridization for transition metal atoms at the substitutional site, but not at the divacancy site. In both cases, the spin polarisation and magnetic stabilization energies are critically dependent on the lattice site and charge state of the transition metal impurity. By allowing magnetic interactions between transition metal atoms, we find that ferromagnetic ordering is likely to be achieved in divacancy Cr+2, Mn+2, Mn+1 and Co0 as well as in substitutional Fe+2 and Fe+1, indicating that transition metal-doped diamond is likely to form a diluted magnetic semiconductor which may successfully be considered for room temperature spintronic applications. In addition, these charge states correspond to p-type diamond, except for divacancy Co0, suggesting that co-doping with shallow acceptors such as B ( will result in an increase of charge concentration, which is likely to enhance mediation of ferromagnetic spin coupling. The highest magnetic stabilization energy occurs in substitutional Fe+1 (33.3 meV), which, also exhibits half metallic ferromagnetic ordering at the Fermi level, with an induced magnetic moment of 1.0 μB per ion, thus suggesting that 100 % spin polarisation may be achieved in Fe-doped diamond. / Physics / D. Litt. et Phil. (Physics)

The ultimate goal in the rapidly burgeoning field of spintronics is to realize semiconductor-based devices that utilize the spin degree of freedom of a single charge carrier (electron or hole) or an ensemble of such carriers to achieve novel and/or enhanced device functionalities such as spin based light emitting devices, spin transistors and femto-Tesla magnetic field sensors. These devices share a common feature: they all rely on controlled transport of spins in semiconductors. A prototypical spintronic device has a transistor-like configuration in which a semiconducting channel is sandwiched between two contacts (source and drain) with a gate electrode sitting on top of the channel. Unlike conventional charge-based transistors, the source electrode of a spin transistor is a ferromagnetic (or half-metallic) material which injects spin polarized electrons in the channel. During transit, the spin polarizations of the electrons are controllably rotated by a gate electric field mediated spin-orbit coupling effect. The drain contact is ferromagnetic (or half-metallic) as well and the transmission probability of an electron through this drain electrode depends on the relative orientation of electron spin polarization and the (fixed) magnetization of the drain. When the spins of the electrons are parallel to the drain magnetization, they are transmitted by the drain resulting in a large device current (ON state of spin FET). However, these electrons will be completely blocked if their spins are antiparallel to the drain magnetization, and ideally, in this situation device current will be zero (OFF state of spinFET). Thus, if we vary the gate voltage, we can modulate the channel current by controlling the spin orientations of the electrons with respect to the drain magnetization. This is how transistor action is realized (Datta-Das model). However, during transport, electrons' velocities change randomly with time due to scattering and hence different electrons experience different spin-orbit magnetic fields. As a result, even though all electrons start their journey with identical spin orientations, soon after injection spins of different electrons point along different directions in space. This randomization of initial spin polarization is referred to as spin relaxation and this is detrimental to the spintronic devices. In particular, for Datta-Das transistor, this will lead to inefficient gate control and large leakage current in the OFF state of the spinFET. The aim of this work is to understand various spin relaxation processes that are operative in semiconductor nanostructures and to indicate possible ways of minimizing them. The theoretical aspect of this work (Chapters 2-5) focuses on the D'yakonov-Perel' process of spin relaxation in a semiconductor quantum wire. This process of spin relaxation occurs because during transport electron spin precesses like a spinning top about the spin-orbit magnetic field. We show that the conventional drift-diffusion model of spin transport, which has been used extensively in literature, completely breaks down in case of a quantum confined system (e.g. a quantum wire). Our approach employs a semi-classical model which couples the spin density matrix evolution with the Boltzmann transport equation. Using this model we have thoroughly studied spin relaxation in a semiconductor quantum wire and identified several inconsistencies of the drift-diffusion formalism.The experimental side of this work (Chapters 6-8) deals with two different issues: (a) performing spin transport experiments in order to extract spin relaxation length and time in various materials (e.g. Cu, Alq3) under one-dimensional confinement, and (b) measurement of the ensemble spin dephasing time in self-assembled cadmium sulfide quantum dots using electron spin resonance technique. The spin transport experiment, as described in Chapter 7 of this dissertation, shows that the spin relaxation time in organic semiconductor (Alq3) is extremely long, approaching a few seconds at low temperatures. Alq3 is the chemical formula of tris- 8 hydroxy-quinoline aluminum, which is a small molecular weight organic semiconductor. This material is extensively used in organic display industry as the electron transport and emission layer in green organic light emitting diodes. The long spin relaxation time in Alq3 makes it an ideal platform for spintronics. This also indicates that it may be possible to realize spin based organic light emitting diodes which will have much higher internal quantum efficiency than their conventional non-spin counterparts. From spin transport experiments mentioned above we have also identified Elliott-Yafet mode as the dominant spin relaxation mechanism operative in organic semiconductors. Electron spin resonance experiment performed on self-assembled quantum dots (Chapter 8) allows us to determine the ensemble spin dephasing time (or transverse spin relaxation time) of electrons confined in these systems. In quantum dots electrons are strongly localized in space. Surprisingly, the ensemble spin dephasing time shows an increasing trend as we increase temperature. The most likely explanation for this phenomenon is that spin dephasing in quantum dots (unlike quantum wells and wires) is dominated by nuclear hyperfine interaction, which weakens progressively with temperature. We hope that our work, which elaborates on all of the above mentioned topics in great detail, will be a significant contribution towards the current state of knowledge of subtle spin-based issues operative in nanoscale device structures, and will ultimately lead to realization of novel nano-spintronic devices.

Research in future generation computing is focused on reducing energy dissipation while maintaining the switching speed in a binary operation to continue the current trend of increasing transistor-density according to Moore’s law. Unlike charge-based CMOS technology, spin-based nanomagnetic technology, based on switching bistable magnetization of single domain shape-anisotropic nanomagnets, has the potential to achieve ultralow energy dissipation due to the fact that no charge motion is directly involved in switching. However, switching of magnetization has not been any less dissipative than switching transistors because most magnet switching schemes involve generating a current to produce a magnetic field, or spin transfer torque or domain wall motion to switch magnetization. Current-induced switching invariably dissipates an exorbitant amount of energy in the switching circuit that nullifies any energy advantage that a magnet may have over a transistor. Magnetoelastic switching (switching the magnetization of a magnetostrictive magnet with voltage generated stress) is an unusual switching paradigm where the dissipation turns out to be merely few hundred kT per switching event – several orders of magnitude less than that encountered in current-based switching. A fundamental obstacle, though, is to deterministically switch the magnetization of a nanomagnet between two stable states that are mutually anti-parallel with stress alone. In this work, I have investigated ways to mitigate this problem. One popular approach to flip the magnetizations of a nanomagnet is to pass a spin polarized current through it that transfers spin angular moment from the current to the electrons in the magnet, thereby switching their spins and ultimately the magnet’s magnetization. This approach – known as spin transfer torque (STT) – is very dissipative because of the enormous current densities needed to switch magnets, We, therefore, devised a mixed mode technique to switch magnetization with a combination of STT and stress to gain both energy efficiency from stress and deterministic 180o switching from STT. This approach reduces the total energy dissipation by roughly one order of magnitude. We then extended this idea to find a way to deterministically flip magnetization with stress alone. Sequentially applying stresses along two skewed axes, a complete 180o switching can be achieved. These results have been verified with stochastic Landau-Lifshitz-Gilbert simulation in the presence of thermal noise. The 180o switching makes it possible to develop a genre of magneto-elastic memory where bits are written entirely with voltage generated stress with no current flow. They are extremely energy-efficient. In addition to memory devices, a universal NAND logic device has been proposed which satisfies all the essential characteristics of a Boolean logic gate. It is non-volatile unlike transistor based logic gates in the sense that that gate can process binary inputs and store the output (result) in the magnetization states of magnets, thereby doubling as both logic and memory. Such dual role elements can spawn non-traditional non-von-Neumann architectures without the processor and memory partition that reduces energy efficiency and introduces additional errors. A bit comparator is also designed, which happens to be all straintronic, yet reconfigurable. Moreover, a straintronic spin neuron is designed for neural computing architecture that dissipates orders of magnitude less energy than its CMOS based counterparts. Finally, an experiment has been performed to demonstrate a complete 180o switching of magnetization in a shape anisotropic magnetostrictive Co nanomagnet using voltage generated stress. The device is synthesized with nano-fabrication techniques namely electron beam lithography, electron beam evaporation, and lift off. The experimental results vindicate our proposal of applying sequential stress along two skewed axes to reverse magnetization with stress and therefore, provide a firm footing to magneto-elastic memory technology.

Le sujet de cette thèse concerne la dynamique auto-entretenue excitée par transfert de spin de vortex couplés, dans des structures de type nano-piliers vannes de spin (Py/Cu/Py). Un premier objectif a été de comprendre les processus de transport polarisé en spin et de transfert de spin associés à des configurations d’aimantation fortement non-homogènes. Cette étude a permis d‘identifier et ainsi de précisément contrôler les configurations magnétiques à base de vortex, et en particulier d’observer l’influence du transfert de spin sur les mécanismes de renversement du cœur de vortex. En combinant des calculs analytiques et des simulations micro-magnétiques, nous avons également pu déterminer les conditions sur les paramètres relatifs des deux vortex (chiralités et polarités) pour obtenir des oscillations gyrotropiques couplées auto-entretenues de deux vortex dans un pilier unique. Un cas très intéressant est prévu pour les piliers de plus grands diamètres (typiquement supérieurs à 200nm) pour lesquels le courant critique est réduit potentiellement à zéro. Les résultats expérimentaux confirment les prédictions sur l’existence d’une dynamique couplée de vortex, avec des largeurs de raies atteignant 200kHz, un record à champ nul (soit un facteur de qualité Q ≈ 5000, un ordre de grandeur plus grand que pour les auto-oscillations de vortex unique) et diminuant même jusqu’à 50kHz sous champ extérieur. Un second objectif de ce travail a été l’étude de la synchronisation de deux auto-oscillateurs à transfert de spin à base de vortex. Nous avons démontré que le verrouillage des phases par couplage dipolaire de deux oscillateurs identiques peut être théoriquement obtenu indépendamment des paramètres des deux vortex. Toutefois un couplage trois fois plus important est prévu dans le cas de vortex de polarités opposées. Du point de vue expérimental, des premiers résultats ont permis de démontrer une faculté de synchronisation de deux oscillateurs présentant un écart en fréquence atteignant jusqu'à 10% de leurs fréquences d'auto-oscillation. Ce travail de thèse, qui s’inscrit dans l’effort de recherche mené pour améliorer les performances rf des nano-oscillateurs à transfert de spin, a permis d’illustrer que l’excitation de modes d’aimantations couplées est une voie à poursuivre dans le but d’aboutir à des largeurs de raies de plus en plus faibles. / My PhD work is dedicated to the spin transfer induced self-sustained dynamics of two coupled vortices, in nano-pillars spin-valves structures (Py/Cu/Py). A first objective was to understand the spin-polarized transport processes as well as spin transfer mechanisms associated to highly non-homogeneous magnetic configurations. This study allows me to identify and then precisely tune the vortex based magnetic configurations, and notably to observe the influence of spin transfer on reversal mechanisms of the vortex core. Combining analytical calculations and micro-magnetic simulations, we determine the conditions on relative parameters for the two vortices (chiralities and polarities) necessary to obtain self-sustained gyrotropic oscillations of the coupled vortices in a single pillar. A very interesting case is predicted for the pillars with larger diameters (typically over 200nm) for which the critical current is reduced to zero. The experimental results confirm the predictions that a coupled dynamics exists with linewidths as narrow as 200kHz, that is a record at zero field (corresponding to a quality factor Q ≈ 5000, an order of magnitude over the self-sustained oscillations of a single vortex), and even down to 50kHz under external field.A second objective was to investigate the synchronization of two vortex based spin transfer oscillators. We demonstrate theoretically that the phase locking through dipolar coupling of two identical oscillators can be achieved for any parameters of the two vortex. However, the coupling is three times stronger when vortices have opposite core polarities. From an experimental point of view, the synchronization capability for two oscillators having a frequency mismatch reaching up to 10 % of the auto-oscillation frequency has been demonstrated. This work, being part of the research effort made to improve the rf properties of spin transfer nano-oscillators emphasizes how the excitation of coupled magnetizations modes is important to reach lower and lower linewidths.

In this thesis we studied precession mechanism in ferromagnetic thin film half-metal NiMnSb. We measured magnetization oscillations using optical pump and probe experiment at temperatures between 15 and 200 K and we evaluated the magnetic anisotropy fields, spin stiffness and Gilbert damping. New setup for ferromagnetic resonance measurement was built utilizing vector network analyser. With this setup we measured FMR at temperatures between 300 and 75 K. We evaluated the same parameters from FMR experiments as from the optical one. We found very good agreement in results obtained by the two methods. Powered by TCPDF (www.tcpdf.org)

L’objectif de cette thèse était de synthétiser des multicouches à base de couches ferromagnétiques GeMn qui sont empilées et séparées par des couches de Ge en utilisant la technique d'épitaxie par jets moléculaires.Outre de nombreuses applications en spintronique issues de cette structure de matériaux, la réalisation de capteurs biochimiques dédiés à la détection moléculaire est l’idée directrice de ce travail. Un tel dispositif présenterait les atouts que ses matériaux constituants apportent : haute sensibilité, sélectivité et compatibilité parfaite avec la technologie de Si-Ge. Dans la première partie de ce manuscrit sont présentés les résultats obtenus de la croissance d’hétérostructures Mn5Ge3, Mn5Ge3Cx sur Ge(111) puis la reprise d’épitaxie de la barrière de Ge sur Mn5Ge3, la première étape avant la croissance de la deuxième couche ferromagnétique. Nous avons également analysé les propriétés structurales et magnétiques de ces couches minces ainsi que les dificultés dues à la croissance de la couche de Ge, notamment la diffusion et la ségrégation. Deux approches utilisant le carbone ont été proposées pour réduire la ségrégation : barrière de diffusion en carbone et remplissage des sites interstitiels du réseau Mn5Ge3 par du carbone. Le second axe alternatif pour la synthèse est consacré à la croissance de la structure colonnaire empilée Ge1-xMnx. Les conditions pour obtenir la structure colonnaire ont été déterminées. Les propriétés structurales et mesures magnétiques ont montré que cette phase était particulièrement intéressant dans la famille des semiconducteurs ferromagnétiques dilués à base de Ge-Mn pour les applications en spintronique et croissance de multicouches. La reprise d’épitaxie de plusieurs couches ferromagnétiques séparées par Ge a été effectuée et l’étude du couplage magnétique a été également menée. Enfin, nous présentons les premiers résultats sur le greffage de porphyrines et de protéines sur diverses surfaces hydrophiles et hydrophobes (Si, Ge), permettant d’accéder aux études de la faisabilité des capteurs Ge/GeMn. L’ensemble de ce travail indique que les multicouches de Ge/GeMn apparaissent comme des candidats à fort potentiel pour la spintronique, notamment pour capteurs bio-chimiques dans les semi-conducteurs du groupe IV. / The objective of this thesis is to synthetize the multilayers based on the sandwiched structure of GeMn ferromagnetic layers by mean of Molecular Beam Epitaxy on Ge substrate. Applications in spintronic field from this study are potential such as structures of spin valves, nanoscale sensors devoted to the detection of biochemical molecules. We actually focus on the biochemical sensors based on GMR (or TMR) phenomenon in stacking layered structure. These devices offer many advantages that the constituent materials may provide : high sensibility, selectivity, and especially, compatibility with Si-Ge technology. The first part of this manuscript presents the results obtained of heterostructure growth of Mn5Ge3, Mn5Ge3Cx on Ge(111), then Ge overgrowth on Mn5Ge3, the first step to study multilayers growth. Also, we have discussed about the structural and magnetic properties of these thin films as well as the problems due to the growth of multilayers, especially the diffusion and segregation. The approaches to reduce the diffusion were proposed by introducing carbon atoms as diffusion barrier or by fulfilling insterstial sites of Mn5Ge3 lattice by carbon atoms. The second axis of materials synthesis is devoted to the growth of multilayers Ge1-xMnx nanocolumn structure. The growth condition of Ge1-xMnx nanocolumns has been determined. We have studied structural and magnetic properties of this phase which are of particular interest to spintronic applications and multilayers growth. The Ge/Ge1-xMnx nanocolumns multilayers have been done and the interlayer exchange coupling between ferromagnetic layers has been studied. Finally, we have presented the preliminary results of porphyrin molecules and protein grafting on hydrophilic and hydrophobic surfaces (Si and Ge). This allows accessing to study the feasibility of Ge/GeMn-based sensors. This work indicates that the Ge/GeMn mutilayers appear to be a potential candidate for spintronics and biochemical sensors in the group IV semi-conductors.

Les états de bord sont des canaux de transport unidimensionnels qui se développent dans des puits quantiques en régime d'Effet Hall entier, avec de remarquables propriétés de chiralité et de cohérence quantique. Dans cette thèse nous présentons l'idée d'une manipulation de courants électroniques mettant en jeu le mélange de deux canaux de bord co-propageants, et nous discutons son impact potentiel pour l'interférométrie quantique et le transport de qubits de spin. Nous présentons les caractéristiques des états de bord et évaluons l'effet de potentiels locaux et non-adiabatiques, et de leur efficacité pour transférer la charge entre les deux canaux. Il est montré que des variations rapides du potentiel, d'amplitude plus petite que le gap de Landau, donnent lieu à un faible mélange, et nous identifions des stratégies expérimentales permettant d'atteindre un bon pourcentage de mélange. Nous développons des techniques de simulation numérique afin de modéliser de expériences qui mettent en jeu des canaux avec mélange, ainsi que des méthodes analytiques permettant de traiter les interactions coulombiennes entre états de bord, en vue de futures expériences d'interférométrie de spin. / Edge states are one-dimensional transport channels, emerging in quantum wells in the integer Quantum Hall regime, with remarkable properties of chirality and quantum coherence. In this thesis we present the idea of manipulating electronic currents mixed over two co-propagating edge channels, and discuss its potential impact for quantum interferometry and transport of spin-qubit states. We introduce the characteristics of edge states and evaluate the effect of local, non adiabatic potentials and their efficiency to transfer charge between two channels. We show that sharp potential variations whose energies are smaller than the Landau gap provide weak mixing, and we identify some experimental strategies that can achieve good mixing percentages. We develop numerical techniques of simulation to model existing experiments that employ mixed edge channels, and analytical methods in order to treat the effect of Coulomb interactions between edge states in a future spin-interferometry experiment.