The manufacturing of nanocontact-based spin-torque oscillators (NC-STOs)has opened the door for spintronic devices to play a part as active microwaveelements. The NC-STO has the capability of converting a direct current intoa microwave signal, and vice versa, by utilizing the spin transfer torque (STT)in ferromagnetic multilayer systems. However, the high-frequency operation ofNC-STOs typically requires high magnetic fields and the microwave power theygenerate is rather limited. As a result, NC-STOs are not yet commercially used,and they require improvements in both material systems and device geometriesbefore they can find actual use in microwave applications. In order to improve and advance this technology, NC-STOs are requiredwith both different nanocontact (NC) sizes and geometries, and using differ- ent stacks of magnetic materials. This dissertation presents experimental in- vestigations into the manufacturing of such devices using different fabrication techniques and a number of different magnetic material stacks. Currently, the fabrication of NC-STOs is limited to advanced laboratories, because NC fabri- cation requires high-resolution lithography tools. In the present work, we have developed an alternative method of fabrication, which does not require such tools and has the capability of fabricating NC-STOs having one to hundreds of NCs in a variety of sizes, possibly  down to 20 nm. Devices fabricated with this method have shown mutual synchronization of three parallel-connected NCs, and pairwise synchronization in devices with four and five NCs. Furthermore, the present work demonstrates low-field operation (down to0.02 Tesla) of NC-STOs at a record high frequency of 12 GHz. This wasachieved by implementing multilayers with a perpendicular magnetic anisotropy(PMA) material in the free layer of the NC-STO. In addition, the fabricateddevices revealed an unexpected dynamic regime under large external appliedfield (above 0.4 Tesla). The new dynamic regime was found to be due to anentirely novel nanomagnetic dynamic object â a so-called magnetic droplet soliton,predicted theoretically in 1977 but not experimentally observed until now.Detailed experiments and micromagnetic simulations show that the droplet hasvery rich dynamics. Finally,  spin-torque-induced  transverse spin wave instabilities have beenstudied.  A NC-STO with  a material stack consisting of a single ferromag- netic metal sandwiched between two non-ferromagnetic metals was fabricated. Prior to this work, evidence of spin wave instabilities was reported as resis- tance switching in nanopillar- and mechanical point contact based STOs. In the present  work, the fabricated NC-STOs showed actual microwave  signals up to 3 GHz under zero applied field with strong current hysteresis. All  the fabricated NC-STOs open up new means of studying STT in different environ- ments, in order to resolve their current drawbacks for industrial applications. / <p>QC 20130527</p>

Les nano-oscillateurs à transfert de spin (STNO) sont des candidats prometteurs pour la réalisation de composants radiofréquence (RF) intégrés, du à leur taille nanométrique, l'importante gamme de fréquences de base qu'ils peuvent couvrir, ainsi qu'à leur accordabilité autour de ces fréquences de base. Le signal RF est obtenu grâce à l'effet de transfert de spin (STT) qui donne lieu à une oscillation non-linéaire de l'aimantation dans un élément magnétorésistif. Jusqu'ici, ces excitations ont été examinées dans le cadre d'une couche magnétique isolée, c'est-à-dire sans prendre en compte le couplage entre couches. Cependant, nombreux aspects du spectre d'excitation ne peuvent pas être expliqués si l'on considère une couche isolée. Dans cette thèse nous nous attacherons à répondre à la question importante du couplage dynamique entre couches magnétiques dans un nanopilier magnétorésistif, afin de développer une meilleure compréhension des spectres d'excitation, et en particulier la dépendance en courant et champ magnétique appliqué des caractéristiques du pic d'émission, telles que la largeur de raie et la fréquence. Une première étude est réalisée pour un système composé de deux couches ferromagnétiques, couplées entre elles par le couplage RKKY (ce système est appelé un ferrimagnétique synthétique (SyF)). Le couplage induit des différences importantes dans la dépendance en courant de la fréquence par rapport aux excitations d'une couche isolée. Ces différences sont expliquées par l'important couplage dynamique RKKY. Une seconde étude prend en considération une interaction plus complexe, ayant lieu dans un nano-pilier STNO standard basé sur jonctions tunnel ou vannes de spin. Ce dispositif est composé d'un SyF ainsi que d'une couche libre(FL) magnétique, séparés par une fine couche métallique ou isolante. Pour ce système, en plus du couplage dynamique RKKY propre au SyF, nous prenons en compte le couplage dynamique généré par le champ dipolaire ainsi que le spin-torque mutuel (MSTT) entre la couche libre et le SyF. Ce couplage multiple donne lieu à deux signatures distinctes. La première est l'apparition d'un " saut " dans le spectre d'excitation dû à l'hybridation des modes SyF and FL dans le régime atténué. Le second est dû à l'interaction entre les excitations en régime entretenu, éventuellement via leurs composantes harmoniques, avec les excitations en régime atténué. Cette interaction donne lieu à des discontinuités dans la dépendance fréquence - champ, ce lorsque les excitations FL sont prédominantes. Il est intéressant de noter que cela mène à des régions ou la largeur de raie est diminuée. De plus, lorsque les excitations SyF sont prédominantes, la largeur de raie est diminuée par rapport aux cas ou les excitations FL sont prédominantes. Partant de ces observations, nous proposons une structure plus complexe, où un seconde couche de type SyF remplace la couche libre. Les résultats obtenus par une combinaison d'expériences, de simulations numériques et d'analyse analytique, montrent le rôle important des interactions dynamiques dans un nano-pilier. Ils ouvrent de nouvelles voies pour la conception de configurations STNO qui mèneront à des améliorations des performances du signal ainsi synthétisé.

The impact of the spin degree of freedom on the transport properties of electrons traveling through magnetic materials has been known since the pioneer work of Mott [1]. Since then it has been demonstrated that the spin angular momentum plays a key role in the scattering process of electrons in magnetic multilayers. This role has been emphasized by the discovery of the Giant Magnetoresistance in 1988 by Fert and Grunberg [2, 3]. Among the numerous applications and effects that emerged in mesoscopic devices two mechanisms have attracted our attention during the course of this thesis: the spin transfer torque and the spin Hall effects. The former consists in the transfer of the spin angular momentum from itinerant carriers to local magnetic moments [4]. This mechanism results in the current-driven magnetization switching and excitations, which has potential application in terms of magnetic data storage and non-volatile memories. The latter, spin Hall effect, is considered as well to be one of the most fascinating mechanisms in condensed matter physics due to its ability of generating non-equilibrium spin currents without the need for any magnetic materials. In fact the spin Hall effect relies only on the presence of the spin-orbit interaction in order to create an imbalance between the majority and minority spins. The objective of this thesis is to investigate the impact of disorder on spin dependent transport phenomena. To do so, we identified three classes of systems on which such disorder may have a dramatic influence: (i) antiferromagnetic materials, (ii) impurity-driven spin-orbit coupled systems and (iii) two dimensional semiconducting electron gases with Rashba spin-orbit coupling. Antiferromagnetic materials - We showed that in antiferromagnetic spin-valves, spin transfer torque is highly sensitive to disorder, which prevents its experimental observation. To solve this issue, we proposed to use either a tunnel barrier as a spacer or a local spin torque using spin-orbit coupling. In both cases, we demonstrated that the torque is much more robust against impurities, which opens appealing venues for its experimental observation. Extrinsic spin-orbit coupled systems - In disordered metals accommodating spin orbit coupled impurities, it is well-known that spin Hall effect emerges due to spin dependent Mott scattering. Following a recent prediction, we showed that another effect coexists: the spin swapping effect, that converts an incoming spin current into another spin current by "swapping" the momentum and spin directions. We showed that this effect can generate peculiar spin torque in ultrathin magnetic bilayers. Semiconductors spintronics - This last field of research has attracted a massive amount of hope in the past fifteen years, due to the ability of coherently manipulating the spin degree of freedom through interfacial, so-called Rashba, spin-orbit coupling. However, numerical simulations failed reproducing experimental results due to coherent interferences between the very large number of modes present in the system. We showed that spin-independent disorder can actually wash out these interferences and promote the conservation of the spin signal. In the course of this PhD, we showed that while disorder-induced dephasing is usually detrimental to the transmission of spin information, in selected situation, it can actually promote spin transport mechanisms and participate to the enhancement of the desired spintronics phenomenon.

Spin-torque nano-oscillators (STNOs) are prospective successors of transistor-based emitters and receivers of radio-frequency signals in commonly used remote communication systems. In comparison to the conventional electronic oscillators, STNOs offer the advantage of being tunable over a wide range of frequencies simply by adjusting the applied current, the smaller lateral size (up to 50 times) and the lower power consumption as the lateral size of the device is reduced. It has already been demonstrated that the output signal characteristics of STNOs are compatible with the requirements for applications: they can provide output powers in the µW range, frequencies of the order of GHz, quality factors Q (equal to df/f, where f is the frequency, and df is the linewidth) up to several thousands (e.g., 3 200), and can be integrated into Phase-Locked Loop (PLL) circuits. The most promising type of spin-torque oscillators is the hybrid geometry STNOs utilizing an in-plane magnetized fixed layer, an out-of-plane magnetized free layer and the MgO tunnel barrier as a spacer. This geometry maximizes the output power, since the full parallel-to-antiparallel resistance variation can be exploited in the limit of large magnetization precession angle (i.e., when the magnetization oscillates fully within the plane of the STNO stack). Moreover, the considered hybrid geometry allows for the reduction of the critical currents, enables functionality regardless of the applied magnetic or current history and requires a simplified fabrication process in comparison to the opposite hybrid geometry, consisting of an in-plane magnetized free layer and an out-of-plane reference layer, which requires an additional read-out layer. Simultaneously, the choice of the spacer material in considered STNOs is motivated by the increase of both the output power (via large magnetoresistance ratios) and the power conversion rate ('output power to input power' ratio), compared to their fully metallic counterparts. Despite the many advantages of MgO-based hybrid geometry STNOs, unexplained issues related to the physics behind their principle of operation remained. In this thesis, the main focus is put on the two key aspects related to the out-of-plane steady-state precession in hybrid STNOs: the precession mechanism (combined with the analysis of the influence of the bias dependence of the tunnel magnetoresistance) and the zero-field oscillations stabilized by an in-plane shape anisotropy. State-of-the-art theoretical studies demonstrated that stable precession in hybrid geometry STNOs can only be sustained if the in-plane component of the spin-transfer torque (STT) exhibits an asymmetric dependence on the angle between the free and the polarizing layer (which is true for fully metallic devices, but not for the MgO-based magnetic tunnel junctions (MTJs)). Nevertheless, recent experimental reports showed that spin-transfer driven dynamics can also be sustained in MgO-based STNOs with this particular configuration. In this thesis, a phenomenological and straightforward mechanism responsible for sustaining the dynamics in considered system is suggested. The mechanism is based on the fact that, in MgO-based MTJs, the strong cosine-type angular dependence of the tunnel magnetoresistance, at constant applied current, translates into an angle-dependent voltage component, which results in an angle-dependent spin-transfer torque giving a rise to the angular asymmetry of the in-plane STT and, thus, enabling steady-state precession to be sustained. Subsequently, the bias dependence of the tunnel magnetoresistance (TMR), which has been so far neglected in similar calculations, is taken into account. According to the results of analytical and numerical studies, the TMR bias dependence brings about a gradual quenching of the dynamics at large applied currents. The theoretical model yields trends confirming our experimental results. The most important conclusion regarding to this part of the thesis is that, while the angular dependence of the tunnel magnetoresistance introduces an angular asymmetry for the in-plane spin transfer torque parameter (which helps maintain steady-state precession), the bias dependence of the resistance works to reduce this asymmetry. Thus, these two mechanisms allow us to tune the asymmetry of the in-plane STT as function of current and to control the dynamical response of the actual device. Except for the precession mechanism, the thesis is also focused on the issue of zero-field oscillations, which would be especially desirable from the point of view of potential applications. According to the state-of-the-art theoretical studies, for hybrid geometry devices with circular cross-section (i.e., exhibiting no other anisotropy terms), current-driven dynamics cannot be excited at zero applied field. Indeed, zero-field oscillations have only been experimentally observed for systems having the free layer magnetization slightly tilted from the normal to the plane, which has usually been achieved by introducing an in-plane shape anisotropy. In the thesis, the influence of the in-plane shape anisotropy of the MTJ on zero-field dynamics in the hybrid geometry MgO-based STNOs is analytically and numerically investigated. In agreement with the previous reports, no zero-field dynamics for circular nano-pillars is observed; however, according to the numerical data, an additional in-plane anisotropy smaller than the effective out-of-plane anisotropy of the free layer enables zero-field steady-state precession. Accordingly, the lack of an in-plane anisotropy component (e.g., for circular cross-section nano-pillars), or the presence of an in-plane shape anisotropy equal or greater than the out-of-plane effective anisotropy, inhibits the stabilization of dynamics in the free layer at zero field. The results of analytical and numerical studies and the general trends identified in the corresponding experimental data are found to be in excellent qualitative agreement.:1. Introduction 1.1. Short history of magnetotransport applications 1.2. Spin-transfer torque induced effects and devices 1.3. Goals of the thesis 2. Fundamentals 2.1. Electronic transport in single transition metal layers 2.2. Tunnel magnetoresistance (TMR) 2.2.1. Electronic transport in magnetic tunnel junctions 2.2.2. Tunnel magnetoresistance versus structural properties of the multilayer 2.2.3. Bias voltage and temperature dependence of tunnel magnetoresistance 2.2.4. Angular dependence of tunnel magnetoresistance 2.3. Spin-transfer torque in GMR/TMR structures 2.3.1. Spin-transfer torque 2.3.2. Landau-Lifshitz-Gilbert (LLG) equation 2.3.3. LLG equation and spin-transfer torques 2.3.4. Bias voltage dependence of spin-transfer torques in MTJs 2.3.5. Angular dependence of spin-transfer torque 2.4. Spin-torque-based phenomena 2.4.1. Current-induced switching 2.4.2. Current-induced dynamics 3. Experimental 3.1. General characteristics of MgO-based magnetic tunnel junctions 3.2. STNO samples 3.2.1. Samples by AIST (Tsukuba, Japan) 3.2.2. Samples by HZDR / SINGULUS (Dresden / Kahl am Main, Germany) 3.3. Magnetotransport measurements 3.3.1. Experimental setup and data analysis 3.3.2. Experimental results 3.4. Aspects to be explained 4 Numerical and analytical calculations 4.1 Out-of-plane steady-state precession in hybrid geometry STNO 4.1.1 Angular dependence of tunnel magnetoresistance as a mechanism of stable precession 4.1.2. Influence of the bias dependence of tunnel magnetoresistance 4.1.3. Comparison with the experimental data 4.1.4. Comparison with the GMR-type counterpart 4.1.5. Summary 4.2. Zero-field dynamics in hybrid geometry STNO stabilized by in-plane shape anisotropy 4.2.1. Effect of the in-plane shape anisotropy 4.2.2. Zero-field dynamics 4.2.3. Summary 5. Conclusions 6. Outlook Appendix Bibliography

Les nano-oscillateurs à transfert de spin sont des composants radiofréquences magnétiques non-linéaires, nanométrique, de faible consommation en énergie et accordables en fréquence. Ce sont aussi potentiellement des candidats prometteurs pour l’élaboration de larges réseaux d’oscillateurs couplés. Ces derniers peuvent être utilisés dans les architectures neuromorphiques qui nécessitent des assemblées très denses d’unités de calcul complexes imitant les neurones biologiques et comportant des connexions ajustables entre elles. L’approche neuromorphique permet de pallier aux limitations des ordinateurs actuels et de diminuer leur consommation en énergie. En effet pour résoudre des tâches cognitives telles que la reconnaissance vocale, le cerveau fonctionne bien plus efficacement en terme d’énergie qu’un ordinateur classique. Au vu du grand nombre de neurone dans le cerveau (100 milliards) une puce neuro-inspirée requière des oscillateurs de très petite taille tels que les nano-oscillateurs à transfert de spin. Récemment, une première démonstration de calcul neuromorphique avec un unique nano-oscillateur à transfert de spin a été établie. Cependant, pour aller au-delà, il faut démontrer le calcul neuromorphique avec plusieurs nano-oscillateurs et pouvoir réaliser l’apprentissage. Une difficulté majeure dans l’apprentissage des réseaux de nano-oscillateurs est qu’il faut ajuster le couplage entre eux. Dans cette thèse, en exploitant l'accordabilité en fréquence des nano-oscillateurs magnétiques, nous avons démontré expérimentalement l'apprentissage des nano-oscillateurs couplés pour classifier des voyelles prononcées avec un taux de reconnaissance de 88%. Afin de réaliser cette tache de classification, nous nous sommes inspirés de la synchronisation des taux d’activation des neurones biologiques et nous avons exploité la synchronisation des nano-oscillateurs magnétiques à des stimuli micro-ondes extérieurs. Les taux de reconnaissances observés sont dus aux fortes accordabilités et couplage intermédiaire des nano-oscillateurs utilisés. Enfin, afin de réaliser des taches plus difficiles nécessitant de larges réseaux de neurones, nous avons démontré numériquement qu’un réseau d’une centaine de nano-oscillateurs magnétiques peut être conçu avec les contraintes standards de nano-fabrication. / Spin-torque nano-oscillators are non-linear, nano-scale, low power consumption, tunable magnetic microwave oscillators which are promising candidates for building large networks of coupled oscillators. Those can be used as building blocks for neuromorphic hardware which requires high-density networks of neuron-like complex processing units coupled by tunable connections. The neuromorphic approach allows to overcome the limitation of nowadays computers and to reduce their energy consumption. Indeed, in order to perform cognitive tasks as voice recognition or image recognition, the brain is much more efficient in terms of energy consumption. Due to the large number of required neurons (100 billions), a neuromorphic chip requires very small oscillators such as spin-torque nano-oscillators to emulate neurons. Recently a first demonstration of neuromorphic computing with a single spin-torque nano-oscillator was established, allowing spoken digit recognition with state of the art performance. However, to realize more complex cognitive tasks, it is still necessary to demonstrate a very important property of neural networks: learning an iterative process through which a neural network can be trained using an initial fraction of the inputs and then adjusting internal parameters to improve its recognition or classification performance. One difficulty is that training networks of coupled nano-oscillators requires tuning the coupling between them. Here, through the high frequency tunability of spin-torque nano-oscillators, we demonstrate experimentally the learning ability of coupled nano-oscillators to classify spoken vowels with a recognition rate of 88%. To realize this classification task, we took inspiration from the synchronization of rhythmic activity of biological neurons and we leveraged the synchronization of spin-torque nano-oscillators to external microwave stimuli. The high experimental recognition rates stem from the weak-coupling regime and the high tunability of spin-torque nano-oscillators. Finally, in order to realize more difficult cognitive tasks requiring large neural networks, we show numerically that arrays of hundreds of spin-torque nano-oscillators can be designed with the constraints of standard nano-fabrication techniques.

<div>Neuromorphic computing has gained tremendous interest because of its ability to overcome the limitations of traditional signal processing algorithms in data intensive applications such as image recognition, video analytics, or language translation. The new computing paradigm is built with the goal of achieving high energy efficiency, comparable to biological systems.</div><div>To achieve such energy efficiency, there is a need to explore new neuro-mimetic devices, circuits, and architecture, along with new learning algorithms. To that effect, we propose two main approaches:</div><div><br></div><div>First, we explore an energy-efficient hardware implementation of a bio-plausible Spiking Neural Network (SNN). The key highlights of our proposed system for SNNs are 1) addressing connectivity issues arising from Network On Chip (NOC)-based SNNs, and 2) proposing stochastic CMOS binary SNNs using biased random number generator (BRNG). On-chip Power Line Communication (PLC) is proposed to address the connectivity issues in NOC-based SNNs. PLC can use the on-chip power lines augmented with low-overhead receiver and transmitter to communicate data between neurons that are spatially far apart. We also propose a CMOS '<i>stochastic-bit</i>' with on-chip stochastic Spike Timing Dependent Plasticity (sSTDP) based learning for memory-compressed binary SNNs. A chip was fabricated in 90 nm CMOS process to demonstrate memory-efficient reconfigurable on-chip learning using sSTDP training. </div><div><br></div><div>Second, we explored coupled oscillatory systems for distance computation and convolution operation. Recent research on nano-oscillators has shown the possibility of using coupled oscillator networks as a core computing primitive for analog/non-Boolean computations. Spin-torque oscillator (STO) can be an attractive candidate for such oscillators because it is CMOS compatible, highly integratable, scalable, and frequency/phase tunable. Based on these promising features, we propose a new coupled-oscillator based architecture for hybrid spintronic/CMOS hardware that computes multi-dimensional norm. The hybrid system composed of an array of four injection-locked STOs and a CMOS detector is experimentally demonstrated. Energy and scaling analysis shows that the proposed STO-based coupled oscillatory system has higher energy efficiency compared to the CMOS-based system, and an order of magnitude faster computation speed in distance computation for high dimensional input vectors.</div>

This work presents a spin torque nano-oscillator (STNO) based binary frequency shift keying (BFSK) modulation schema implemented on a printed circuit board (PCB). Maximal input data rate reaches 20 Mbit/s. Depending on the STNO used, carrier frequency can range from 1 to 10 GHz. Both DC and AC currents flowing through the STNO can be tuned between 0 to 4 mA. Using one magnetic tunnel junction (MTJ) STNO, a 380 MHz frequency shift around the center frequency 9 GHz was observed, when the modulated current was toggled between 0.8 mA and 1.2 mA at a rate of 20 Mbit/s. This is the first work demonstrating that the STNOs are applicable for BFSK modulation on the wireless application level.

The objective of this dissertation is to classify the opportunities, advantages, and limits of novel interconnects for post-CMOS logic that can augment or eventually replace the CMOS logic. Post-CMOS devices are envisaged on the idea of using state variables other than the electron charge to store and manipulate information. In the first component of the thesis, a comprehensive analysis of the performance and the energy dissipation of novel logic based on various state variables is conducted, and it is demonstrated that the interconnects will continue to be a major challenge even for post-CMOS logic. The second component of the thesis is focused on the analysis of the interconnection aspects of spin-based logic. This research goal is accomplished through the development of physically-based models of spin-transport parameters for various metallic, semiconducting, and graphene nanoribbon interconnects by incorporating the impact of size effects for narrow cross-sectional dimensions of all-spin logic devices. Due to the generic nature of the models, they can be used in the analysis of spin-based devices to study their functionality and performance more accurately. The compact nature of the models allows them to be easily embedded into the developing CAD tools for spintronic logic. These models then provide the foundation for (i) analyzing the spin injection and transport efficiency in an all-spin logic circuit with various interconnect materials, and (ii) estimating the repeater-insertion requirements in all-spin logic, and (iii) estimating the maximum circuit size for all-spin logic. The research is crucial in pinpointing the implications of the physical limits of novel interconnects at the material, device, circuit, and architecture levels.

En électronique de spin, le couple de transfert de spin (STT) et la magnétorésistance tunnel (TMR) dans les jonctions tunnel magnétiques à électrodes ferromagnétiques (F) sont deux phénomènes physiques essentiels. Dans cette thèse, nous présentons une étude théorique du STT dans des jonctions tunnel antiferromagnétiques (AF), où deux électrodes non-plus F mais AF sont séparées par une barrière isolante non-magnétique. Plus concrètement, les comportements du STT et de la TMR sont étudiés dans des jonctions tunnel AF cristallines, et ce, à l´aide de calculs de liaisons fortes dans le cadre du formalisme de Keldysh. Nous avons observé une distribution spatiale de la composante perpendiculaire du STT régulière et de signe alternatif, ce qui est similaire au comportement de la composante parallèle. Ces variations spatiales de la composante perpendiculaire sont cependant spécifiques à l'utilisation d'une barrière tunnel et contrastent avec les effets observés par le passé pour le cas de couches séparatrices métalliques. De plus, contrairement aux jonctions tunnel F conventionnelles, nous avons montré que la TMR peut augmenter avec la tension appliquée et atteindre des valeurs du même ordre de grandeur que pour des vannes de spin usuelles : tout-métallique et à électrodes F.L´analyse effectuée pour des AF est ensuite étendue aux matériaux ferrimagnétiques (FI), pour lesquels les AF constituent, somme toute, des cas limites. La complexité magnétique additionnelle inhérente aux FI se traduit par un comportement spatial du STT beaucoup plus riche dans les jonctions tunnel FI. Nous observons notamment que les paramètres électroniques tels que les largeurs et les décalages de bandes ont une très forte influence sur le STT. Plus particulièrement, la différence entre les couplages d'échange inter-spin locaux des deux sous-réseaux du FI donne lieu à une distribution spatiale du STT ondulatoire qui est modulée par la densité locale de spin. Il est possible d'ajuster cet effet en jouant sur la tension appliquée aux bornes de la jonction tunnel FI. Nous trouvons de plus que la différence entre les couplages d'échange inter-spin locaux constitue un paramètre fondamental pour la quantification des longueurs caractéristiques du STT dans les FIs. Ce paramètre peut être considéré comme un champ d´échange effectif, par similitude avec le cas usuel des Fs qui présentent un champ d´échange homogène.Pour finir, nous avons sondé expérimentalement les longueurs caractéristiques du STT dans des AFs. Pour de l'Ir20Mn80 et du Fe50Mn50, nous avons déterminé les longueurs de pénétration de spin et les mécanismes d'absorption de courants de spin à température ambiante en utilisant la résonance F et le pompage de spin. Plus précisément, nous avons associé les profondeurs de pénétration critiques à deux mécanismes d'absorption distincts: du déphasage pour l´Ir20Mn80 et du retournement de spin pour le Fe50Mn50. / Spin transfer torque (STT) and tunnelling magnetoresistance (TMR) in magnetic tunnel junctions with ferromagnetic (F) leads are two essential underlying phenomena of modern spintronics. We present here a theoretical study of STT in antiferromagnet (AF) based tunnel junctions, where two AF metal electrodes are separated by a thin nonmagnetic insulating barrier. In particular, the behaviour of STT and TMR in epitaxial AF-based tunnel junctions is investigated using tight binding calculations in the framework of the Keldysh formalism. The spatial distribution of the STT out-of-plane component is found to be staggered, similar to the in-plane component. This behaviour is specific to the use of a tunnel barrier and significantly differs from the out-of-plane torques reported in previous works using a metallic spacer. Additionally, we show that unlike conventional ferromagnetic-based tunnel junctions, the TMR can increase with applied bias and reach values comparable to typical magnetoresistances found for usual spin valves.Next, the analysis carried out for AFs is extended to ferrimagnets (FI), for which AFs constitute simpler limiting cases. The additional magnetic complexity inherent to FI materials yields to a richer physics concerning the STT spatial behaviour in FI based tunnel junctions.Electronic structure parameters such as band widths and exchange splittings of the FI are shown to have a strong influence on STT. In particular, the STT spatial distribution within the leads exhibits a striking spin-modulated wave-like behaviour resulting from the interplay between the exchange splittings of the two FI sublattices. This wave-like behaviour can also be tuned via the applied voltage across the junction. Furthermore, the fundamental intrinsic parameter for quantifying STT characteristic lengths in FI metals is identified. This fundamental parameter can be considered as an effective exchange field in FIs, similar to the homogeneous exchange field in the F case.Finally, the STT characteristic lengths in AF materials are investigated experimentally. Here, room temperature critical depths and absorption mechanisms of spin currents in Ir20Mn80 and Fe50Mn50 are determined by F-resonance and spin pumping. In particular, room temperature critical depths are observed to be originated from different absorption mechanisms: dephasing for Ir20Mn80 and spin flipping for Fe50Mn50.

Spin torque oscillator (STO) technology has a unique blend of features, including but not limited to octave tunability, GHz operating frequency, and nanoscaled size, which makes it highly suitable for microwave and radar applications. This thesis studies the fundamentals of STOs, utilizes the state-of-art STO's advantages, and proposes two STO-based microwave systems targeting its microwave applications and measurement setup, respectively. First, based on an investigation of possible STO applications, the magnetic tunnel junction (MTJ) STO shows a great suitability for microwave oscillator in multi-standard multi-band radios. Yet, it also imposes a large challenge due to its low output power, which limits it from being used as a microwave oscillator. In this regard, different power enhancement approaches are investigated to achieve an MTJ STO-based microwave oscillator. The only possible approach is to use a dedicated CMOS wideband amplifier to boost the output power of the MTJ STO. The dedicated wideband amplifier, containing a novel Balun-LNA, an amplification stage and an output buffer, is proposed, analyzed, implemented, measured and used to achieve the MTJ STO-based microwave oscillator. The proposed amplifier core consumes 25.44 mW from a 1.2 V power supply and occupies an area of 0.16 mm2 in a 65 nm CMOS process. The measurement results show a S21 of 35 dB, maximum NF of 5 dB, bandwidth of 2 GHz - 7 GHz. This performance, as well as the measurement results of the proposed MTJ STO-based microwave oscillator, show that this microwave oscillator has a highly-tunable range and is able to drive a PLL. The second aspect of this thesis, firstly identifies the major difficulties in measuring the giant magnetoresistance (GMR) STO, and hence studying its dynamic properties. Thereafter, the system architecture of a reliable GMR STO measurement setup, which integrates the GMR STO with a dedicated CMOS high frequency IC to overcome these difficulties in precise characterization of GMR STOs, is proposed. An analysis of integration methods is given and the integration method based on wire bonding is evaluated and employed, as a first integration attempt of STO and CMOS technologies. Moreover, a dedicated high frequency CMOS IC, which is composed of a dedicated on-chip bias-tee, ESD diodes, input and output networks, and an amplification stage for amplifying the weak signal generated by the GMR STO, is proposed, analyzed, developed, implemented and measured. The proposed dedicated high frequency circuits for GMR STO consumes 14.3 mW from a 1.2 V power supply and takes a total area of 0.329 mm2 in a 65 nm CMOS process. The proposed on-chip bias-tee presents a maximum measured S12 of -20 dB and a current handling of about 25 mA. Additionally, the proposed dedicated IC gives a measured gain of 13 dB with a bandwidth of 12.5 GHz - 14.5 GHz. The first attempt to measure the (GMR STO+IC) pair presents no RF signal at the output. The possible cause and other identified issues are given. / <p>QC 20140114</p>