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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Spin valves and spin-torque oscillators with perpendicualr magnetic anisotropy

Mohseni Armaki, Seyed Majid January 2012 (has links)
Researches in spintronics, especially those remarkably classified in the current induced spin-transfer torque (STT) framework, circumvent challenges with different materials and geometries. Perpendicular magnetic anisotropy (PMA) materials are showing capability of holding promise to be employed in STT based spintronics elements, e.g. spin-torque oscillators (STOs), STT-magnetoresistive random access memories (STT-MRAMs) and current induced domain wall motion elements. This dissertation presents experimental investigations into developing sputter deposited Co/Ni multilayers (MLs) with PMA and employs these materials in nano-contact STOs (NC-STOs) based on giant magnetoresistance (GMR) effect and in pseudo-spin-valve (PSV) structures. The magnetostatic stray field coupling plays an important role in perpendicular PSVs. The temperature dependent coupling mechanism recommends that this coupling can be tailored, by i) the saturation magnetization and coercivity of the individual layers, ii) the coercivity difference in layers, and iii) the GMR spacer thickness, to get a well decoupled and distinguishable switching response. Moreover, this thesis focused on the implementation and detailed characterization of NC-STOs with strong PMA Co/Ni ML free layers and in-plane Co reference layers as orthogonal (Ortho) magnetic geometry in so-called Ortho-NC-STOs. The primary target of reaching record high STO frequencies, 12 GHz, at close to zero field, 0.02 Tesla, was achieved. However, in large external fields, &gt;0.4 Tesla, an entirely new magnetodynamic object, a “magnetic droplet”, theoretically predicted in 1977, was discovered experimentally. Detailed experiments, combined with micromagnetic simulations, demonstrate the formation of a magnetic droplet with a partially reversed magnetization direction underneath the NC, and a zone of large amplitude precession in a region bounding the reversed magnetization. The magnetic droplet exhibits a very rich dynamics, including i) auto-modulation as a combine of droplet frequency with a slow time evolution (few GHz) of un-centering the droplet mode under the NC, ii) droplet breathing as reversible deformation of droplet mode with ½ droplet frequency. All observation of droplet opens a new mechanism of excitation for future fundamental studies as well as experiments especially for domain wall electronics and nano-scopic magnetism. / <p>QC 20121119</p>
2

Spin momentum transfer effects for spintronic device applications

Zhou, Yan January 2009 (has links)
The recent discovery that a spin-polarized current can exert a large torque on a ferromagnet, through direct transfer of spin angular momentum, offers the possibility of electrical current controlled manipulation of magnetic moment in nanoscale magnetic device structures. This so-called spin torque effect holds great promise for two applications, namely, spin torque oscillators (STOs) for wireless communication and radar communication, and spin transfer torque RAM (STT-RAM) for data/information storage.   The STO is a nanosized spintronic device capable of microwave generation at frequencies in the 1-65 GHz range with high quality factors. Although the STO is very promising for future telecommunication, two major shortcomings have to be addressed before it can truly find practical use as a radio-frequency device. Firstly, its very limited output power has to be significantly improved. One possibility is the synchronization of two or more STOs to both increase the microwave power and further increase the signal quality. Synchronization of serially connected STOs has been suggested in this thesis. In this configuration, synchronization relies on phase locking between the STOs and their self-generated alternating current. While this locking mechanism is intrinsically quite weak, we find that the locking range of two serially connected spin-valve STOs can be enhanced by over two orders of magnitude by adjusting the circuit I-V phase to that of an intrinsic preferred phase shift between the STO and an alternating current. More recently, we have also studied the phase-locking of STOs based on magnetic tunnel junctions (MTJ-STO) to meet the power specifications of actual application where the rf output levels should be above 0 dBm (1 mW). In addition to the spin torque terms present in GMR spin valves, MTJs also exhibit a significant perpendicular spin torque component with a quite complex dependence on both material choices and applied junction bias. We find that the perpendicular torque component modifies the intrinsic preferred I-V phase shift in single MTJ-STOs in such a way that serially connected STOs synchronize much more readily without the need for additional circuitry to change the I-V phase.   Secondly, equal attention has been focused on removing the applied magnetic field for STO operation, which requires bulky components and will limit the miniaturization of STO-based devices. Various attempts have been made to realize STOs operating in zero magnetic field. By using a tilted (oblique angle) polarizer (fixed layer) instead of an in-plane polarizer (standard STO), we show zero field operation over a very wide polarizer angle range without sacrificing output signal. In addition, the polarizer angle introduces an entirely new degree of freedom to any spin torque device and opens up for a wide range of additional phenomena.   The STT-RAM has advantages over other types of memories including conventional MRAM in terms of power consumption, speed, and scalability. We use a set of simulation tools to carry out a systematic study on the subject of micromagnetic switching processes of a device for STT-RAM application. We find that the non-zero k spin wave modes play an important role in the experimentally measured switching phase boundary. These may result in telegraph transitions among different spin-wave states, and be related to the back-hopping phenomena where the switching probability will decrease with increasing bias in tunnel junctions. / QC 20100819
3

Energy Efficient Neuromorphic Computing: Circuits, Interconnects and Architecture

Minsuk Koo (8815964) 08 May 2020 (has links)
<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>
4

CMOS High Frequency Circuits for Spin Torque Oscillator Technology

Chen, Tingsu January 2014 (has links)
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>
5

Synthèse de fréquence par couplage d'oscillateurs spintroniques

Zarudniev, Mykhailo 28 January 2013 (has links)
La tendance actuelle dans le domaine des télécommunications mène à des systèmes capables de fonctionner selon plusieurs standards, et donc plusieurs fréquences porteuses. La synthèse de la fréquence porteuse est un élément clef, dont les propriétés reposent essentiellement sur les performances de l’oscillateur employé. Pour assurer le fonctionnement de systèmes compatibles avec plusieurs standards de télécommunication, la solution conventionnelle consiste à intégrer plusieurs oscillateurs locaux. Cette solution est coûteuse, d’autant plus que, malgré le fait que les technologies actuelles atteignent des niveaux d’intégration très importants, la surface occupée par des oscillateurs traditionnels de type LC ne peut pas être diminuée, alors que le coût de fabrication au millimètre carré devient de plus en plus élevé. Il serait donc très intéressant de remplacer les oscillateurs LC, ce qui nous amène à rechercher des solutions alternatives parmi de nouvelles technologies. L’oscillateur spintronique (STO) est un nouveau dispositif issu des études sur les couches minces magnétiques. Il apparait comme un candidat potentiel de remplacement des oscillateurs LC du fait de sa grande accordabilité en fréquence et de son faible encombrement. Toutefois des mesures effectuées sur les STOs ont montré que la performance en puissance et en bruit de phase d’un oscillateur seul ne permet pas de remplir les spécifications pour des applications de télécommunication. Nous proposons de remplir ces spécifications en couplant un nombre d’oscillateurs spintroniques important. Dans ce cadre se posent plusieurs questions qui concernent les procédures de modélisation, d’analyse et de synthèse des systèmes interconnectés. Les procédures de modélisation incluent la démarche de recherche de modèles à complexité croissante qui décrivent les propriétés entrée-sortie d’un oscillateur spintronique, ainsi que la démarche de généralisation des modèles des oscillateurs dans le cadre du réseau. Les procédures d’analyse cherchent à vérifier la stabilité et évaluer la performance des systèmes interconnectés. Les procédures de synthèse permettent de concevoir des interconnexions sophistiquées pour les oscillateurs afin d’assurer toutes les spécifications du cahier des charges. Dans ce document, nous établissons tout d’abord le problème de la synthèse de fréquence par couplage avec un cahier des charges formalisé en termes de gabarits fréquentiels sur des densités spectrales de puissance. Le cahier des charges posé amène la nécessité de modéliser l’oscillateur spintronique pour pouvoir simuler et analyser son comportement. Ici, nous proposons une modélisation originale selon des degrés de complexité croissante. Ensuite, nous discutons de la structure de la commande de l’ensemble des oscillateurs afin de remplir les spécifications du cahier des charges. La structure de commande proposée nécessite de développer une méthode de conception des interconnexions du réseau d’après les critères de performance. Dans les deux derniers chapitres, nous proposons deux méthodes fréquentielles de synthèse originales pour résoudre le problème de synthèse de fréquence par couplage. La première méthode de synthèse permet de prendre en compte un critère mathématique du cahier des charges, qui correspond à un gabarit fréquentiel à respecter, et permet d’obtenir une matrice d’interconnexion des sous-systèmes, telle que le module de la réponse fréquentielle du réseau approxime le gabarit imposé par le cahier des charges. La deuxième méthode de synthèse permet de prendre en compte plusieurs gabarits fréquentiels à la fois. La solution obtenue est une matrice d’interconnexion des sous-systèmes, qui résout le problème de la synthèse de fréquence par couplage d’oscillateurs spintroniques. / The current trends in telecommunication are leading to systems that are compatible with multiple standards and consequently multiple carrier frequencies. The frequency synthesis is a key element influenced by the local oscillator performance. In order to ensure the system compatibility with multiple telecommunication standards, the conventional solution consists in using one local oscillator for each standard. This solution is expensive, even more, since the cost per squared millimetre is increasing, while the silicon area occupied by the traditional LC-tank oscillators cannot be reduced in spite of the fact that technology is going to higher integration levels. Thus, it should be interesting to find a substitution to the LC-tank oscillators which leads to research for alternative solutions among new technologies. The spin torque oscillator (STO) is a new device issued from the ferromagnetic thin-film research. Due to its frequency accord ability and its capability to occupy relatively small volume, it appears as a potential candidate for the LC-tank oscillator replacement. However, a set of measurements prove that these devices exhibit poor power and phase noise performance, making them unable to fulfill the technical specification of the radiofrequency applications. We propose to reach these specifications by coupling of a large number of spin torque oscillators. In this scope, numerous questions appear regarding the procedures of modelling, analysis and synthesis of the complex interconnected systems. The modelling procedures are dedicated to the increasing complexity models that describe the input-output behaviour of a spin torque oscillator and its behaviour within the interconnected network. The analysis procedures are targeted to verify the stability and to evaluate the performance level of the interconnected systems. The synthesis procedures allow to design the interconnection law for spin torque oscillators in order to fulfill the technical requirements. In this document, the frequency synthesis problem by spin torque oscillator coupling with technical specification description in terms of power spectral densities is established. The formulated specifications introduce the problem of the oscillator modelling in order to perform a simulation and an analysis of the oscillator behaviour. Here, we propose an original model using several conventional models with increasing complexity. An original oscillator network model that describes qualitative properties of the oscillator synchronisation is introduced. Afterwards, the control law architecture for an oscillator set is established in order to accomplish the technical requirement specifications. The suggested control architecture needs to be developed with quantitative systematic and efficient design method for the network interconnection taking into account the formulated performance criteria. In the last two chapters we propose two original frequency domain design methods allowing the resolution of our frequency synthesis problem. The first design method allows to consider explicitly a performance criterium corresponding toa desired frequency constraint. The method allows to obtain a suitable sub-system interconnection matrix that fits the frequency specification constraint. The second design method allows to find an interconnection matrix and to take into account simultaneously several frequency specification constraints. The interconnection matrix obtained with the proposed method solves the problem of frequency synthesis by coupling of spin torque oscillators.

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