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Magnetic Logic Devices: Design, Simulation and Measurement
No full textThe constant scaling of transistor sizes that has driven the extraordinary develo- pment of CMOS technology in the last decades. The reduction of transistor sizes has many advantages: the circuit area and power consumption decrease and the clock frequency increases. However the scaling of transistor sizes is rapidly moving to- wards its physical limits. The two major factors limiting the development of CMOS technology are the difficulties in the fabrication process and the unavoidable impact of leakage losses, mainly due to the gate tunnel current. To overcome the limitations of CMOS transistors and to create circuits even denser and faster, many alternative technologies have been studied. Among these possibilities, the technologies based on magnetic devices are really interesting and promising: NanoMagnet Logic (NML), Domain Wall (DW) and Spin Wave (SW) are all part of a new family of emerging magnetic logic devices. Their magnetic nature represents the main reason of this increasing interest: they can mix logic, they can have a very low dynamic power consumption. NML and DW have also no stand-by power dissipation. The main purpose of this thesis is to investigate and to design logic circuits and interconnections based on magnetic technologies. After a complete introduc- tion analyzing different magnetic technologies, the thesis work focuses on the design, validation through simulations and analysis of logic circuits and innovative intercon- nection methods. In fact interconnections between logic elements represent a limit in many of these magnetic circuits. The research work here presented has led to the development of advanced interconnections based on different magnetic technologies. At the beginning the attention has been focused on Domain Wall technology. From the magnetic point of view, a domain wall is an interface which divides diffe- rent magnetic domains, which are defined as regions with a uniform magnetization. A domain is a region inside a magnetic material where the magnetic moments of atoms have the same direction and versus between each other. A domain wall is a region of transition between different magnetic moments in which occurs a gra- dual orientation of the magnetic moments. A domain wall has been used to interface NML logic elements. The traditional interconnection composed by nanomagnets has been substituted by a new interconnection system based on domain walls. Different structures have been presented and validated through accurate simulations. A para- metrical analysis of the new interconnection methods has been done, considering the length and the width of the line. This analysis has been used to better understand and to validate the working principle, the efficiency in terms of domain wall speed and the power consumption of the new interconnection system. This analysis has been also fundamental in finding the best trade-off between power dissipation and operation frequency to achieve a circuit based on domain wall with the best possible performance. In the second part of the thesis the attention has been directed to the Spin Wave technology. A spin wave is a collective oscillation of spins along the magnetiza- tion direction of a magnetic structure. In this case, in addition to an innovative interconnection method based on spin wave, pure spin wave logic circuits have been analyzed and validated through simulations. Moreover spin wave devices based on reconfigurable magnetic patterning structures have been presented and validated through micromagnetic simulations. This innovative technique can be used in order to pattern complex structures in a continuous film, avoiding possible scattering at interfaces and introducing a reconfigurability and tunability. This last part has been carried on in collaboration with the University of Notre Dame. The last part of the thesis work focuses on the Magneto-Optic Kerr Effect (MO- KE). This activity, carried on at the University of Notre Dame, consisted in the design and assembly of a MOKE system, with the aim of measuring magnetic sam- ples with logic gates and interconnections to validate the simulation results. The reflected optical response from a magnetic media has been introduced, using the theory of light propagation inside and at interfaces between the media. Taking ad- vantage of the electromagnetic theory, the optical rotation phenomenon observed in magnetic media is described with the expression of the reflectivity. The complete experimental set-up for longitudinal MOKE, assembled at the University of Notre Dame, is described in detail, with the correspondent operating manual useful to perform a complete measurement of a magnetic hysteresis. The research work of this thesis represents an important milestone in order to improve the NML interconnections and to design innovative logic devices. Never- theless work is still necessary to reach a complex system, analyzing and validating the behavior of other interconnection structures with different geometries. Many changes are necessary to guarantee the correct operation and to further improve a complete system, composed by logic gates and interconnection wires. Innovative approaches, for example to combine all the magnetic technologies presented in this work in a new kind of magnetic circuit, is an indispensable requirement to develop really innovative technologies and systems
Domain Magnet Logic (DML): A new approach to magnetic circuits
Full text linkIn the post CMOS scenario NanoMagnets Logic (NML) has attracted a considerable attention due to its characteristic features. The ability to combine logic and memory in the same device, and a possible low power consumption, allows NML to overcome some of the CMOS intrinsic limitations. However, considering realistic circuit implementations where both theoretical and technological constraints are kept into account, performance could not be reduced with respect to the expectations. The reason lies in the fact that a huge area is wasted with interconnection wires. In this paper we propose a new approach to the conception of magnetic circuits, that we have baptized Domain Magnet Logic (DML). We embed domain walls in NML circuits in a technologically compatible solution, with the aim of improving interconnection performance. We have validated our solution with physical level simulations, and we show the improvements designing as a case study a complex and realistic circuit, a 32 bit Pentium-4 tree-adder. DML logic allows to reduce the circuit area up to 50%, with consequent dramatic improvements on circuit latency and power dissipation. This is a very good result itself, that represents just the tip of the iceberg of the amazing possibilities opened by this innovative approac
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
A reconfigurable array architecture for NML
No full textNanoMagnet Logic (NML) is one of the most promising emerging technologies, in particular for its low power consumption and for the capability to mix logic and memory in the same device. At the same time this technology has some drawbacks, the most important of which is the long delay of wires and the correlation between layout and circuit timing. From a technological point of view, MagnetoElastic NML (ME-NML) is one of the proposed improvements that could address some of these drawbacks. From an architectural point of view, instead, to exploit the peculiar characteristics of NML and reduce the impact of its drawbacks, parallel solutions like Systolic Arrays can be adopted. Systolic Arrays are commonly used as hardware accelerators dedicated to a single algorithm, and for this reason their field of use has been extremely limited. Reconfigurable Arrays can overcome this limitation. In this article we first introduce our Reconfigurable Systolic Array. It can be configured to execute different algorithms and it is therefore an ideal architecture for NML. The Reconfigurable Systolic Array has been first designed at a RTL level in CMOS and synthesized using a 28nm technology. Then, it has been synthesized and simulated in classic NML using ToPoliNano, the first existing tool for NML. Finally, a custom layout based on ME-NML has been designed and we have estimated area and power dissipation. Comparison among the technologies show that ME-NML is extremely promising in terms of area occupation and power dissipation. Even if the technology is not yet mature it can already compete with CMOS
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Domain Wall Interconnections for NML
No full textNanoMagnet Logic is one of the most novel solutions studied as complementary technology to CMOS transistors. Information propagation involves only a change in spin orientation, no charge movement is present. Since the basic element is a nanomagnet, NML circuits have no stand-by power consumption and the ability to mix logic and memory in the same device. While CMOS is a multilayer technology, until now NML is confined to one single physical layer. The consequence is that circuit area grows exponentially due to interconnections overhead. In this paper we present an innovative solution that drastically reduces the area wasted for interconnection wires relying on the properties of Domain Walls (DWs). We mix DWs and NML technologies in a unique Doman Wall Logic (DWL) solution that exploits the advantages of both technologies. The proposed solution is technologically compatible with up-to-date fabrication processes. All the results here presented for the NML logic blocks and the DWs interconnections and their combination are obtained through rigorous micromagnetic simulations. Moreover, we implemented as a case study an high performance adder (Pentium-4 Adder) and evaluated its features with increasing parallelism and compared to the simple NML implementation in order to explore the potential of DWL technology at circuit and architectural level. The reduction in circuit area corresponds to a notable reduction in both latency and power consumption. The improvements in NML technology are shown by both the remarkable performance improvement and new possibilities offered by this novel solution
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