The Spin-Transfer Torque Magnetoresistive Random Access Memory (STT-MRAM) has opened new doors as an emerging technology with high potential to replace traditional CMOS-based memory technology. This has come true due to the density, speed and non- volatility that have been demonstrated. The STT-MRAM uses Magnetic Tunnel Junction (MTJ) elements as non-volatile memory storage devices because of the recent discovery of spin-torque phenomenon for switching the magnetization states. The magnetization of the free layer in STT-MRAM can be switched from logic "1" to logic "0" by the use of a spin-transfer torque. However, the STT-MRAMs have till now only been used as universal memory. As a result, STT-MRAMs are not yet commercially used as computing elements, though they have the potential to be used as Logic-In-Memory computation applications. In order to advance this STT-MRAM technology for computation, we have used different MRAM devices that are available as memory elements with different geometries, to use it as computing elements. This dissertation presents design and implementation of such devices using different multilayer magnetic material stacks for computation. Currently, the design of STT-MRAMs is limited to only memory architectures, and there have been no proposals on the viability of STT-MRAMs as computational devices. In the present work, we have developed a design, which could be implemented for universal logic computation. We have utilized the majority gate architecture, which uses the magneto-static interaction between the freelayers of the multilayer nanomagnets, to perform computation. Furthermore, the present work demonstrates the study of dipolar interaction between nanomagnetic disks, where we observed multiple magnetization states for a nanomagnetic disk with respect to its interaction energy with its neighboring nanomagnets. This was achieved by implementing a single layer nanomagnetic disk with critical dimension selected from the phase plot of single domain state (SDS) and vortex state (VS). In addition, we found that when the interaction energy between the nanomagnetic disks with critical dimension decreases (increase in center-to-center distance) the magnetization state of the nanomagnetic disks changes from single domain state to vortex state within the same dimension. We were able to observe this effect due to interaction between the neighboring nanomagnets. Finally, we have presented the design and implementation of a Spin-Torque driven Re- configurable Array of Nanomagnets (STRAN) that could perform Boolean and non-Boolean computation. The nanomagnets are located at every intersection of a very large crossbar array structure. We have placed these nanomagnets in such a way that the ferromagnetic free layers couple with each other. The reconfigurable array design consists of an in-plane (IP) free layer and a fixed polarizer [magnetized out-of-plane (OP)]. The cells that need to be deselected from the array are taken to a non-computing oscillating state.

Excessive energy dissipation in CMOS devices during switching is the primary threat to continued downscaling of computing devices in accordance with Moore’s law. In the quest for alternatives to traditional transistor based electronics, nanomagnet-based computing [1, 2] is emerging as an attractive alternative since: (i) nanomagnets are intrinsically more energy-efficient than transistors due to the correlated switching of spins [3], and (ii) unlike transistors, magnets have no leakage and hence have no standby power dissipation. However, large energy dissipation in the clocking circuit appears to be a barrier to the realization of ultra low power logic devices with such nanomagnets. To alleviate this issue, we propose the use of a hybrid spintronics-straintronics or straintronic nanomagnetic logic (SML) paradigm. This uses a piezoelectric layer elastically coupled to an elliptically shaped magnetostrictive nanomagnetic layer for both logic [4-6] and memory [7-8] and other information processing [9-10] applications that could potentially be 2-3 orders of magnitude more energy efficient than current CMOS based devices. This dissertation focuses on studying the feasibility, performance and reliability of such nanomagnetic logic circuits by simulating the nanoscale magnetization dynamics of dipole coupled nanomagnets clocked by stress. Specifically, the topics addressed are: 1. Theoretical study of multiferroic nanomagnetic arrays laid out in specific geometric patterns to implement a “logic wire” for unidirectional information propagation and a universal logic gate [4-6]. 2. Monte Carlo simulations of the magnetization trajectories in a simple system of dipole coupled nanomagnets and NAND gate described by the Landau-Lifshitz-Gilbert (LLG) equations simulated in the presence of random thermal noise to understand the dynamics switching error [11, 12] in such devices. 3. Arriving at a lower bound for energy dissipation as a function of switching error [13] for a practical nanomagnetic logic scheme. 4. Clocking of nanomagnetic logic with surface acoustic waves (SAW) to drastically decrease the lithographic burden needed to contact each multiferroic nanomagnet while maintaining pipelined information processing. 5. Nanomagnets with four (or higher states) implemented with shape engineering. Two types of magnet that encode four states: (i) diamond, and (ii) concave nanomagnets are studied for coherence of the switching process.

Since its inception, Moore's Law has been a reliable predictor of computational power. This steady increase in computational power has been due to the ability to fit increasing numbers of transistors in a single chip. A consequence of increasing the number of transistors is also increasing the power consumption. The physical properties of CMOS technologies will make this powerwall unavoidable and will result in severe restrictions to future progress and applications. A potential solution to the problem of rising power demands is to investigate alternative low power nanotechnologies for implementing logic circuits. The intrinsic properties of these emerging nanotechnologies result in them being low power in nature when compared to current CMOS technologies. This thesis specifically highlights quantum dot celluar automata (QCA) and nanomagnetic logic (NML) as just two possible technologies. Designs in NML and QCA are explored for simple arithmetic units such as full adders and subtractors. A new multilayer 5-input majority gate design is proposed for use in NML. Designs of reversible adders are proposed which are easily testable for unidirectional stuck at faults.

A recent International Technology Roadmap for Semiconductors (ITRS) report (2.0, 2015 edition) has shown that Moore’s law is unlikely to hold beyond 2028. There is a need for alternate devices to replace CMOS based devices, if further miniaturization and high energy efficiency is desired. The goal of this dissertation is to experimentally demonstrate the feasibility of nanomagnetic memory and logic devices that can be clocked with acoustic waves in an extremely energy efficient manner. While clocking nanomagnetic logic by stressing the magnetostrictive layer of a multiferroic logic element with with an electric field applied across the piezoelectric layer is known to be an extremely energy-efficient clocking scheme, stressing every nanomagnet separately requires individual contacts to each one of them that would necessitate cumbersome lithography. On the other hand, if all nanomagnets are stressed simultaneously with a global voltage, it will eliminate the need for individual contacts, but such a global clock makes the architecture non-pipelined (the next input bit cannot be written till the previous bit has completely propagated through the chain) and therefore, unacceptably slow and error prone. Use of global acoustic wave, that has in-built granularity, would offer the best of both worlds. As the crest and the trough propagate in space with a velocity, nanomagnets that find themselves at a crest are stressed in tension while those in the trough are compressed. All other magnets are relaxed (no stress). Thus, all magnets are not stressed simultaneously but are clocked in a sequentially manner, even though the clocking agent is global. Finally, the acoustic wave energy is distributed over billions of nanomagnets it clocks, which results in an extremely small energy cost per bit per nanomagnet. In summary, acoustic clocking of nanomagnets can lead to extremely energy efficient nanomagnetic computing devices while also eliminating the need for complex lithography. The dissertation work focuses on the following two topics: Acoustic Waves, generated by IDTs fabricated on a piezoelectric lithium niobate substrate, can be utilized to manipulate the magnetization states in elliptical Co nanomagnets. The magnetization switches from its initial single-domain state to a vortex state after SAW stress cycles propagate through the nanomagnets. The vortex states are stable and the magnetization remains in this state until it is ‘reset’ by an external magnetic field. 2. Acoustic Waves can also be utilized to induce 1800 magnetization switching in dipole coupled elliptical Co nanomagnets. The magnetization switches from its initial single-domain ‘up’ state to a single-domain ‘down’ state after SAW tensile/compressive stress cycles propagate through the nanomagnets. The switched state is stable and non-volatile. These results show the effective implementation of a Boolean NOT gate. Ultimately, the advantage of this technology is that it could also perform higher order information processing (not discussed here) while consuming extremely low power. Finally, while we have demonstrated acoustically clocked nanomagnetic memory and logic schemes with Co nanomagnets, materials with higher magnetostriction (such as FeGa) may ultimately improve the switching reliability of such devices. With this in mind we prepared and studied FeGa films using a ferromagnetic resonance (FMR) technique to extract properties of importance to magnetization dynamics in such materials that could have higher magneto elastic coupling than either Co or Ni.

Esta tese descreve o desenvolvimento de aplicações de nanopartículas superparamagnéticas de magnetita, devidamente funcionalizadas, nas seguintes áreas: Sensoriamento - As nanopartículas superparamagnéticas foram utilizadas em conjunto com um pequeno sistema analítico especialmente desenvolvido para esse trabalho, na análise de diversos metais pesados de grande relevância ambiental. O ponto de destaque da metodologia é o grande incremento em intensidade do sinal analítico em função da pré-concentração que pode ser realizada mediante utilização das nanopartículas superparamagnéticas e aplicação de um campo magnético externo. Remediação ambiental - A grande afinidade das nanopartículas por diversos materiais foi aproveitada na obtenção de um sistema de remediação ambiental que combina as propriedades magnéticas das nanopartículas e as propriedades adsorventes de carvão ativado. Esse sistema foi testado em contaminantes orgânicos e inorgânicos e permite uma maneira eficiente e inovadora de remoção de contaminantes em meio aquoso. Nanohidrometalurgia magnética - Os efeitos de pré-concentração de nanopartículas superparamagnéticas modificadas foram utilizados para melhorar a eficiência dos processos hidrometalúrgicos atuais para produção de cobre metálico a partir de minérios com baixa concentração do metal, chegando - se ao estado da arte da técnica. / This thesis describes the development of applications of superparamagnetic magnetite nanoparticles, properly functionalized, in the following areas: Sensing - The superparamagnetic nanoparticles were used in conjunction with a small analytical system specially developed in this work for analysis of several heavy metals of great environmental significance. The important point of this methodology is the general increase in intensity of the analytical signal as a function of pre-concentration may be performed by use of superparamagnetic nanoparticles under the influence of an external magnetic field. Environmental Remediation - The high affinity of nanoparticles for various materials was used to obtain an environmental remediation system that combines the properties of magnetic nanoparticles and the properties of activated carbon adsorbents. This system was tested in organic and inorganic contaminants and provides a novel and efficient removal of contaminants in the aqueous medium. Magnetic Nanohydrometalurgy - The effects of pre-concentration fro the funcionalized superparamagnetic nanoparticles modified were used to improve the efficiency of the actual hydrometallurgical processes used for production of metallic copper from ores with low concentration of metal, showing good perspectives of improving the state of the art of this technique.

Esta tese descreve o desenvolvimento de aplicações de nanopartículas superparamagnéticas de magnetita, devidamente funcionalizadas, nas seguintes áreas: Sensoriamento - As nanopartículas superparamagnéticas foram utilizadas em conjunto com um pequeno sistema analítico especialmente desenvolvido para esse trabalho, na análise de diversos metais pesados de grande relevância ambiental. O ponto de destaque da metodologia é o grande incremento em intensidade do sinal analítico em função da pré-concentração que pode ser realizada mediante utilização das nanopartículas superparamagnéticas e aplicação de um campo magnético externo. Remediação ambiental - A grande afinidade das nanopartículas por diversos materiais foi aproveitada na obtenção de um sistema de remediação ambiental que combina as propriedades magnéticas das nanopartículas e as propriedades adsorventes de carvão ativado. Esse sistema foi testado em contaminantes orgânicos e inorgânicos e permite uma maneira eficiente e inovadora de remoção de contaminantes em meio aquoso. Nanohidrometalurgia magnética - Os efeitos de pré-concentração de nanopartículas superparamagnéticas modificadas foram utilizados para melhorar a eficiência dos processos hidrometalúrgicos atuais para produção de cobre metálico a partir de minérios com baixa concentração do metal, chegando - se ao estado da arte da técnica. / This thesis describes the development of applications of superparamagnetic magnetite nanoparticles, properly functionalized, in the following areas: Sensing - The superparamagnetic nanoparticles were used in conjunction with a small analytical system specially developed in this work for analysis of several heavy metals of great environmental significance. The important point of this methodology is the general increase in intensity of the analytical signal as a function of pre-concentration may be performed by use of superparamagnetic nanoparticles under the influence of an external magnetic field. Environmental Remediation - The high affinity of nanoparticles for various materials was used to obtain an environmental remediation system that combines the properties of magnetic nanoparticles and the properties of activated carbon adsorbents. This system was tested in organic and inorganic contaminants and provides a novel and efficient removal of contaminants in the aqueous medium. Magnetic Nanohydrometalurgy - The effects of pre-concentration fro the funcionalized superparamagnetic nanoparticles modified were used to improve the efficiency of the actual hydrometallurgical processes used for production of metallic copper from ores with low concentration of metal, showing good perspectives of improving the state of the art of this technique.

Even though silicon technology is dominant today, the physics (quantum electron tunneling effect), design (power dissipation, wire delays) and the manufacturing (lithography resolution) limitations of CMOS technology are pushed towards the scaling end. These issues motivated us towards a new paradigm that contributes to a continued advancement in terms of performance, density, and cost. The magnetic field coupled computing (MFC) paradigm, which is one of the regimes where we leverage and utilize the neighbor interaction of the nanomagnets to order the single-domain magnetic cells to perform computational tasks. The most important and attractive features of this technology are: 1) room temperature operation, which has been a limitation in electrostatic field coupled devices, 2) high density and nonetheless 3) low static power dissipation. It will be intriguing to address queries like, what are the challenges posed by the technology with such exotic features? Answer to such questions would become the focus of this doctoral research. The fundamental problem with magnetic field coupled devices is the directional flow of information from input to output. In this work, we have proposed a novel spatially moving Landauer clock system for MFC nanomagnet array which has an advantage over existing adiabatic clock system. Extensive simulation studies were done to model and validate the clock for different length, size, and shape of nanomagnet array. Another key challenge is the manufacturing defect, which leads to uncertainty and unreliability issues. We studied the different dominant types of geometric defects (missing material, missing cell, spacing, bulge, and merging) in array (used as interconnects) based on our fabrication experiments. We also studied effect of these defects on different segments (locations) of the array with spatially moving clock. The study concluded that a spatially moving clock scheme constitutes a robust MFC architecture as location of defect and length of arrays does not play any role in error masking as opposed to conventional clock. Finally, the work presents the study on the 2D nanomagnet array for boolean logic computation and vision logic computation. The effect of dipole-dipole interaction on magnetization state transition in closely spaced 2D array of ferromagnetic circular nanomagnet was explored. The detailed design space to demarcate the boundary between single domain state and vortex state reveals that the single domain state space is desirable for Boolean logic computation while the space around the boundary would be appropriate for vision logic computing.

Made available in DSpace on 2014-12-17T14:06:54Z (GMT). No. of bitstreams: 1 SheilaBG_DISSERT.pdf: 1652158 bytes, checksum: ad653b0a72284122fdd7e2e051380eb2 (MD5) Previous issue date: 2010-04-23 / Universidade Federal do Rio Grande do Norte / In this study barium hexaferrite was (general formulae BaFe12O19) was synthesized by the Pechini method under different conditions of heat treatment. Precursors like barium carbonate and iron nitrate were used. These magnetic ceramic, with magnetoplumbite type structure, are widely used as permanent magnet because of its excellent magnetic properties, such as: high Curie temperature, good magnetic anisotropy, high coercivity and corrosion resistance. The samples were characterized by thermal analysis (DTA and TG), X- ray Diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM) end Vibrating sample Magnetometer (VSM). The results confirm the expected phase, which was reinforced according to our analysis. A single phase powder at relatively high temperatures with particle sizes around 100 nm was obtained. The characteristic magnetic behavior one of the phases has been noted (probably superparamagnetic material), while another phase was identified as a ferrimagnetic material. The ferrimagnetic phase showed vortex configuration with two central and slightly inclined plateaus. In general, increase of heat treatment temperature and time, directly influenced the technological properties of the samples / Neste trabalho foi sintetizado o composto da Hexaferrita de B?rio de f?rmula geral BaFe12O19, preparado pelo m?todo Pechini (sob diferentes condi??es de tratamento t?rmico), utilizando-se como precursores o carbonato de b?rio e o nitrato de ferro. Estes materiais cer?micos magn?ticos, com estrutura do tipo magnetoplumbita, s?o amplamente utilizados como magnetos permanentes, devido as suas excelentes propriedades magn?ticas, tais como: alta temperatura de Curie, boa anisotropia magn?tica, alta coercividade e resist?ncia ? corros?o. As amostras obtidas foram caracterizadas pelas t?cnicas: An?lises T?rmicas (DTA e TG), Difra??o de raios-X (DRX), Espectroscopia de absor??o na regi?o do infravermelho (FTIR), Microscopia Eletr?nica de Varredura (MEV) e Magnet?metro Vibracional (VSM). Os resultados mostraram a fase desejada, onde foi intensificada de acordo com as an?lises realizadas, resultando na obten??o de um p? monof?sico nas temperaturas relativamente altas com tamanhos de part?culas em torno de 100 nm. Os materiais possuem fases de comportamento magn?tico caracter?stico, provavelmente de um material superparamagn?tico e outro ferrimagn?tico que em temperaturas mais elevadas apresentaram uma configura??o de v?rtice, com dois plat?s centrais com pequenas inclina??es. De modo geral, os aumentos da temperatura de tratamento t?rmico e do tempo, influenciaram diretamente nas propriedades tecnol?gicas das amostras

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.

Nanomagnetic logic, incorporating logic bits in the magnetization orientations of single-domain nanomagnets, has garnered attention as an alternative to transistor-based logic due to its non-volatility and unprecedented energy-efficiency. The energy efficiency of this scheme is determined by the method used to flip the magnetization orientations of the nanomagnets in response to one or more inputs and produce the desired output. Unfortunately, the large dissipative losses that occur when nanomagnets are switched with a magnetic field or spin-transfer-torque inhibit the promised energy-efficiency. Another technique offering superior energy efficiency, “straintronics”, involves the application of a voltage to a piezoelectric layer to generate a strain which is transferred to an elastically coupled magnetrostrictive layer, causing magnetization rotation. The functionality of this scheme can be enhanced further by introducing magnetocrystalline anisotropy in the magnetostrictive layer, thereby generating four stable magnetization states (instead of the two stable directions produced by shape anisotropy in ellipsoidal nanomagnets). Numerical simulations were performed to implement a low-power universal logic gate (NOR) using such 4-state magnetostrictive/piezoelectric nanomagnets (Ni/PZT) by clocking the piezoelectric layer with a small electrostatic potential (~0.2 V) to switch the magnetization of the magnetic layer. Unidirectional and reliable logic propagation in this system was also demonstrated theoretically. Besides doubling the logic density (4-state versus 2-state) for logic applications, these four-state nanomagnets can be exploited for higher order applications such as image reconstruction and recognition in the presence of noise, associative memory and neuromorphic computing. Experimental work in strain-based switching has been limited to magnets that are multi-domain or magnets where strain moves domain walls. In this work, we also demonstrate strain-based switching in 2-state single-domain ellipsoidal magnetostrictive nanomagnets of lateral dimensions ~200 nm fabricated on a piezoelectric substrate (PMN-PT) and studied using Magnetic Force Microscopy (MFM). A nanomagnetic Boolean NOT gate and unidirectional bit information propagation through a finite chain of dipole-coupled nanomagnets are also shown through strain-based "clocking". This is the first experimental demonstration of strain-based switching in nanomagnets and clocking of nanomagnetic logic (Boolean NOT gate), as well as logic propagation in an array of nanomagnets.