Studying dynamics of magnetization relaxation in excited magnetic materials is important both for understanding the rates and pathways of magnetization relaxation and for the potential use in spin-based electronics and data storage devices in the future. Previous studies have demonstrated that the size of nanocrystals is an important factor for energy relaxation in quantum dots and metal nanoparticles. Since magnetization relaxation is one of energy relaxation pathways, the size of nanocrystals may be also an important factor for magnetization relaxation in nanoscale magnetic materials. The goal of this study is to have a better understanding of magnetization relaxation in nanoscale magnetic materials. In particular, we focused on the correlation between the nanocrystal size and the rates of spin-lattice relaxation (SLR), a magnetization relaxation pathway, in magnetic nanocrystals. The size-dependent magnetization relaxation rate after optically induced demagnetization in colloidal Fe3O4 nanocrystals was measured by using time-resolved Faraday rotation (FR). Fe3O4 nanocrystals were chosen as the model system to study the correlation between the size of nanocrystals and the rates of SLR due to the wellestablished synthetic procedure of making nanocrystals with various sizes and narrow size dispersion. Faster SLR rates were observed in smaller Fe3O4 nanocrystals. The results suggested the surface of nanocrystals have higher efficiency of SLR than the interior region by using a simple model to analyze the SLR rates of Fe3O4 nanocrystals with various sizes. Higher efficiency of SLR at the surface may be due to the stronger spin-orbit coupling at the surface relative to the interior region. In addition to magnetization dynamics studies, the effect of oxidation on static FR in iron oxide nanocrystals (between Fe3O4 and y-Fe2O3) was studied. The results indicated FR signal is linearly correlated to the strength of optical transition between Fe2 and Fe3 in Fe3O4 for a given size of nanocrystals.

A model for studying magnetisation during FC-PEDRI experiments has been developed and verified experimentally. The optimum EPR irradiation frequency, which maximises the Overhauser enhancement while minimising the RF power deposition in sample, was investigated using the FC-DNP technique. Experiments to verify the model were carried out in the EPR frequency range between 20 and 180 MHz and with different applied ERP RF power for free radicals with different EPR linewidths. The optimum EPR irradiation frequency was found to be dependent on the applied power level and on the EPR linewidths. Methods to obtain NMR images with T₁-weighted were also investigated. The most successful method was found to be a combination of Inversion Recovery and FC-PEDRI. This technique yields both high <I>T</I>₁ contrast between samples and good contrast between images with different <I>T</I>₁ values. The free radical distribution is recorded with the same SNR as those collected with the conventional FC-PEDRI techniques. Rapid imaging of FC-PEDRI has also been studied. The number of EPR irradiation periods can be optimised to give a good free radical distribution image while reducing the power deposition in the sample. With 4 EPR irradiation periods, the difference image displays the free radical distribution comparable with the conventional technique (64 EPR irradiation periods). The power deposition in the sample is reduced by the factor of 16 and acquisition time is reduced by a factor of 4. Finally, inversion recovery spin preparation has been added to the hybrid fast sample. It was found that an IR hybrid FISP FC-PEDRI pulse sequence could be used to generate <I>T</I>₁ contrast in images which show similar detail and intensity of the free radical distribution as those obtained using hybrid FISP-PEDRI.

Every engineer that has worked on designing an integrated circuit has to leverage an under- standing of device physics. Understanding device physics is essential when optimizing a design for speed, power, etc. These characteristics affect the bottom line when considering an integrated circuit used in a particular application. In order for there to be an under- standing of device physics, there must be a device model that is developed for a device of interest. The development of a device model often involves utilizing fundamental physical equations in a manner that is solvable by either analytical or numerical means. This typically begins by simplifying fundamental physical equations, possibly spanning multiple domains, and considering the physical quantities of interest. In order to make simplifications, assumptions about the underlying physics must be made. It is the process of transitioning from known physics laws to simplified mathematical models that a device modeler spans. This thesis will cover the device modeling aspects of a new classification of computing devices, spintronics. It will begin by stating the physical assumptions necessary for the operation of spintronic devices. Then it will go the process of deriving the underlying physical equations and stating them in a tractable form with the appropriate boundary conditions. Then these equations will be manipulated and mapped into an equivalent circuit. The equivalent circuits will them be validated against analytical solutions provided from other works. It will then finish by providing example devices that can be simulated with the develop device models, and some optimization results are proposed based off a simplified circuit model.

Thesis (Ph.D.)--Tufts University, 1998 . / Adviser: Yaacov Shapira. Submitted to the Dept. of Physics. Includes bibliographical references. Access restricted to members of the Tufts University community. Also available via the World Wide Web;

Thesis (M.S.)--Rochester Institute of Technology, 2007. / Typescript. Includes bibliographical references (leaves 92-98).

Neste trabalho é investigada a relaxação longitudinal da magnetização nuclear de água impregnada em pastilhas de pó de alumina (Al IND. 2 O IND. 3) com grau de pureza de 99,8%, objetivando a determinação do diâmentro de poro de uma espuma cerâmica do mesmo material a partir de medidas do tempo de relaxação longitudinal T IND. 1. O trabalho fundamenta-se no fato dos tempos de relaxação da magnetização nuclear T IND. 1 e T IND. 2, spin-rede e spin-spin respectivamente, de um fluido confinado em um meio poroso serem menores que tempos T IND. 1B e T IND. 2B para o mesmo fluido não confinado. Mediram-se os tempos T IND. 1 para graus de impregnação de água nas amostras entre 10 e 100% usando um espectrômetro montado a partir de módulos comerciais e de um gerador de pulsos desenvolvido no IFSC-USP. A partir dos tempos T IND. 1 e dos diâmetros de poro das pastilhas de pó, determinou-se o valor da relaxatividade longitudinal ou intensidade de relaxação superficial (mu IND. 1), um parâmetro que caracteriza a interface água-alumina nestas amostras. Usando o valor de (mu IND. 1) obtido para o pó e o tempo de relaxação T IND. 1 medido nas amostras de espuma cerâmica foi determinado o diâmetro de poro desta. Última, o qual é comparado com os valores obtidos com a microscopia eletrônica e com a injeção de mercúrio. O caráter não invasivo desta técnica em contraste com as outras, faz dela a única possível em algumas aplicações. / Nuclear magnetization relaxation of water imbibed on 99,8% purê alumina (Al IND. 2 O IND. 3) grain packs is investigated aiming to determine the mean pore diameter of ceramic foam pieces of the same material. The work rely on the shortening of the relaxation times T IND. 1 spin-lattice and T IND. 2 spin-spin of the nuclear magnetization of fluids confined on porous media as compared with their T IND. 1B e T IND. 2B bulk values. Relaxation times T IND. 1 were measured, using a spectrometer built from commercial modules and a pulse generator developed at the IFSC-USP, varying the content of water imbibed from 10 to 100%. The longitudinal relaxation (mu IND. 1), or surface sink strength, was determined from the measured T IND. 1 values and the mean pore diameter of the powder packs. This parameter characterizes the water-alumina interface of these samples. Using the (mu IND. 1) value from the powder and the relaxation times T IND. 1 measured on the ceramic foam pieces, the mean pore diameter of these samples were determined and are compared with the values obtained from mercury intrusion and electron microscopie. The non-invasive character of this technique distinguishes it from the other, making it the only possible in some applications.

Orientador: Flavio Cesar Guimarães Gandra / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-10T12:45:58Z (GMT). No. of bitstreams: 1 Loula_GeorgeDourado_M.pdf: 2173914 bytes, checksum: ff1208aeb34d1619646a01ea6f3f1ec8 (MD5) Previous issue date: 2005 / Resumo: Neste trabalho realizamos medidas de difração de raios-X, magnetização, calor específico e resistividade elétrica nos compostos da série U(Cu,Au)Sn, além do composto UPtSn, que pode cristalizar-se tanto na forma hexagonal (sem tratamento térmico) quanto cúbica, com tratamento térmico de 850ºC por uma semana. Na série UCu(1-x)A u(x)Sn verificamos que todos os compostos apresentam ordenamento antiferromagnético, sendo que a temperatura de ordenamento magnético ( TN) decresce com o aumento da concentração do substituinte. Além disso, percebe-se um pequeno aumento no calor especifico eletrônico ( d ) e no volume da célula unitária ao se inserir Au. O momento efetivo do urânio diminui de 3.44 m B para x=0 até 3 m B para x=1, devido a uma maior hibridização 5f-s,d decorrente da substituição do Cu por Au. Essa hibridização propicia diversas alterações nas propriedades físicas dos compostos. Quanto ao sistema UPtSn, verificamos que o tratamento térmico ao qual a amostra é submetida influencia na formação estrutural do composto e as propriedades físicas de cada estrutura são muito distintas. A fase cúbica ordena antiferromagneticamente em 35K e o urânio apresenta momento magnético efetivo igual a 3.24 m B, enquanto que a fase hexagonal ordena ferromagneticamente em 27K, e apresenta momento magnético efetivo de 2.82 m B. Esta redução no momento efetivo pode ser devida ao efeito Kondo, mas os resultados experimentais não são conclusivos quanto a isto / Abstract: In this work we report on measurements of x-ray diffraction, magnetization, specific heat and electrical resistivity for the series U(Cu,Au)Sn and for UPtSn, which crystallize either with hexagonal (as-cast) or with cubic simmetry (annealed). In the series UCu(1-x)Ausub(x)Sn we verify that all the compounds order antiferromagnetically and the magnetic ordering temperature (TN) decreases when the substitute's concentration is increased. We also noticed a small increase of the electronic specific heat coefficient and unit cell volume by replacing Au for Cu. The effective moment is reduced from 3.44 m B at x=0 to 3 m B at x=1 due to a more effective 5f-s,d hybridization. This hybridization is responsible for the phisical properties of these compounds. For UPtSn we found that the thermal treatment can define the structure of the compound. The phisical properties of both compound are quite different. The cubic phase is AF with TN = 35K and present m eff = 3.24 m B while the hexagonal phase orders at 27K with m eff = 3B. Such reduction of the effective moment may be due to the Kondo effect but the experiments where not conclusive about this / Mestrado / Física da Matéria Condensada / Mestre em Física

Neste trabalho foram obtidas barras da liga (Fe0,80Al0,20)97B3 por solidificação direcional (SD), nos diâmetros de 3 e 4 mm, com o objetivo de melhorar a magnetostricção desse material através da introdução de textura. As etapas de produção das barras envolveram a fabricação de lingotes da liga por fusão a arco L1, L2 e L6, seguida de usinagem (U) ou forjamento rotativo (F) a quente. O maior comprimento de barra obtido foi de 50 mm. Através de microscopia eletrônica de varredura e análise de composição por espectroscopia de energia dispersiva, detectou-se a presença de boro nas amostras pela visualização do microconstituinte eutético formado da fase ? e da fase Fe2B. Resultados obtidos por difração de raios X indicam que a amostra da liga L1 para o diâmetro de 4 mm apresentou orientação preferencial nas direções , o que causou uma diminução expressiva da magnetostricção se comparada a da amostra do mesmo lingote para o diâmetro de 3 mm. Medidas de magnetização em função do campo magnético aplicado (Ha) indicaram que a magnetização de saturação das amostras que têm boro são em média 181 Am2/kg, sugerindo que as amostras tem ~ 2% de B. A barras obtidas por SD, tratadas termicamente (CTT) a 1100°C por 6 h, alcançaram uma maior magnetostricção (? = ?l/l), comparada as amostras sem tratamento térmico (STT), no qual o maior valor foi de 73 ? 10-6 na direção longitudinal da barra L1 forjada no diâmetro de 3 mm (L1F3-CTT). O maior fator de sensibilidade d?/dH obtido em função do comprimento da barra L1F3 foi de d?/dH = 0,9 x 10-9 m/A para a razão de aspecto ? = 12. Valor da constante anisotrópica K1 calculada a partir dos resultados experimentais se comparadas com dados da literatura, sugerem que em algumas amostras ocorre ordenamento parcial da fase cúbica da matriz, havendo assim a coexistência das fases Fe3Al (D03) e ?. / In this work, bars of the alloy (Fe0,80Al0,20)97B3 were obtained by directional solidification (DS) in diameters of 3 and 4 mm, with the goal of improving the magnetostriction of the material through the introduction of texture. Production steps of the bars involved the manufacture of the alloy ingots, L1, L2, L6, by arc melting, then machining (M) or hot swaging (F). The greatest length that was achieved in a bar was 50 mm. The presence of boron in the samples was identified by scanning electron microscopy and energy dispersive spectroscopy, due to the presence of an eutectic microstructure that has dendrites and a microconstituent formed of phases ? and Fe2B. Results obtained by X-ray diffraction indicates that the bar L1 at the diameter of 4 mm showed preferential orientation in the directions , which caused a significant decrease of the magnetostriction compared to the same sample of the bar of diameter 3 mm. Magnetization measurements as a function of applied magnetic field (Ha) reveal that the saturation magnetization of the samples having boron is in average 181 Am2/kg, suggesting that the samples have ~ 2% of boron. The bars obtained by DS, thermally treated (CTT ) at 1100 ° C for 6 h, achieved higher magnetostrictions (? = ?l/l) compared to the samples without heat treatment.The highest value was 73 ? 10-6 was obtained in the longitudinal direction of the bar L1F in the diameter of 3 mm (L1F3-CTT). The maximum sensitivity factor d?/dH found as a function of the bars lengths was d?/dH = 0.9 x 10-9 m/A for an aspect ratio of ? = 12. Values of anisotropic constant K1, calculated using experimental results suggest that in some samples occur partial ordering of the cubic phase matrix , so there is the coexistence of phases Fe3Al ( D03 ) and ?.

Orientador: Vivian Vanessa França / Banca: Erica regina Filletti Nascimento / Banca: Tamiris trevisani Negri / Resumo: A superfluidez convencional, cuja descrição foi desenvolvida em 1957 por Bardeen-Cooper-Schrieffer (BCS), é fundamentada em um sistema não polarizado e na ausência de campos magnéticos. Porém outros regimes de superfluidez foram propostos na presença de campos magnéticos ou de população de spin desbalanceada. Neste caso, haveria a coexistência de magnetismo e superfluidez. Tais fenômenos de superfluidez heterogênea têm atraído a atenção de diversas áreas da ciência, pois também podem explicar a existência de propriedades não-convencionais como, por exemplo, variações súbitas no período de rotação de estrelas de nêutrons. Devido às diversas propostas existentes para o fenômeno de superfluidez heterogênea, há grande controvérsia em relação aos regimes de parâmetros em que o fenômeno se manifesta e se mantem estável. Diversos cálculos exatos e aproximados têm sido realizados a fim de detectar e descrever tais fases, como DMRG (Density Matrix Renormalization Group) e DFT (Density Functional Theory). Nessa monografia, através de cálculos exatos de DMRG, exploramos as propriedades de superfluidez exótica em modelos de nanoestruturas. Investigamos em particular a propriedade de emaranhamento quântico, conhecida por revelar os parâmetros em que transições de fase quânticas ocorrem. Nossos resultados demonstram que o emaranhamento de sistemas superfluidos é um bom parâmetro para a detecção da fase FFLO e que pode ser quantificado experimentalmente em superfluidos através de sua surpre... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Conventional superfluidity, whose description was developed in 1957 by Bardeen-Cooper-Schrieffer (BCS), is based on a non-polarized system and the absence of magnetic fields. However, other superfluidity regimes were proposed in the presence of magnetic fields or spin-imbalanced population. In this case, there would be the coexistence of magnetism and superfluidity. Such phenomenon of heterogeneous superfluidity has attracted attention of several scientific areas, as they may also explain the existence of unconventional properties such as, for example, sudden variations in the period of neutron stars rotation. Due to the several proposals for the phenomenon of heterogeneous superfluidity, there is great controversy regarding the parameter regimes in which the phenomenon manifests itself and remains stable. Several exact and approximate calculations have been performed to detect and describe such phases, such as DMRG (Density Matrix Renormalization Group) and DFT (Density Functional Theory). In this monograph, through exact DMRG calculations, we explore the properties of exotic superfluidity in nanostructures models. We investigate in particular the quantum entanglement property known to reveal the parameters in which quantum phase transitions occur. Our results demonstrate that entanglement of superfluid systems is a good parameter for detecting the FFLO phase and that it can be quantified experimentally in superfluids by exploring its surprising linear relation with the magn... (Complete abstract click electronic access below) / Mestre

La découverte de la magnétorésistance géante en 1988 est considérée comme la date de naissance d’un nouveau et dynamique champ de recherche appelé l’électronique de spin. La riche physique associée au transport de spin devrait révolutionner le futur de la nanoélectronique. Dans ce cadre les nano-oscillateurs à transfert de spin (STOs) se sont positionnés comme des candidats sérieux pour le développement d’une nouvelle génération de dispositifs rf basés sur l’électronique de spin.Au début de ma thèse, l’important bruit de phase des STOs restait une contrainte majeure limitant les perspectives technologiques à ce type d’oscillateurs. Dans cette thèse nous avons cherché à contrôler la dynamique des STOs et à réduire leur bruit phase en développant différentes stratégies : (i) l’optimisation des propriétés des matériaux magnétiques utilisés (ii) l’excitation de modes couplés dans des systèmes hybridés (iii) la stabilisation de la dynamique de la phase d’un STO avec un signal extérieur de référence (iv) la synchronisation mutuelle de différents oscillateurs pour améliorer la cohérence spectrale et la puissance des STOs. Nous focalisons en particulier sur le cas de STO à base de vortex magnétique qui présentent intrinsèquement des cohérences spectrales plus élevées que celles d’autres types d’oscillateurs.Dans une première partie, nous nous proposons d’identifier et d’étudier les différents mécanismes qui régissent et contrôle la dynamique d’un STO à base de vortex magnétique dans les régimes auto-oscillant et non-autonomes. Nous mettons tout d’abord en évidence que l’excitation de modes couplés permet de contrôler les propriétés rf d’un oscillateur unique en prenant l’exemple d’un STO à base de deux vortex magnétiques couplés. Par la suite, nous étudions la synchronisation « parfaite » de ces STOs à base de vortex avec un courant rf de référence. Nous corrélons l’observation d’une largeur de raie d’un hertz et d’un bruit de phase minimum de -90 dBc/Hz à 1kHz de la porteuse dans l’état synchronisé à une absence de glissement de phase, i.e. à l’absence de phénomène de désynchronisation. Le fort couple de Field-like planaire mesuré dans ces STOs représente un outil précieux pour contrôler le processus de synchronisation. Dans le cas des STOs à double vortex, un tel contrôle nous permet d’observer des phénomènes physiques exotiques allant de la synchronisation simultanée de plusieurs modes, à de l’auto-synchronisation en passant par des dynamiques de synchronisation incohérentes.Dans une seconde partie, nous proposons différents concepts innovants de dispositifs rf à base de vortex magnétique. Nous présentons tout d’abord les bases d’une boucle à retard de phase permettant d’asservir un STO. En prenant avantage du fort couple de Field Like, nous développons un nouveau schéma de détection rf, plus efficace que les actuelles diodes Schottky, basé sur un renversement d’aimantation en expulsant réversiblement le cœur de vortex à l’aide d’un courant rf. Finalement, nous démontrons qu’il est possible de synchroniser électriquement deux STOs connectés directement en parallèle ou en série, ou à l’aide d’une ligne à retard. Nous montrons ainsi qu’une forte amélioration de la cohérence spectrale (d’un facteur 2) et de la puissance (d’un facteur 4 pour un maximum de 1.6 μW) peut être obtenues dans l’état synchronisé. A l’aide de la ligne à retard, nous mettons par ailleurs en évidence le rôle crucial du déphasage entre les deux STOs sur les propriétés de l’état synchronisé. Ces résultats prometteurs ouvrent la voie vers la synchronisation de réseaux de STOs sans champ appliqué et sans ligne à retard entre les oscillateurs.Dix ans après leur découverte, les oscillateurs à transfert de spin n’ont toujours pas dévoilé tout leur potentiel et de nouvelles applications sont maintenant envisagées, allant de dispositif rf classiques à des circuits logiques et dispositifs bio-inspirés basés sur les STOs. / The discovery of the giant magnetoresistance in 1988 is considered as the birth date of a new and dynamic research field called spintronics. The rich physics associated with spin transport has created a breakthrough for the future of nano-electronics. In the magnetism roadmap, spin-torque oscillators (STOs) are candidates for future generation of spintronic based rf-devices.At the beginning of this thesis, one major issue of spin-torque oscillators remained their poor spectral coherence. To overcome this issue, we have investigated different approaches: (i) the development of magnetic materials with a low damping and large spin-polarization, (ii) the study of collective mode dynamics in hybridized magnetic systems (iii) the stabilization of the STO dynamics with a reference external signal (iv) the synchronization of multiple STOs to enhance both their power and spectral coherence. We focus our work on vortex based STOs which present higher spectral coherences than other kinds of STOs.In a first part, we study the different mechanisms that can drive and stabilize the dynamics of a vortex based STO in the autonomous and non-autonomous regimes. We first highlight that the excitation of collective modes allows the harnessing the rf-properties of a single and isolated in a double vortex based STO. Then we report the ``perfect'' phase-locking of a STO with an external rf-current. To go beyond this analysis, we notice that a 1 Hz minimum linewidth and a flat phase noise level of -90 dBc/Hz at 1 kHz from the offset frequency in the locked state could be associated with the absence of phase slips, i.e desynchronization events. We demonstrate that the locking process is driven by a Field-like in-plane torque which gives the possibility to control with precision the STO locking process. In our double vortex based STO, we can even observe exotic behaviors such as multi-mode synchronization, self-resonance and eventually incoherent motion. Such a degree of control, unexpected for a nano-scale oscillator, is particularly promising for the development of STO based nanodevices.In a second part, we propose different concepts of spintronic rf-devices based on vortex STOs. We describe the basis of an on-chip STO based phase locked loop. By taking advantage of the large Field-like torque in our STOs, we develop a new radio-frequency detection scheme, more efficient than the state of the art Schottky diode, based on magnetization switching through the resonant and reversible expulsion of the vortex core. Finally, we show the first experimental observation of the electrical synchronization of two STOs connected directly in parallel or in series, or with an electrical delay line. In the synchronized state, we show a strong improvement of both the spectral coherence (by a factor 2) and the output power (by a factor 4, up to 1.6 μW). We also demonstrate, with an electrical delay line, the strong impact of the phase shift between the two STOs on the synchronized regime. These promising results open the way for the synchronization of STO arrays at zero field and without electrical delays.Ten years after their discovery, spin-torque oscillators have thus not yet revealed all their potential and promising applications could be soon targeted, in order to realize a spin logic circuit, bio-inspired spintronic devices and more classical rf-applications.