No description available.

The high frequency dynamic magnetic responses of FeCo thin films and structures have been investigated mainly using ferromagnetic resonance (FMR) technique. The FMR resonance condition and linewidth are first derived from the dynamic Landau- Lifshitz-Gilbert equation, followed by a study of the conversion between FMR field and frequency linewidths. It is found that the linewidth conversion relation based on the derivative of resonance condition is only valid for samples with negligible extrinsic linewidth contribution. The dynamic magnetic properties obtained by using FMR measurements of FeCo thin films grown on Si/SiO2 substrates with varying deposition temperatures is then presented. The effective Landé g-factor, extrinsic linewidth, and Gilbert relaxation rate are all found to decrease in magnitude with increasing sample growth temperature from 20oC to about 400–500oC and then on further increase of the growth temperature to increase in magnitude. Samples grown at about 400–450oC display the smallest coercivity, while the smallest value of the Gilbert relaxation rate of about 0.1 GHz is obtained for samples grown at 450–500oC. An almost linear relation between extrinsic linewidth and coercivity is observed, which suggests a positive correlation between magnetic inhomogeneity, coercivity and extrinsic linewidth. Another major discovery in this study is that the Gilbert relaxation decreases with increasing lattice constant, which is ascribed to the degree of structural order in the films. A micromagnetic model is established for an asymmetric trilayer system consisting of two different ferromagnetic (FM) layers separated by thin non-magnetic (NM) layer, treating the magnetization in each FM layer as a macrospin. Based on the model, numerical simulations of magnetization curves and FMR dispersion relations, of both the acoustic mode where magentizations in the two FM layers precess in phase and the optic mode where they precess out-of-phase, have been carried out. The most significant implication from the results is that the coupling strength can be extracted by detecting only the acoustic mode resonances at many different unsaturated magnetic states using broadband FMR technique. Finally, trilayer films of FeCo(100 Å)/NM/FeNi(100 Å) with NM=Ru or Cu were prepared and studied. The thickness of the Ru and Cu spacer was varied from 0 to 50 Å. For the Ru spacer series, the film with 10 Å Ru spacer shows antiferromagnetic coupling while all other films are ferromagnetically coupled. For the Cu spacer trilayers, it is found that all films are ferromagnetically coupled and that films with thin Cu spacer are surprisingly strongly coupled (the coupling constant is 3 erg/cm2 for the sample with 5 Å Cu spacer). The strong coupling strength is qualitatively understood within the framework of a combined effect of Ruderman-Kittel- Kasuya-Yosida interaction and pinhole coupling, which is evidenced by transmission electron microscopy analysis. The magnetic coupling constant decreases exponentially with increasing Cu spacer thickness, without showing an oscillatory thickness dependence. The results have implications for the design of multilayers for spintronic applications.

The field of Spintronics, which utilizes the spin polarization of electrons as a means to transport energy and information, is currently undergoing a massive expansion due to the numerous recent discoveries of electron spin-based effects only visible at the micro and nano scale. With all these new material and current based properties being discovered, it has become increasingly difficult to experimentally isolate the effects of each of them individually. In this work one of the main proposals is an experimental method for separating the voltage signals generated by the spin rectification effect and spin pumping in bilayer samples, a topic currently of much interest to the spintronics community. After demonstrating this new method for separating voltage signals, it is utilized to reveal new details about how the layers of ferromagnetic bilayer samples interact with each other and the behaviours of spin currents within these systems.

Master of Science / Department of Electrical and Computer Engineering / W. B. Kuhn / An analysis of frequency pulling in a varactor-tuned LC VCO under coupling from an on-chip PA is presented. The large-signal behavior of the VCO's inversion-mode MOS varactors is outlined, and the susceptibility of the VCO to frequency pulling from PA aggressor signals with various modulation schemes is discussed. We show that if the aggressor signal is aperiodic, band-limited, or amplitude-modulated, the varactor-tuned LC VCO will experience frequency pulling due to time-modulation of the varactor capacitance. However, if the aggressor signal has constant-envelope phase modulation, VCO pulling can be eliminated, even in the presence of coupling, through careful choice of VCO frequency and divider ratio. Additional mitigation strategies, including new inductor topologies and system-level architectural choices, are also examined. The analysis is then applied to improve a fully-integrated half-duplex UHF micro- transceiver in which signal coupling between the LO and PA caused frequency pulling that prevented the use of QPSK signaling at certain data rates. We determine that a VCO operating at 4x transmit frequency will be naturally insensitive to pulling from QPSK signals. To validate the proposed solution, a prototype IC containing a pair of QPSK transmitters with integrated 100mW Class-C PAs was designed and fabricated in 0.18um SOI. The transmitters--one utilizing a 2x VCO, one utilizing a 4x VCO-- were designed to closely match the performance of the original microtransceiver when transmitting QPSK data. The transmitter with the 2x VCO experienced frequency pulling from the PA while transmitting QPSK data, but the transmitter with the 4x VCO did not, thereby confirming the analysis in this work. A revision of the microtransceiver was designed in 0.5 [mu]m SOS utilizing an off- chip PA inductor to reduce signal coupling with the VCO. A second revision of the microtransceiver with two prototype transmitters was designed in 0.25 [mu]m SOS uti- lizing 4x VCOs and figure-8 VCO inductors for maximum insensitivity to pulling from QPSK and band-limited modulation, as well as other design improvements that leverage the higher f[subscript]t of the smaller process. Both revisions also include a hardware FSK modulator, a new charge pump, and a redesigned fractional-N synthesizer to attenuate a divided-reference spur in the IF output. These revisions of the radio will enable future researchers to focus on system-level applications where highly-integrated medium-power transceivers with fully-functioning IQ modulation are needed.

The SiC MOSFET has attracted interest due to its superior characteristics compared to its Si counterpart. Several SiC MOSFETs are usually paralleled to increase current capability, considering cost effectiveness and manufacturability. Current unbalance among the MOSFETs is a concern as it affects reliability. The two main causes are asymmetrical layout and parameter mismatch. The variation in parameters, unlike circuit or module layout, is unavoidable during production. Among all the parameters of MOSFET, the spreads in on-state resistance (Rds(on)) and threshold voltage (Vth) are the major concerns during paralleling. The disparity in Rds(on) causes static current unbalance which is self-limited due to the positive temperature coefficient of Rds(on). Its influence is not investigated here. The threshold voltage Vth has a negative temperature coefficient, forcing the MOSFET with lower Vth to carry more current during switching transient. Paralleled MOSFETs are usually de-rated to guarantee safe operation. Balancing of peak currents during switching transient isthe goal of this work. Integration of current/voltage sensors into paralleled structure is difficult in real application. Complicated feedback loop design and separate gate drivers also need to be avoided in perspective of cost and volume. Passive balancing solutions are investigated in this dissertation. The inductors and resistors most effective in improving current sharing are identified by parametric analysis. Their current balancing mechanisms are analyzed in circuit point of view. The design guidelines involving the magnitude of Vth mismatch, current rise time, and unbalance percentage are derived for the selection of passive components. The theory upholds well when substantial parasitics from device package and layout exist. Several passive balancing structures are analyzed and compared in terms of current balancing capability, voltage stress, total switching loss, and switching loss difference. All of them can provide much better current and power balancing without increasing switching loss. Some of the them may increase the stress-inducing inductance, which can be reduced by negative magnetic coupling. Perfect coupling between power-source inductors would enable current matching without penalty on voltage stress. Common-source inductance (Lcm) is effective in dynamic balancing, but at the expense of higher switching loss. It is not considered in power module application because Kelvin connection is normally applied. However, wire bond inside the package of discrete MOSFETs and part of the external leads are inevitable and add to Lcm. Peak-current and switching energy mismatches vary with operating conditions (including input voltage, input current, and switching speed). Design guidelines and procedures that are valid for wide operating range are provided for cases with and without Lcm. This dissertation also models the switching energy and switching energy mismatch of paralleled MOSFETs. The influence of operating conditions, passive balancing components, layout and package parasitic inductances, nonlinear channel performance, and voltage dependent parasitic capacitors are included in the modeling process. The resulting high order system is simplified by reducing the number of passive components and number of devices without losing accuracy. The influence of current balancing components and magnitude of threshold voltage mismatch on sharing are discussed based on modeling results. In conclusion, this dissertation balances the transient currents between paralleled SiC MOSFETs automatically by inductance, resistance and magnetic coupling. This procedure is done utilizing one gate driver without current/voltage sensors and feedback loop. Those solutions work for both polarities of Vth mismatch and force balancing from the first current peak. Design guidelines involving the magnitude of Vth mismatch, current rise time, and maximum peak-current difference are derived to guide the choice of passive components. The detail design procedures are recommended to force currents to share over wide operating range. The aforementioned benefits are demonstrated by two paralleled SiC MOSFETs (C2M0160120D) tested at variant operating conditions. The difference of peak currents can be reduced below 5% of steady-state current in every switching transient. Switching energy mismatch percentage can be reduced by 6 times without increasing total switching energy. / Ph. D.

Nesse trabalho é apresentada uma estratégia de controle para manter a potência entregue a carga do link indutivo de um sistema de transferência de potência sem fios maximizada continuamente. Inicialmente, foi desenvolvido o modelo do link indutivo do sistema, de forma a encontrar uma expressão para a potência na carga. Em seguida, essa expressão foi analisada, buscando os pontos de potência máxima correspondentes às frequências ressonantes quando o fator de acoplamento magnético do link indutivo era variado. Assim, foi estabelecido um método para estimar esse fator de acoplamento k e consequentemente da indutância mútua M a partir do monitoramento direto da fase e do módulo da corrente de entrada do link e indireto da impedância de entrada. Portanto, a estratégia de controle proposta consiste em monitorar a corrente da fonte de entrada do link para estimar o fator de acoplamento magnético k e finalmente, sintonizar a fonte na frequência de ressonância associada ao coeficiente de acoplamento magnético pela função obtida fres(k). Os resultados experimentais mostraram que a estratégia proposta aumenta significativamente a potência entregue a carga em comparação a uma fonte de alimentação sintonizada em uma frequência fixa. / A control strategy is presented in this work to maintain the power delivered to a load continuously maximized at the output of a wireless power transfer system. Initially, the inductive link model of the system was developed in order to find an expression for the power delivered to the load. Thus, the maximum power delivered to the load points were obtained experimentally while the magnetic coupling factor of the inductive link was varied and associated with the frequency of the source. The frequency and magnetic coupling variables were related to the maximum power on the load by a simple analytic function. Thus, a method has been established to estimate the coupling factor k and consequently the mutual inductance M by direct monitoring of the phase and the module of the input current of the link and indirect monitoring of the input impedance. Therefore, the proposed control strategy is to monitor the current of the input source of the link to estimate the magnetic coupling factor k and finally to tune the source at the resonance frequencies associated to the magnetic coupling coefficient by the obtained function fres(k). The experimental results showed that the proposed strategy increases significantly the power delivered to the load compared to a power source tuned at a fixed frequency.

La tesi estudia l'acoblament magnètic en sistemes de diferent dimensionalitat amb mètodes multireferencials. L’objectiu principal del treball és calcular propietats macroscòpiques, com la dependència de la susceptibilitat magnètica amb la temperatura, a partir de la constant d'intercanvi magnètic calculada, J. Aquest paràmetre microscòpic quantifica la interacció magnètica entre dos centres i es pot extreure per ajust de la corba de susceptibilitat experimental en sistemes finits però això no és possible en sistemes magnètics infinits com cadenes o capes 2D. L’estratègia del treball és calcular J en petits clusters i simular els sistemes estesos utilitzant aquesta J en l’Hamiltonià de Heisenberg en models de 8 a 16 centres. Amb l’espectre obtingut es construeixen les corbes de dependència tèrmica de la susceptibilitat magnètica que, comparades amb les experimentals, donen la possibilitat de quantificar les interaccions magnètiques dels materials estudiats a nivell microscòpic. S'han estudiat diferents tipus de sistemes estesos com cadenes i xarxes hexagonals, on els centres magnètics són ions de metalls de transició. / The thesis studies the magnetic coupling in systems of different dimensionality, by using multireference methods. The aim of the work is to determine macroscopic properties such as the thermal dependency of magnetic susceptibility, from the calculated magnetic exchange constant J. This microscopic parameter quantifies the magnetic interaction between two magnetic sites and can be extracted from the experimental susceptibility curve in finite systems. However this extraction is not possible in extended magnetic systems such as chains or 2D-layers. The strategy followed consists in calculating J in small clusters and in simulating the extended systems by introducing the calculated J in the Heisenberg Hamiltonian of 8 to 16 site models. From the spectrum, the thermal dependency of the magnetic susceptibility is the calculated. When compared to the experimental one, this curve gives a quantification of the magnetic interactions of the studied materials at the microscopic level. We have studied different types of extended systems such as chains and hexagonal lattices, where the magnetic sites are transition metal ions.

A series of 1,2,3-dithiazolyl radicals have been prepared as spin-bearing radical ligands for coordination to paramagnetic transition metal and lanthanide ions to afford both interesting magnetic and potential conductive properties. The novel syntheses and full characterization of the ligands are described with the electronic properties investigated by both solution & solid-state EPR and cyclic voltammetry. The ligand structures are characterized by X-ray crystallography and the magnetic properties investigated. The ligands described herein represent the first reported 1,2,3-dithiazolyl radical ligands to coordinate to paramagnetic transition metal or lanthanide ion(s). The 1,2,3-dithiazoyl ligands described have been used to prepare a number of metal complexes and the structures have been fully characterized by X-ray crystallography. A diverse range of complexes including monomers, trimers and polymers have been synthesized and characterized. The solid-state and solution magnetic properties of the metal complexes have been investigated using a range of appropriate instrumental techniques, and are reported. The chelating properties of the 1,2,3-dithiazolyl ligands described are via an unprecedented motif. A para-naphthoquinone backbone is utilized with the fused 1,2,3dithiazolyl heterocyclic ring. The coordination geometry between the nitrogen atom and the quinone oxygen(s) allows for the variety of complexes reported, including the design objective of a polymeric paramagnetic coordination complex. A series of 1,2,5-thiadiazolyl radical anions have been prepared as spin-bearing radical ligands for the coordination to paramagnetic transition metal and lanthanide ions to enable both interesting magnetic and conductive properties. The syntheses and full characterizations are described with the electronic properties investigated by solution EPR and cyclic voltammetry. The ligand structures are characterized by X-ray crystallography and both the magnetic properties and conductivity (including Extended Hückel Theory calculations) are investigated and reported. A series of novel fluorinated 1,2,5-thiadiazole closed shell neutral ligands have been prepared and characterized. The radical anion is studied in situ by EPR. All materials generated in the synthesis were sent to the National Cancer Institute (USA) for collaboration involving clinical cancer inhibition studies and the results are reported.

Nesse trabalho é apresentada uma estratégia de controle para manter a potência entregue a carga do link indutivo de um sistema de transferência de potência sem fios maximizada continuamente. Inicialmente, foi desenvolvido o modelo do link indutivo do sistema, de forma a encontrar uma expressão para a potência na carga. Em seguida, essa expressão foi analisada, buscando os pontos de potência máxima correspondentes às frequências ressonantes quando o fator de acoplamento magnético do link indutivo era variado. Assim, foi estabelecido um método para estimar esse fator de acoplamento k e consequentemente da indutância mútua M a partir do monitoramento direto da fase e do módulo da corrente de entrada do link e indireto da impedância de entrada. Portanto, a estratégia de controle proposta consiste em monitorar a corrente da fonte de entrada do link para estimar o fator de acoplamento magnético k e finalmente, sintonizar a fonte na frequência de ressonância associada ao coeficiente de acoplamento magnético pela função obtida fres(k). Os resultados experimentais mostraram que a estratégia proposta aumenta significativamente a potência entregue a carga em comparação a uma fonte de alimentação sintonizada em uma frequência fixa. / A control strategy is presented in this work to maintain the power delivered to a load continuously maximized at the output of a wireless power transfer system. Initially, the inductive link model of the system was developed in order to find an expression for the power delivered to the load. Thus, the maximum power delivered to the load points were obtained experimentally while the magnetic coupling factor of the inductive link was varied and associated with the frequency of the source. The frequency and magnetic coupling variables were related to the maximum power on the load by a simple analytic function. Thus, a method has been established to estimate the coupling factor k and consequently the mutual inductance M by direct monitoring of the phase and the module of the input current of the link and indirect monitoring of the input impedance. Therefore, the proposed control strategy is to monitor the current of the input source of the link to estimate the magnetic coupling factor k and finally to tune the source at the resonance frequencies associated to the magnetic coupling coefficient by the obtained function fres(k). The experimental results showed that the proposed strategy increases significantly the power delivered to the load compared to a power source tuned at a fixed frequency.

Submitted by Emanuel Varela Cardoso (emanuel.varela@ufcg.edu.br) on 2018-08-28T20:15:07Z No. of bitstreams: 1 DIEGO ALBERTO ACEVEDO BUENO – DISSERTAÇÃO (PPGEEI) 2015.pdf: 8647076 bytes, checksum: c16b323a227c1c3224d8f023324db513 (MD5) / Made available in DSpace on 2018-08-28T20:15:07Z (GMT). No. of bitstreams: 1 DIEGO ALBERTO ACEVEDO BUENO – DISSERTAÇÃO (PPGEEI) 2015.pdf: 8647076 bytes, checksum: c16b323a227c1c3224d8f023324db513 (MD5) Previous issue date: 2015-08 / CNPq / Esta dissertação examina seis topologias hibridas assimétricas com acoplamento magnético. O termo topologia hibrida assimétrica se refere a qualquer topologia que combine os princípios das topologias clássicas com duas fontes de tensão isoladas ou mais com tensões diferentes. Das topologias propostas, quatro possuem saída única conectada ao ponto intermediário do barramento e as restantes dispõem de saída diferencial usando outro braço de dois níveis. Para simplificar o estudo destas topologias, desenvolveu-se um modelo elétrico e magnético detalhado dos indutores acoplados usando componentes de modo comum e diferencial. Posteriormente, esse modelo foi inserido no modelo do conversor para o desenvolvimento de uma estratégia de controle da corrente de saída e da corrente de modo diferencial garantindo a operação em modo de condução continua com número reduzido de componentes. Os modos de operação e as respectivas estratégias de modulação são apresentados. Além disso, o efeito da diferença das tensões de polo máximas sobre o desempenho dos conversores e na escolha do indutor acoplado foi analisado. Para avaliar o desempenho destas topologias, foram realizadas simulações com os modelos completos das chaves e dos diodos usando o software PSIM fixando a potência de saída para todas as topologias em 1 kVA. Também, alguns resultados experimentais em malha aberta são apresentados para validar os modelos desenvolvidos / This paper examines six asymmetric hybrid topologies with magnetic coupling. The term “asymmetric hybrid topology” refers to any topology that combines the principles of classical topologies with two isolated voltage sources or more with different voltages. From these topologies, four have single-ended output connected to the midpoint of dc-link and the remaining has differential output employing another two-level arm. To simplify the analysis, a detailed electric and magnetic model of the coupled inductors was developed appealing to common- and differential-mode components. Subsequently, this model was inserted into the model of the converter for developing a control strategy for the output and the differential mode currents, ensuring the operation in continuous conduction mode with a reduced number of components. The modes of operation and the modulation strategies are presented. In addition, the difference of the maximum pole voltages was analyzed for determining its effect on the converter performance and the coupled inductor selection. To evaluate the performance of these topologies, simulations were performed with the complete models of switches and diodes using the PSIM software by setting the output power at 1 kVA for all topologies. Also, experimental open loop results are presented to validate the developed models.