Doctora en Ciencias, Mención Física / El objetivo general de este trabajo es estudiar la dinámica de la magnetización, tanto en el régimen lineal como no lineal, de la capa ferromagnética libre de una estructura nanopilar, cuando se inyecta una corriente polarizada en spin. Para ello, se desarrolla un modelo que utiliza un método alternativo a las simulaciones micromagnéticas usuales, siguiendo un formalismo Hamiltoniano estándar. La capa libre corresponde a un disco de un material ferromagnético suave (específicamente permalloy), de sección circular, y se aplica un campo magnético paralelo al plano que determina una magnetización de equilibrio cuasi-uniforme en su misma dirección. Si la corriente continua supera un cierto umbral, se excitan las ondas de spin y es posible observar auto-oscilaciones de la magnetización. En particular, se estudia la estabilidad de la auto-oscilación del modo que es excitado a la menor corriente crítica, considerando dos versiones del modelo: una que incluye los efectos del borde del disco, y otra que no. Además, se estudia la dinámica lineal de las ondas de spin bajo corrientes alternas. Esta tesis se organiza de la siguiente manera. En la introducción se exponen de manera resumida la motivación, los objetivos y la metodología a utilizar. El capítulo 1, que corresponde al marco teórico, introduce los conceptos básicos para comprender el problema en cuestión. En el capítulo 2 se presenta en detalle el modelo, basado en un formalismo Hamiltoniano, el cual se utiliza para estudiar la dinámica de la magnetización. En el capítulo 3 se estudia la estabilidad de la auto-oscilación del modo con corriente crítica más baja, que corresponde al modo uniforme o macro-spin. Se considera que la capa libre es muy delgada, lo cual permite aproximar el campo demagnetizante al de un plano infinito, y se determina la corriente continua crítica por sobre la cual es posible observar la auto-oscilación del modo uniforme. Luego, se determinan los modos normales de oscilación de la magnetización no uniformes y se estudia la interacción entre la auto-oscilación del modo uniforme con los modos no uniformes. Se determina cuándo comienza el crecimiento exponencial de los modos no uniformes, lo cual permite conocer el rango del espacio de parámetros donde sólo existe la oscilación del modo macro-spin de manera aislada. En el capítulo 4 se introducen los efectos que tienen los bordes del disco, considerando el campo demagnetizante completo. Primero se determina la magnetización de equilibrio que ya no es uniforme. Luego se estudian los modos normales de oscilación de un disco ferromagnético con un campo aplicado en el plano, determinándose sus formas y frecuencias. En el capítulo 5 se estudia la excitación de las ondas de spin del capítulo anterior, mediante la inyección de una corriente continua, y se determina la corriente crítica necesaria para observar auto-oscilaciones de la magnetización. Luego, se determina la región del espacio de parámetros en la cual es posible observar un sólo modo auto-oscilando. En el capítulo 6, se considera que la corriente continua es menor a la corriente crítica y se le agrega una componente alterna. Se estudia la excitación de modos normales bajo este forzamiento: se consideran los efectos del campo de Oersted y la componente perpendicular del torque por transferencia de spin, y se estudian resonancias directas y paramétricas. Finalmente se presenta una sección de conclusiones. / Este trabajo ha sido parcialmente financiado por Becas Conicyt 2012 folio núm. 21120160, CEDENNA y FONDECYT N° 1130192

Orientador: José Geraldo T. Brandão / Coorientador: Jose Cláudio Caraschi / Banca: José Elias Tomazini / Banca: Renato José Soares / Resumo: O constante aumento no número de praticantes de atividade física, muitos deles sem a devida orientação, vem fazendo com que a incidência de lesões relacionadas ao esporte também cresça. Exercícios físicos realizados com sobrecarga e sem a periodização correta podem acarretar desequilíbrios musculares importantes, modificando a biomecânica normal do praticante, o que acaba impondo ao corpo cargas excessivas e muitas vezes lesivas. Acompanhando essa evolução, a medicina esportiva juntamente com a engenharia biomédica tem avançado muito no que diz respeito à tecnologia para avaliar e quantificar inúmeras variáveis do desempenho humano. As avaliações musculares estão certamente entre as mais comuns e mais utilizadas pelos profissionais da área da reabilitação esportiva. Porém o dinamômetro isocinético que é o dispositivo mais indicado para tais avaliações é um equipamento de difícil acesso, tanto pelo alto custo quanto pela dificuldade de portabilidade. Com isso há necessidade da busca por dispositivos com boa confiabilidade, com preço acessível e que possam gerar dados quantitativos a respeito do desempenho muscular. O objetivo desse trabalho foi desenvolver um dinamômetro isométrico digital da articulação do joelho adaptando uma célula decarga de tração e compressão a uma cadeira extensora utilizada em academias. Após o desenvolvimento do dinamômetro foram realizadas avaliações musculares em 20 voluntários do sexo masculino, com idade entre 18 e 19, sem história prévia de lesões nos membros inferiores. Com os dados obtidos foi possível avaliar o equilíbrio muscular entre os músculos extensores e flexores do joelho dos voluntários, cumprindo assim o objetivo inicial proposto na idealização desse projeto / Abstract: The constant increase in the number of physically active, many of them without proper guidance, is causing the number of sports-related injuries also grow. Exercises performed with overload and without the correct periodization can cause significant muscle imbalances, modifying the normal biomechanics of the practitioner, which ends up imposing excessive loads on the body and often detrimental. Following this trend, the sports medicine with biomedical engineering has advanced greatly in regard to technology toassess and quantify several variables of human performance. Muscle assessments are certainly among the most common and most used by professionals in the sports rehabilitation. But the isokinetic dynamometer that is most indicated device for those evaluations is a difficult equipment to access, both the high cost and the difficulty of portability. Thus there is need to search for devices with good reliability, with affordable price and that can generate quantitative data regarding the muscle performance. The objective of this study was todevelop a digital isometric dynamometer for the knee joint adapting a tension and compression load cell to a knee extension chair used in gyms. After the development of the dynamometer 20 healthy male volunteers, aged between 18 and 19, with no history of lower limb injuries, had muscle evaluated.With the data obtained it was possible to evaluate the muscle balance between the extensor and flexor muscles of vonlunteers knees, thus fulfilling the initial proposed objective of this project / Mestre

Magnetic random­access memory (MRAM) devices have been widely studied since the 1960s. During this time, the size of spintronic devices has continued to decrease. Conse quently, there is now an urgent need for new low­dimensional magnetic materials to mimic the traditional structures of spintronics at the nanoscale. We also require new effective mechanisms to conduct the main functions of memory devices, which are: reading, writ ing, and storing data. To date, most research efforts have focused on MRAM devices based on magnetic tun nel junction (MTJ), such as a conventional field­driven MRAM and spin­transfer torque (STT)­MRAM devices. Consequently, many efforts are currently focusing on new alterna tives using different techniques, such as spin­orbit torque (SOT) and magnetic skyrmions (a skyrmion is the smallest potential disruption to a uniform magnet required to obtain more effective memory devices). The most promising memory devices are SOT­MRAMs and skyrmion­based memories. This study investigates the magnetic properties of 1T­phase vanadium dichalcogenide (VXY) Janus monolayers, where X, Y= S, Se, or Te (i.e., monolayers that exhibit inversion symme try breaking due to the presence of different chalcogen elements). This study is developed along four directions: (I) the nature of the magnetism and the SOT effect of Janus mono layers; (II) the Dzyaloshinskii Moriya interaction (DMI); (III) investigation of stability en hancement by adopting practical procedures for industry; and (IV) study of the effect of a hexagonal boron nitride (h­BN) monolayer as an insulator on the magnetism of the VXY monolayer. This study provides a clear perspective for the next generation of memory de vices, such as SOT­MRAMs based on transition metal dichalcogenide monolayers.

This thesis deals with the design of the magnetorheological (MR) clutch. The extensive literature research with particular conclusions to each design variant of MR clutches or brakes has been done in the first part of the thesis. The second part of the thesis describes the design of the actual MR clutch based on the results of the analytical calculation of the magnetic circuit. Finally, the experiments verifying the function of the key segments of the clutch, and the whole clutch are summarized in the third part of the thesis.

This diploma thesis deals with the problematic of measuring motorcycle performance output using dynamometers primary designed for measuring performance output of four wheel personal vehicles, proposals and comparison of possible designs of device that enable this method of motorcycle performance output measuring by using FEA methods.

abstract: As the world population continues to age, the demand for treatment and rehabilitation of long-term age-related ailments will rise. Healthcare technology must keep up with this demand, and existing solutions must become more readily available to the populace. Conditions such as impairment due to stroke currently take months or years of physical therapy to overcome, but rehabilitative exoskeletons can be used to greatly extend a physical therapist’s capabilities. In this thesis, a rehabilitative knee exoskeleton was designed which is significantly lighter, more portable and less costly to manufacture than existing designs. It accomplishes this performance by making use of high-powered and weight-optimized brushless DC (BLDC) electric motors designed for drones, open-source hardware and software solutions for robotic motion control, and rapid prototyping technologies such as 3D printing and laser cutting. The exoskeleton is made from a series of laser cut aluminum plates spaced apart with off-the-shelf standoffs. A drone motor with a torque of 1.32 Nm powers an 18.5:1 reduction two-stage belt drive, giving a maximum torque of 24.4 Nm at the output. The bearings for the belt drive are installed into 3D printed bearing mounts, which act as a snug intermediary between the bearing and the aluminum plate. The system is powered off a 24 volt, 1,500 MAh lithium battery, which can provide power for around an hour of walking activity. The exoskeleton is controlled with an ODrive motor controller connected to a Raspberry Pi. Hip angle data is provided by an IMU, and the knee angle is provided by an encoder on the output shaft. A compact Rotary Series Elastic Actuator (cRSEA) device is mounted on the output shaft as well, to accurately measure the output torque going to the wearer. A Proportional-Derivative (PD) controller with feedforward relates the input current with the output torque. The device was tested on a treadmill and found to have an average backdrive torque of 0.39 Nm, significantly lower than the current state of the art. A gravity compensation controller and impedance controller were implemented to assist during swing and stance phases respectively. The results were compared to the muscular exertion of the knee measured via Electromyography (EMG). / Dissertation/Thesis / Masters Thesis Engineering 2020

In the past two decades, the interest in materials with strong spin-orbit coupling has attracted substantial attention because of the novel physical mechanisms they display and their potential for applications. The interface displaying large spin-orbit coupling has been recognized as a powerful platform to investigate the spin transport in ferromagnetic, antiferromagnetic, and non-magnetic materials, as well as their interfaces. Besides its rich physics, the related applications are also worth studying. The current-induced spin-orbit-torque arising from angular momentum transfer from the lattice to the spin system has substantial potential in recent state-of-art spin-orbit torque magnetic random access memory. In this dissertation, we have been interested in better understanding and characterizing the spin-orbit torque and spin Hall transport in various heterostructures of interest. We used the second harmonic method to determine the magnitude of the spin currents generation and transmission in Cu-Au alloy and Ir-Mn compound, respectively. We also characterized the device performance in selected heterostructures displaying either perpendicular MgO-based tunnel magnetoresistance or unusual surface states. Finally, we used these properties to approach spin-orbit torque magnetic random access memory through designing, fabricating, and characterizing the devices that focused on current-induced spin-orbit-torque magnetization switching.

One of the most remarkable achievements of modern condensed matter physics is the discovery of topological phases of matter. Materials in a non-trivial topological phase or the topological insulators can be distinguished by their unique electronic and transport properties which are indifferent to different types of perturbations and thus open new routes towards the dissipationless transport. Explaining their properties requires proper involvement of relativistic approach as well as topological analysis. Among different classes of topological insulators, the Z2 topological insulators have drawn special attention due to their strong spin-orbit coupling which makes them a promising candidate for spintronics application, especially for magnetic memory devices. Due to their inherent strong spin-orbit coupling, they provide an efficient way to manipulate electronic spin with an applied electric field via spin orbit torque. The topological insulators have been found to be far more superior in manipulating the magnetic order parameter of a ferromagnet compared to the conventional heavy metals like platinum or tantalum. Another milestone in magnetic memory devices is marked by the introduction of antiferromagnetic memory devices which has not drawn any attention for long time as they cannot be controlled by an applied magnetic field. Recently it has been found that in case of a non-centrosymmetric antiferromagnet, the magnetic order parameter can be manipulated by with spin-orbit torque which also have been verified experimentally. The advantages of antiferromagnetic devices over ferromagnetic devices are that they allow faster switching speed and they are immune to an external magneticfield which are two highly solicited properties for next generation spintronic devices. This thesis is focused on understanding the transport properties in topologically nontrivial materials and their interface with different magnetic material. We use simplified continuum model as well as tight binding models to capture the salient features of these systems. Using non-equilibrium Green's function we explore their transport properties as well as spin-charge conversion mechanism. Our finding would provide a better understanding of these new class of materials and thus would be instrumental to discover new mechanisms to manipulate their properties.

The purposes of these studies were to determine if an acute static stretch influenced isokinetic peak torque (IPT), and to examine if the order in which the warm up routine was performed affected peak knee extension torque. Twenty trained college male students performed maximal isokinetic knee extensions under four conditions: a control consisting of no stretching, a stretch only trial, jog then stretch, and stretch then jog conditions. Each stretch was held for a total volume of 360 s. Measurements were taken on a Biodex System 3 isokinetic dynamometer at speeds of 60º s-1 and 300º s-1. Data were analyzed using t-tests to compare the stretch condition with the control. The results indicated that there was a significant difference between the stretch and the control at 300 º s-1 (p = 0.03 t = 2.42) but not at 60 º s-1 (p = 0.16). A 2 x 3 ANOVA (300 º s-1 x 60 º s-1, and control x stretch then jog x jog then stretch) yielded no significance at either speed (p > 0.05). Conclusions from this study indicate that stretching should not be the sole exercise in a warm-up routine as previous research confirms the decrease in IPT after stretching. Another finding of this study is that the negative effects of stretching can be diminished when combined with an aerobic activity such as jogging prior to performance. Further research is needed to determine the underlying factors that contribute to the post stretch decrease in IPT and the factors that lead to the restoration of force after aerobic activity. Caution is advised since these were controlled tests in a laboratory and results may vary with actual performance.

A great deal of research has shown decrements in force and power following static stretching. There has been little research investigating the acute effects of static stretching of the antagonist on the expression of strength and power. The purpose of this study was to investigate the effects of static stretching of the antagonist muscles on a variety of strength and power measures. Sixteen active males were tested for vertical jump height and isokinetic torque production in a slow knee extension (KES) at 60°/s and a fast knee extension (KEF) at 300°/s. Electromyography was taken during knee extension tests for the vastus lateralis and the biceps femoris muscles. Participants performed these tests in a randomized counterbalanced order with and without prior antagonist stretching. All variables for stretching and non-stretching treatments were compared using paired t tests at an alpha of .05. Paired samples t tests revealed a significant (p = .034) difference between stretch KEF and non-stretch KEF conditions. There was no significant (p > .05) difference between KES stretch and non-stretch conditions. Vertical jump height was significantly (p = .011) higher for the stretching treatment than the non-stretching treatment. Vertical jump power was also significantly higher (p = .005) in the stretch versus the non-stretch condition. Paired samples t test indicated no significant (p > .05) difference between testing conditions for electromyography, represented as a percentage of maximal voluntary contraction (MVC). These results suggest that stretching the antagonist hamstrings prior to high speed isokinetic knee extension increases torque production. It also demonstrated that stretching the hip flexors and dorsi flexors may enhance jump height and power. Practitioners may use this information to acutely enhance strength and power performances.