Understanding magnetization reversal is very important in designing high density and high data transfer rate recording media. This research has been motivated by interest in developing new nonvolatile data storage solutions as magnetic random access memories - MRAMs. This dissertation is intended to provide a theoretical analysis of static and dynamic magnetization switching of magnetic systems within the framework of critical curve (CC). Based on the time scale involved, a quasi-static or dynamic CC approach is used. The static magnetization switching can be elegantly described using the concept of critical curves. The critical curves of simple uncoupled films used in MRAM are discussed. We propose a new sensitive method for CC determination of 2D magnetic systems. This method is validated experimentally by measuring experimental critical curves of a series of Co/SiO2 multilayers systems. The dynamics switching is studied using the Landau-Lifshitz-Gilbert (LLG) equation of motion. The switching diagram so-called dynamic critical curve of Stonerlike particles subject to short magnetic field pulses is presented, giving useful information for optimizing field pulse parameters in order to make ultrafast and stable switching possible. For the first time, the dynamic critical curves (dCCs) for synthetic antiferromagnet (SAF) structures are introduced in this work. Comparing with CC, which are currently used for studying the switching in toggle MRAM, dCCs show the consistent switching and bring more useful information on the speed of magnetization reversal. Based on dCCs, better understanding of the switching diagram of toggle MRAM following toggle writing scheme can be achieved. The dynamic switching triggered by spin torque transfer in spin-torque MRAM cell has been also derived in this dissertation. We have studied the magnetization's dynamics properties as a function of applied current pulse amplitude, shape, and also as a function of the Gilbert damping constant. The great important result has been obtained is that, the boundary between switching/non-switching regions is not smooth but having a seashell spiral fringes. The influence of thermal fluctuation on the switching behavior is also discussed in this work.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2018-08-22T15:35:41Z No. of bitstreams: 1 evertonluizmartinsdapaixao.pdf: 15592539 bytes, checksum: 3e2c3d43b62b9fa0edea517213c63a12 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2018-09-03T16:33:20Z (GMT) No. of bitstreams: 1 evertonluizmartinsdapaixao.pdf: 15592539 bytes, checksum: 3e2c3d43b62b9fa0edea517213c63a12 (MD5) / Made available in DSpace on 2018-09-03T16:33:20Z (GMT). No. of bitstreams: 1 evertonluizmartinsdapaixao.pdf: 15592539 bytes, checksum: 3e2c3d43b62b9fa0edea517213c63a12 (MD5) Previous issue date: 2018-02-26 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Entender e controlar o movimento de parede de domínio em nanofios é extremamente im-portante para o desenvolvimento de novas tecnologias para a aplicação em dispositivos de ar-mazenamento de dados. É conhecido que defeitos como entalhes ("notches") em nanofios são úteis para fixar paredes de domínio. No entanto, a intensidade de potencial de aprisionamento gerado com esse tipo de defeito é muito forte, e para desprender a parede de domínio é ne-cessário aplicar uma densidade de corrente muito elevada. Entretanto, pode-se criar armadilhas para paredes de domínios variando localmente propriedades magnéticas do nanofio, tais como: tais como constante de troca, magnetização de saturação, constante de anisotropia, parâmetro de amortecimento de Gilbert. Definimos essas regiões como impurezas magnéticas por ter propriedades magnéticas diferentes do nanofio. Neste trabalho, realizamos simulações micro-magnéticas para investigar a dinâmica de uma parede de domínio transversal (PDT) aprisionada em um defeito magnético usando pulsos de corrente elétrica polarizada em spin. Afim de criar armadilhas de aprisionamento para a PDT, consideramos um modelo de impureza magnética variando localmente a constante de troca. Ao ajustar o potencial de interação entre impure-zas magnéticas e uma PDT, verificamos que pulsos de corrente de baixa intensidade e de curta duração são capazes de desprender a PDT. Por fim, demonstramos que é possível controlar a posição de uma PDT aplicando pulsos de corrente sequenciais em uma nanofita contendo uma distribuição linear de impurezas magnéticas igualmente espaçadas. / Understand and control the domain wall movement in nanowires is extremely important for the development of new technologies for an application in data storage devices. It is known that defect as notches in nanowires are useful to pinning domain walls. Nevertheless, the pinning potential intensity generated by this type of defect is strong, and for depinning the domain wall it is necessary to apply a high current density. However, it is possible to create traps for domains walls by locally varying magnetic properties of the nanowire, such as: the exchange constant, saturation magnetization, anisotropy constant, Gilbert damping parameters. We define those regions as magnetic impurities once their magnetic properties differ from the nanowire proper-ties. In this study, we realized micromagnetic simulations in order to investigate the dynamics of a transverse domain wall (TDW) trapped in a magnetic defect using electric current pulses of spin-polarized. In order to create traps to TDW pinning, we have modeled the magnetic impurities by varying the exchange constant locally. When we adjusted the interaction poten-tial between the magnetic impurities and the nanowire we showed that low intensity and short duration current pulses are capable of depinning the TDW. At last, we demonstrated that it is possible to control the TDW position applying sequential current pulses in a nanowire planar containing a linear distribution of magnetic impurities equally distributed.