A study of magnetization dynamics experiments in nanostructured materials using the rf susceptibility tunnel diode oscillator (TDO) method is presented along with a extensive theoretical analysis. An original, computer controlled experimental setup that measures the change in susceptibility with the variation in external magnetic field and sample temperature was constructed. The TDO-based experiment design and construction is explained in detail, showing all the elements of originality. This experimental technique has proven reliable for characterizing samples with uncoupled magnetic structure and various magnetic anisotropies like: CrO2 , FeCo/IrMn and Co/SiO2 thin films. The TDO was subsequently used to explore the magnetization switching in coupled magnetic systems, like synthetic antiferromagnet (SAF) structures. Magnetoresistive random access memory (MRAM) is an important example of devices where the use of SAF structure is essential. To support the understanding of the SAF magnetic behavior, its configuration and application are reviewed and more details are provided in an appendix. Current problems in increasing the scalability and decreasing the error rate of MRAM devices are closely connected to the switching properties of the SAF structures. Several theoretical studies that were devoted to the understanding of the concepts of SAF critical curve are reviewed. As one can notice, there was no experimental determination of SAF critical curve, due to the difficulties in characterizing a magnetic coupled structure. Depending of the coupling strength between the two ferromagnetic layers, on the SAF critical curve one distinguishes several new features, inexistent in the case of uncoupled systems. Knowing the configuration of the SAF critical curve is of great importance in order to control its switching characteristics. For the first time a method of experimentally recording the critical curve for SAF is proposed in this work. In order to overcome technological limitations, a new way of recording the critical curve by using an additional magnetic bias field was explored.

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.

Static techniques to measure different magnetic properties of coupled magnetic nanostructured systems is researched and documented with an extensive analysis of the tunnel diode oscillator (TDO). The VSM was used to obtain the major hysteresis loop for the samples and the TDO was used to measure the magnetic susceptibility. The magnetic susceptibility was employed to conceive the static critical curve. The thesis describes both equipments, VSM and TDO, that were used to obtain data for our experiments. Albeit a more comprehensive outlook on the TDO is provided. The theoretical functionality of TDO, previous successful applications for experiments, and the physical setup in the laboratory is explored. The novel addition of the double Helmholtz coil in this setup is described. The viability of replacement of the big electromagnet and the advantages of the Helmholtz coil are discussed. Magnetization dynamics in a series of FeCoB/Ru/FeCoB synthetic antiferromagnetic samples were investigated via reversible susceptibility measurements acquired through the TDO. The major hysteresis loop generated by the VSM were used to calculate the coercivity and magnetic saturation of the sample. The VSM and TDO were subsequently used to explore the magnetization switching in a di_erent coupled magnetic system, the exchange bias samples. A range of NiFe/FeMn samples were studied with varying thickness of the antiferromagnetic layer.

A study is presented detailing experimental investigations of magnetization dynamics in nanostructured systems which are coupled magnetically. This work seeks to characterize the anisotropy of such systems through experimental techniques which probe microwave resonant absorption in the materials. A custom-built experimental setup, designed and assembled in our labs, is explained in detail. This setup allows for angular-dependent ferromagnetic resonance (FMR) measurements in the sample plane through vector network analyzer spectroscopy and is adaptable to two different types of coplanar waveguides. This technique has proven effective for characterization of multiple types of magnetic systems, including multilayered structures as detailed here, with different types of anisotropies while allowing us to draw analogies with more common characterization techniques. The angular FMR setup has been used to study coupled systems, such as those coupled through the Ruderman–Kittel–Kasuya–Yosida interaction as well as exchange-biased structures. These types of coupled systems have technological impacts and are highly applied in the components of magnetoresistive random access memory. Using this new characterization technique, properties of synthetic antiferromagnets have been revealed which had not been observed before. In addition to these experiments, magnetic susceptibility and FMR in exchange biased systems have been investigated at temperatures as low as 2 K. This investigation used a new FMR spectrometer and was one of the first studies to use this instrument. For the first time a new method of identifying several types of coupling which can be present in layered nanostructures is presented and supported through comparison with known techniques, thus connecting a new characterization technique for layered structures with decades-old procedures. Many results within this work are also supported theoretically with computer simulations.

Cette thèse est consacrée à l’étude du modèle d’accrochage en environnementfaiblement corrélé. Le modèle d’accrochage s’applique à de multiples situationstelles que la localisation d’un polymère au voisinage d’une interface unidimensionnelle,la transition de mouillage ou encore la dénaturation de l’ADN, le pointcommun étant la présence d’une transition entre une phase localisée et une phasedélocalisée.Nous commençons par donner un aperçu des résultats disponibles sur lescourbes et exposants critiques pour le modèle homogène puis pour le modèledésordonné lorsque le désordre est une suite de variables aléatoires indépendanteset identiquement distribuées (i.i.d.). Dans ce dernier cas, nous donnons égalementune borne sur la courbe critique quenched à haute température, dans un régimeoù le désordre est dit pertinent.Nous étudions ensuite le modèle d’accrochage désordonné dans le cas où ledésordre est gaussien et les corrélations ont une portée finie, à l’aide de la théoriedes processus de renouvellement markoviens. Nous donnons dans ce cas une expressionde la courbe annealed à l’aide de la plus grande valeur propre d’une matricede transfert ainsi que l’exposant critique annealed. Nous généralisons ensuite lescritères de pertinence et de non pertinence du désordre prouvés dans le cas i.i.d.Nous nous intéressons ensuite à des désordres dont les corrélations ont uneportée de corrélation infinie. Dans un premier temps, nous généralisons la démarcheutilisée dans le cas d’une portée de corrélations finie et obtenons le comportementcritique annealed dans le cas d’un désordre gaussien sous des hypothèses dedécroissance forte des corrélations. Nous utilisons pour cela les propriétés spectralesdes opérateurs de transfert pour des décalages sur des suites d’entiers etdes potentiels à variations sommables. Dans un deuxième temps, nous donnonsquelques résultats dans le cas où le désordre est donné par une chaîne de Markov. / In this dissertation we study the pinning model with weakly correlated disorder.The pinning model applies to various situations such as localization of a polymernear a one-dimensional interface, wetting transition and DNA denaturation, whichall display a transition between a localized phase and a delocalized phase.We start by giving a survey of the available results concerning critical pointsand exponents, first for the homogeneous setup and then for the inhomogeneousone, in the case when disorder is given by a sequence of independent and identicallydistributed (i.i.d.) random variables. In the latter case, we also provide a hightemperaturebound on the quenched critical curve in a case of relevant disorder.We then study the random pinning model when disorder is gaussian and hascorrelations with finite range, using the theory of Markov renewal processes. Weexpress the annealed critical curve in terms of the largest eigenvalue of a transfermatrix and we give the annealed critical exponent. We then generalize the criteriafor disorder relevance/irrelevance that were proved for the i.i.d. case.Next we are interested in disorder sequences with infinite range correlations.At first we generalize the method used to deal with finite range correlations andobtain the annealed critical behaviour in the case of gaussian disorder assumingfast decay of correlations. We use to this end the spectral properties of transferoperators for shifts on integer sequences and potentials with summable variations.Secondly we provide some results when disorder is a Markov chain.