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Ces travaux de thèse rentrent dans le cadre de l’étude de la dynamique ultra rapide de l’aimantation. Tout d’abord sont présentés des aspects théoriques, puis les aspects expérimentaux de ces expériences. Pour ce faire, nous avons étudié la réponse d’alliages ferrimagnétiques à composition variables à l’aide d’un dispositif de mesure d’effet Kerr résolu en temps, puis dans une seconde partie, la dynamique de l’aimantation et de la bande de valence du gadolinium épitaxié sur tungstène. Dans ce cadre rentre une étude de l’oxydation de ce matériau, limitant dans le temps les études approfondies. Pour finir, il est mis l’accent sur un phénomène contraignant lors des études de dynamique électronique en photoémission, l’effet de charge-espace. Ceci a pour effet de générer des photoélectrons à partir de métaux, à l’aide d’un processus multiphotonique. Nous proposons dans cette partie un modèle théorique expliquant ce phénomène.Ces travaux sont inscrits dans le cadre du développement du synchrotron SOLEIL, pour permettre le développement du FEMTOSLICING, qui permettra prochainement de mesurer des dynamiques rapides résolues en éléments, à une résolution de l’ordre de la centaine de femtosecondes. / Those thesis works are included in the framework of the study of ultrafast magnetization dynamics. First of all I introduce theoretical aspects, then experimental aspects of this kind of experiments.In this aim, we have studied the answer of ferromagnetic alloys of different compositions with a bench of time resolved magneto optical Kerr effect measurement, then in a second part, the magnetization and valence band dynamics of the epitaxial Gadolinium on tungsten. In this framework, we studied the oxidization of the Gd, which limits in the time the studies. In the end, we focus on a disturbing process that happens during the study of electrons dynamics in photoemission, the space charge effect. This can generate photoelectrons from metals, with a multiphotonic process. We propose in this last part a theoretical model to explain this phenomenon.These works are included in the development of SOLEIL synchrotron facility, in order to allow the development of the FEMTOSLICING, that will next allow to perform element resolved experiments within a time resolution of a hundredth of femtoseconds.

This research work focuses on the geometry and shape effects on submicron magnetic material. A web based micromagnetics program is written to model the hysteresis loop of nano magnetic samples with arbitrary geometry shapes and multiple magnetic materials. Three material samples have been modeled with this program along with nano magnets with a variety of geometric shapes. Shape anisotropy has been introduced to a permalloy ring by adding a cross-tie structure with various widths. The in-plane hysteresis loop and reversal behavior have no notable difference in direction parallel to the cross-tie, but greatly changed in perpendicular and diagonal directions. The switching field distribution is significantly reduced. The two distinct "onion" bit states of the modified ring elements are stabilized in the hysteresis in the diagonal direction. The changes in the modified rings make them better candidates for Magnetic Random Access Memory elements. Two Pac-Man elements, PM I and PM II, geometrically modified from disc and half disc respectively, are modeled. The PM I element undergoes a magnetic reversal through a two-stage mechanism that involves nucleation in the left and right middle areas followed by vortex core formation and vortex core motion in the lower middle area. The reversal process of the PM II element lacks the vortex core formation and motion stage. The switching field of the PM I and PM II elements are the same but the switching field distribution of the PM II elements is much narrower than that of the PM I element. Only the PM II element meets MRAM application requirements. The thickness dependence of the magnetic properties of a core-shell structure has been studied. The nano particles have a cobalt core and a permalloy shell. The nano spheres are the same size but with various shell thickness. Simulations reveal a multi-stage reversal process without the formation of a Bloch wall for thin-shell structure and smooth reversal process with the formation and motion of a Bloch wall for thick-shell structure. Gradual transition of the hysteresis loop patterns has been observed.

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.

The Interactions and magnetization reversal of Ni nanowire arrays and synthetic anit-ferromagnetic coupled thin film trilayers have been investigated through first order reversal curve (FORC) method. By using a quantitative analysis of the local interaction field profile distributions obtained from FORC, it has proven to be a powerful characterization tool that can reveal subtle features of magnetic interactions.

We present a time-resolved Kerr microscope, capable of measuring the magnetization dynamics of samples grown on transparent, double-side-polished substrates.The magnetization is excited by a current pulse, using a coplanar waveguide placed beneath the samples. The Kerr rotation is detected with the stroboscopic pump-probe technique, using a probing laser, synchronized with the current pulse.We report benchmark measurements of the time-resolved Kerr instrument for magnetization dynamics in thin permalloy and FePd films. The experimental results for ferromagnetic resonance peaks have been compared with the values predicted by Kittel. / Vi presenterar ett tidsupplöst Kerr-mikroskop, kapabel till att mäta magnetiseringsdynamiken hos magnetiska prov tillverkade på transparenta, dubbelsdigt polerade substrat. Magnetiseringen exiteras med en strömpuls via en koplanär vågguide placerad under provet. Kerr-rotationen detekteras med hjälp av en stroboskopisk ”pump-probe” teknik som använder en ”probing” laser synchroniserad med strömpulsen. Vi rapporterar prestandatest av det tidsupplösta kerr-instrumentet för magnetiseringsdynamiken i tunna permalloy - och FePd - filmer. De experimentella resultaten för de ferromagnetiska resonans-peakarna har jämförts med beräknade värden från Kittel.

A 2014 study by the US Department of Energy conducted at Lawrence Berkeley National Laboratory estimated that U.S. data centers consumed 70 billion kWh of electricity[1]. This represents about 1.8% of the total U.S. electricity consumption. Putting this in perspective 70 billion kWh of electricity is the equivalent of roughly 8 big nuclear reactors, or around double the nation's solar panel output[2]. Developing new memory technologies capable of reducing this power consumption would be greatly beneficial as our demand for connectivity increases in the future. One newly emerging candidate for an information carrier in low power memory devices is the magnetic skyrmion. This magnetic texture is characterized by its specific non-trivial topology, giving it particle-like characteristics. Recent experimental work has shown that these skyrmions can be stabilized at room temperature and moved with extremely low electrical current densities. This rapidly developing field requires new measurement techniques capable of determining the topology of these textures at greater speed than previous approaches. In this dissertation, I give a brief introduction to the magnetic structures found in Fe/Gd multilayered systems. I then present newly developed techniques that streamline the analysis of Lorentz Transmission Electron Microscopy (LTEM) data. These techniques are then applied to further the understanding of the magnetic properties of these Fe/Gd based multilayered systems. This dissertation includes previously published and unpublished co-authored material.