Magnetic minerals are abundant within our Earth's crust and can retain, through one of a number of processes, a remanent magnetisation induced by the Earth's magnetic field. Analyses of palaeomagnetic samples have been used for the past fifty years to improve our understanding of many of the Earth's major processes. Recent studies utilising newly developed imaging techniques, namely holographic transmission electron microscopy, have for the first time allowed direct observations of the magnetic structure in palaeomagnetic samples on a nanoscale. It is commonly observed that igneous rocks contain closely packed magnetic lamellae with a non-magnetic matrix, a result of the chemical process of exsolution. However, the results of current micromagnetic models, generated to predict the magnetic structure within such samples, are not in agreement with these direct observations. The results do, however, show strong similarities to the direct observations. The discrepancies between the direct observations and micromagnetic models indicate a lack of understanding of the magnetic interactions within such samples. To examine this two distinct hypotheses have been tested. Firstly, the geometry of the system has been altered to examine the effect of this on the magnetic structure of the grains. Secondly, a multiphase model has been produced. This multiphase model allows the simulation of more complicated systems that include more than one magnetic material in direct contact. This multiphase model has allowed us to examine the effect of varying the exchange in these multiphase structures and its effect on the modelled magnetic structure. Further, this multiphase model has allowed us to examine theoretical systems involving combinations of magnetic materials commonly found in palaeomagnetic samples.

Dans les dispositifs actuels de sauvegarde de données, les bits d'informations sont stockées sous la forme de paroi de domaines dans une couche mince, voire des media "patternés". Le support reste donc 2D. De nos jours, la densité de stockage tend vers une valeur maximale qu'il est difficile de dépasser pour des raisons fondamentales et technologiques. Ainsi, récemment des efforts ont été réalisés pour développer des dispositifs 3D qui allient la polyvalence de la mémoire RAM solide avec un coût comparable à celui des disques durs actuels.Un nouveau concept théorique particulièrement intéressant pour une mémoire magnétique en 3D a été proposé en 2004 par S. Parkin et al.. Cette mémoire de type registre à décalage est constituée d'un réseau de nanofils magnétiques verticaux avec une section transversale cylindrique ou bien rectangulaire. Dans ce nouveau type de mémoire, les bits sont codés sous forme d'une série de parois de domaine. Cette dernière peut être déplacée vers une tête de lecture intégrée par des impulsions de courant polarisé en spin de quelques nanosecondes.Les parois de domaines magnétiques dans des nanofils cylindriques ont suscité l'intérêt de la communauté scientifique en raison de leur application possible dans un dispositif fonctionnel ainsi qu'en raison de nouvelles propriétés intéressantes qui résultent du confinement géométrique des parois. A ce jour, seules quelques études expérimentales sur de telles parois de domaines existent. Elles ont mis en évidence la difficulté de maîtriser la propagation de parois dues à des forts effets de piégeage. Jusqu'à présent, l'origine microscopique de ce piégeage n'a été que partiellement comprise. On s'attend à ce qu’indépendamment de la qualité géométrique du fil, la microstructure du matériau puisse jouer un rôle non négligeable.Dans le cadre du projet européen FP7 m3D, l'objectif de mon travail de thèse a été d'étudier la propagation des parois de domaine dans des nanofils cylindriques avec des modulations de diamètre. L'énergie de ces parois de domaine augmentant avec le diamètre du fil, on s'attend à ce que des excroissances (ou des constrictions) agissent comme des barrières d'énergie artificielles (respectivement puits). Par conséquent, une propagation de paroi de domaine contrôlée via la géométrie du fil semble possible.La première partie de mon travail concerne l'optimisation des matériaux. Des fils d'un alliage de NiCo (diamètre de 100-200nm et longueur de plusieurs dizaines de micromètres) avec deux géométries distinctes ont été fabriqués par électrodéposition en collaboration avec le groupe du Prof. J. Bachmann à l' Université d'Erlangen. Pour chaque géométrie, j'ai exploré l'effet de la composition de l'alliage ainsi que d'un recuit sur la microstructure du matériau. Par la suite, la propagation des parois de domaine dans des nanofils individuels a été étudiée sous l'influence d'un champ magnétique quasi-statique ou d'une impulsion de champ magnétique avec une durée d'impulsion de l'ordre de la nanoseconde. Dans la dernière partie de ma thèse, j'ai effectué des simulations micromagnétiques complémentaires pour étudier l'effet de la géométrie des modulations sur le piégeage de ces parois de domaine magnétiques. / In all current data storage devices, the information bits are stored in form of domain walls in a thin film or in patterned media on a two-dimensional surface . Within the next decade, further increase of the storage density in these devices is expected to come to a halt due to several fundamental and technological issues. Thus there have recently been efforts to develop three-dimensional devices combining the versatility of solid state RAM with the cost efficiency of common hard disk drives.A particularly interesting theoretical concept for a three-dimensional magnetic memory has been proposed in 2004 by S. Parkin et al. . Their racetrack memory consists of a vertical array of magnetic nanowires with either cylindrical or rectangular cross section. The bits are encoded in a series of up to 100 domain walls per wire. Using nanosecond spin polarized current pulses these walls are shifted past an integrated read head.Magnetic domain walls in cylindrical nanowires have raised the interest of the scientific community due to their possible application in a functional device as well as due to exciting new properties which arise from the geometric confinement. Up to date, only a few pioneering experimental studies on such domain walls exist. They indicate strong pinning effects preventing a deterministic domain wall propagation. So far the microscopic origin of this pinning has only partially been understood. It is expected however that beside the wire geometry the material microstructure may play a considerable role.Situated within the framework of the European FP 7 project m3D, the objective of my work has been to investigate the domain wall propagation in cylindrical nanowires with diameter modulations by means of magnetic force microscopy and micromagnetic simulation. As the domain wall energy increases with the wire diameter, protrusions (resp. notches) are expected to act as an artificial energy barrier (resp. well). Consequently, a deterministic domain wall propagation controlled via the wire geometry seems possible.A first part of my work concerns material optimization. For this, NiCo alloy wires (100-200nm diameter and multiple tens of micrometers in length) with two distinct geometries have been fabricated by template assisted electrodeposition (Chemist collaborators at Univ. Erlangen, Prof. J.Bachmann). I have then explored the impact of the alloy composition as well as of possible post-fabrication annealing on the material microstructure. Subsequently, domain wall propagation in individual nanowires has been investigated under the influence of either a quasistatic magnetic field or a nanosecond magnetic field pulse. In addition I have performed complementary micromagnetic simulations to study the effect of the modulation geometry on the domain wall pinning.

Spintronics is the study of electron spins and their utilization in electronic devices. Within this field, spin-based oscillators have shown promise for mi- crowave signal generation as they can operate at high frequencies, are small in scale and are compatible with modern fabrication techniques. Among these oscillators are the spin Hall nano-oscillators (SHNOs). They are nanoscale thin-film structures driven by pure spin-current injection from a primary con- ductor into a ferromagnetic material. This process can be used to generate microwave signals through oscillations in the material’s magnetization. By constraining the current flow in the device to individual constrictions, an ar- ray arrangement of multiple oscillators can be realized. These oscillators can then be coupled together via their internal interactions to achieve mutual syn- chronization and improve their characteristics.In this work, a versatile micromagnetic modelling procedure for simulating constriction-based SHNOs and their synchronization dynamics in different ar- ray arrangements is presented. A case study of various 2x2 array geometries is conducted along with an exploration of higher-order networks of 4x4, 6x6 and 8x8 oscillators. A perturbative optimization algorithm is developed to improve excitation conditions and drive geometries into a synchronized regime. Lastly, a comparison to nonlinear auto-oscillator theory is presented to illustrate the dependence of generated signals on constriction sizes and the spacing between oscillators. Mutual synchronization between multiple oscillators is achieved and favourable geometry and excitation conditions are defined. The conducted simulations show good agreement with experimental results and illustrate the potential for future studies of SHNO characteristics through micromagnetic modelling. / Spinntronik är ett forskningsområde, som handlar om hur elektronens s.k. spinn kan användas i elektroniska komponenter. Inom detta område har spinnbaserade oscillatorer visat sig ha lovande egenskaper för generering av mikrovågssignaler, eftersom de har höga arbetsfrekvenser, liten storlek och är kompatibla med moderna tillverkningstekniker. En typ av dessa oscillatorer kallas spinn-Hall nano-oscillatorer (SHNO). De är nanometerstora tunnfilms- strukturer, vilka drivs av en ren spinnström, som injiceras från en (metallisk) ledare till en ett ferromagnetiskt material. Denna mekanism kan användas för att skapa mikrovågssignaler genom oscillationer i materialets magnetisering. Genom att begränsa strömflödet i komponenten till enskilda gap kan man skapa en matris med ett stort antal oscillatorer. Dessa oscillatorer kan sedan kopplas till varandra genom interna utbytesmekanismer och på så sätt uppnår man en ömsesidig koppling och förbättrade egenskaper.I detta arbete presenteras ett mångsidigt mikromagnetiskt modelleringsflö- de, för att simulera SHNO:er, baserade på nano-gap, och deras synkronisering i olika matriskonfigurationer. En fallstudie som inkluderar olika 2x2 matris- geometrier har genomförts tillsammans med explorativ utforskning av högre ordnings nätverk, såsom 4x4, 6x6 och 8x8 oscillatorer. En störnings-baserad optimerings-algoritm har utvecklats för att förbättra exciterings-parametrarna och för att tvinga geometrierna in i en synkroniserad regim. Som en avslutning presenteras en jämförelse med icke-linjär auto-oscillatorteori för att visa den genererade signalens beroende på gapens storlek och avståndet mellan dem. Ömsesidig synkronisering mellan flera oscillatorer kunde uppnås och en för- delaktig geometri samt lämpliga värden på exciterings-parametrarna kunde definieras. Simuleringarna i studien hade bra överensstämmelse med experi- mentella resultat och visar på potentialen för vidare studier av SHNO egen- skaper med hjälp av mikromagnetisk modellering.

L'auteur n'a pas fourni de résumé en français / Nowadays in medicine and biotechnology a wide range of applications involves magnetic micro/nano-object manipulation including remote control of magnetic beads, trapping of drug vectors, magnetic separation of labelled cells and so on. Handling and positioning magnetic particles and elements functionalized with these particles has greatly benefited from advances in microfabrication. Indeed reduction in size of the magnet while maintaining its field strength increases the field gradient. In this context, arrays made of permanent micromagnets are good candidates for magnetic handling devices. They are autonomous, suitable for integration into complex systems and their magnetic action is restricted to the region of interest.In this thesis we have elaborated an original approach based on AFM and MFM for quantitative study of the magnetic force and associated force gradients induced by TMP micromagnet array on an individual magnetic micro/nano-object. For this purpose, we have fabricated smart MFM probes where a single magnetic (sub)micronic sphere was fixed at the tip apex of a non-magnetic probe thanks to a dual beam FIB/SEM machine equipped with a micromanipulator.Scanning Force Microscopy conducted with such probes, the so-called Magnetic Particle Scanning Force Microscopy (MPSFM) was employed for 3D mapping of TMP micromagnets. This procedure involves two main aspects: (i) the quantification of magnetic interaction between micromagnet array and attached microsphere according to the distance between them and (ii) the complementary information about micromagnet array structure. The main advantage of MPSFM is the use of a probe with known magnetization and magnetic volume that in combination with modelling allows interpreting the results ably.We conducted MPSFM on TMP sample with two types of microparticle probes: with superparamagnetic and NdFeB microspheres. The measurements carried out with superparamagnetic microsphere probes reveal attractive forces (up to few tens of nN) while MFM maps obtained with NdFeB microsphere probes reveal attractive and repulsive forces (up to one hundred of nN) for which the nature of interaction is defined by superposition of microsphere and micromagnet array magnetizations. The derived force and its gradient from MFM measurements are in agreement with experiments on microparticle trapping confirming that the strongest magnetic interaction is observed above the TMP sample interfaces, between the areas with opposite magnetization. Thanks to 3D MFM maps, we demonstrated that intensity of magnetic signal decays fast with the distance and depends on micromagnet array and microsphere properties.Besides the magnetic interaction quantification, we obtained new information relevant to TMP sample structure: we observed and quantified the local magnetic roughness and associated fluctuations, in particular in zones of reversed magnetization. The variation of detected signal can reach the same order of magnitude as the signal above the micromagnet interfaces. These results complete the experiments on particle trapping explaining why magnetic microparticles are captured not only above the interfaces, but also inside the zones of reversed magnetization.Quantitative measurements of the force acting on a single (sub)microsphere associated to the modelling approach improve the understanding of processes involved in handling of magnetic objects in microfluidic devices. This could be employed to optimize the parameters of sorting devices and to define the quantity of magnetic nanoparticles required for labelling of biological cells according to their size. More generally these experimental and modelling approaches of magnetic interaction can meet a high interest in all sorts of applications where a well-known and controlled non-contact interaction is required at micro and nano-scale.

В магистерской диссертации представлены результаты моделирования магнитных гистерезисных свойств ансамбля обменносвязанных частиц. Для выполнения компьютерного моделирования разработан пакет компьютерных программ в среде MATLAB, позволяющий моделировать и анализировать предельные и частные петли магнитного гистерезиса для однофазных и многофазных ансамблей с различными типами магнитной анизотропии, упорядочением фаз, а также при параметрической зависимости микроскопических констант от температуры. Все это реализовано в рамках модели однодоменных нанокристаллитов, имеющих однородную намагниченность, процесс изменения которой осуществляются за счет когерентного вращения. Разработан алгоритм параллельного расчета, позволяющий основные арифметические и логически операции выполнять на графических ускорителях. Полученные с его помощью численные результаты качественно соответствуют экспериментальным данным для системы Nd-Fe-B. / The results of magnetic properties modelling of the ensemble of exchange coupled particles are represented in this master's thesis. The computer modelling is realized in the MATLAB programming environment and allows performing calculations and analysis of major and minor magnetic hysteresis loops of single-phase or multiphase ensembles with different types of magnetic anisotropy, phase arrangement and with the parametric dependency of microscopic constants on temperature. The latter is based on the model of single domain nanocrystallites. Each of them has a uniform magnetization and any changes of one are happened by coherent rotation. The algorithm of parallel calculations using a graphic processing unit is also shown in the work. The obtained numeric results qualitatively are in compliance with the Nd-Fe-B experimental data.