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In this work the dynamics of permalloy circular magnetic islands of 225 nm radius and 10 nm thickness arranged into square lattices was investigated employing micromagnetic simulations.The simulations of the vortex magnetization loops and the ferromagnetic resonance (FMR) spectra were carried out using a free micromagnetics simulation software Mumax3. The obtained data was analyzed using Matlab. The simulations were carried out on a single vortex island as well as on two different lattices. The first lattice is comprised of interacting islands, while the second lattice - of non-interacting islands, separated by 25 nm and by 450 nm edge-to-edge distance, respectively. The magnetization loops were simulated by applying the static magnetic field in-plane of the single island or the lattice. The FMR simulations were carried out by applying the static magnetic field in-plane of the lattice and after the system reached the ground state in that field, the excitation as a sinc pulse was sent out along the out-of-plane direction of the lattice. The analysis of the obtained FMR spectra revealed that the several resonant modes are present for the single vortex island and the lattice, comprised of such islands. However, the physical explanation of the origin of those modes is a subject for further investigations.

L'imagerie de champs magnétiques de faible amplitude avec une résolution spatiale à l'échelle nanométrique est un enjeu important dans de nombreux domaines de la physique et pour de multiples applications, que ce soit par exemple en science des matériaux pour le stockage magnétique de l'information, ou bien en optique quantique afin de pouvoir contrôler un spin individuel utilisé comme bit quantique, ou encore en biophysique pour l'étude structurelle de protéines par résonance magnétique. Dans ce contexte, cette thèse décrit la réalisation d'un magnétomètre à balayage fondé sur la réponse magnétique du spin électronique d'un centre coloré NV du diamant. Un tel magnétomètre présente des propriétés sans équivalent, en combinant une résolution spatiale sub-nanométrique, assurée par la dimension atomique du capteur, et une très haute sensibilité (< 1 µT/Hz^(-1/2)), ceci même à température ambiante. De plus la mesure de champ magnétique est quantitative et non perturbative, offrant ainsi un avantage majeur par rapport à la microscopie à force magnétique couramment utilisée pour l'imagerie magnétique de nanostructures. Nous aborderons, dans un premier temps, les problématiques liées à la fabrication de la sonde magnétique, constituée par un centre coloré NV unique dans un nanocristal de diamant positionné à l'extrémité d'une pointe AFM. Les propriétés de ce magnétomètre seront caractérisées en imageant le champ de fuite d'un disque dur magnétique. Cette étude nous permettra d'introduire différentes méthodes d'imagerie magnétique et de comparer leurs performances. Le magnétomètre à centre NV sera par la suite utilisé pour imager des distributions d'aimantation vortex dans des plots ferromagnétiques, dont le cœur est connu pour être l'un des objets les plus petits du micromagnétisme, le rendant extrêmement difficile à observer. Les propriétés du magnétomètre à centre coloré NV du diamant, démontrées dans cette thèse, ouvrent la voie à de nombreuse études en nanomagnétisme et en spintronique. / Imaging weak magnetic fields at the nanoscale is a challenge for many field of reasearch, and for a wide range of applications, such as in material science, for the magnetic storage of the information, or in quantum optics with the opportunity to control a single spin used as a quantum bit, or in biophysics where magnetic resonance can enable structural imaging of a protein. In that context, this thesis describes the realisation of a scanning probe magnetometer based on the electron spin resonance of a single NV colour centre in diamond. Such a magnetometer provides unprecedented properties, consisting of a sub-nanometric spatial resolution, given by the effective atomic size of the defect, combined with a high sensitivity (< 1 µT/Hz^(-1/2)), even at room temperature. Moreover magnetic field ca be measured quantitatively, with no perturbation induced, which is a major asset comparing to what can be done using the usual magnetic force microscopy technique. We will first describe the realisation of the magnetic probe, consisting of a single NV colour centre in a diamond nanocristal, grafted at the apex of an AFM tip. Properties of this magnetometer will be then characterized by imaging the stray field generated by a magnetic hard disk. Different imaging techniques will be presented, and compared during this study. Finally, magnetic vortex in patterned ferromagnetic thin film will be imaged, especially the vortex core, which is known to be one of the smallest object of micromagnetism. The exceptional properties of NV based magnetometry, demonstrated in ths work, might enable various applications in nanomagnetisms and spintronics.

The ability of a ferromagnet to maintain its magnetic state in the absence of an external magnetic field has made ferromagnetic materials an important subject of study in physics since the end of the 19th century. Moreover, ferromagnetic materials are the cornerstone for data storage systems such as magnetic tapes, magnetic disk drives and magnetic random access memory. The discovery of the Giant Magneto Resistance (GMR) in 1988 suggested that, since the magnetic state of the electrical conductor has an important effect upon the current flow, there may also be an inverse influence of the current upon the magnetization. In this effect, predicted in 1989 [1] by Slonczewski and called Spin Transfer Torque, angular momentum transferred by a spin polarized current can exert a torque on the magnetization of a ferromagnetic material, changing the local magnetization and stimulating the precession of the magnetic moments, generating microwave signals. This provides a new method of manipulating magnetization without applying an external field. Large polarized currents lead to spin transfer effects which are the driving force for the magnetic dynamics of devices known as Spin Transfer Oscillators (STO). In this new kind of nano-device the emission of microwaves is stimulated by a DC electrical current and measured as a change in the output voltage due the GMR effect. The specific characteristics of these devices such as working frequency and DC current ranges, microwave emission linewidth, and maximum emission power among others, are given by the design and size of the device,and the nature of the magnetic oscillations producing the emission. Among the multiple types of STO that now exist , I have focused my research upon three of them: Spin Transfer Vortex Oscillators (STVO), Single Layer Spin Transfer Oscillators (SL-STO) and Orthogonal Pseudo Spin Valves. Within STVOs and SL-STOs we can nucleate what is called a magnetic vortex. A magnetic vortex is a curling of the in-plane of a magnetic layer with its centre pointing out of the magnetization plane. The gyration of this vortex due to STT produces a microwave emission < 1GHz with a greater emission power than that produced by the precession of magnetic moments in STOs. The phase-locked synchronisation of multiple vortices is expected to exhibit enhanced microwaved power and phase stability compared to a single vortex device, providing a solution to the drawbacks of the STO in the low frequency regime. On the other hand, Orthogonal Pseudo Spin Valves promote the nucleation of magnetic dissipative solitons, also called magnetic droplets. This type of magnetic structure has an opposite out of plane magnetization to the layer that contains it. Compared to the microwave emission of magnetic vortices , magnetic droplets have a higher frequency range and emission power. However, their nucleation is subject to large external fields being applied to the sample. In this thesis, I electrically characterized these devices and applied magnetic imaging techniques in order to go further in the understanding of the spatial features and dynamic behaviour of these magnetic structures. It is not possible to acquire this knowledge by only using electrical characterization. Understanding the magnetization dynamics in these devices is crucial for the design of STO based devices while imaging studies are required to prove the existence of these magnetic structures, as in case of the magnetic droplet. In chapter 2 I will introduce the background concepts of magnetism that are relevant to this thesis. I will go from the basics principles of ferromagnetism, its quantum mechanical treatment, and the theory that explain the dynamics of the magnetisation. I will also present the state of the art in experimental research in the field of spin transfer oscillators. My aim is to give the basic background needed to understand the results presented in this thesis. In chapter 3 I will introduce the two main experimental techniques used for imaging the magnetisation of the devices presented: Holography with Extended Reference by Autocorrelation Linear Differential Operator (HERALDO) and Time Resolved Scanning Kerr Microscopy (TRSKM). I will revise the theoretical background concepts and the development of the techniques in order to demostrate the uniqueness of each technique and how they were used in this thesis. It is interesting to note that while MOKE is a well-known and widely-used technique, far fewer laboratories in the world area able to perform time resolved measurements using MOKE, with the University of Exeter being one of them. Furthermore, HERALDO is a novel technique that is used for the first time to image magnetic structures within multilayer systems in this thesis, which is a milestone in the development of the techinque. In chapter 4 I present an investigation of the magnetization dynamics of a SL-STO. Electrical transport measurements provided an initial characterization of the device. We then used HERALDO for the first time to investigate the magnetization dynamics in an intermediate layer of a multilayer stack. We present time averaged measurements of the magnetisation of a magnetic vortex formed underneath a nano contact (NC) positioned on top of the multilayer, using a combination of x-ray holography and x-ray magnetic circular dichroism. In chapter 5 I present the first direct measurement at the time of a magnetic dissipative droplet, using holography with extended reference autocorrelation by linear differential operator (HERALDO). I studied the out of plane magnetisation of the free layer under a NC within an orthogonal pseudo spin salve. In chapter 6 I present and study STVO devices with pairs of NCs of 100 nm diameter and centre-to-centre separation D = 200 to 1100 nm, by a combination of electrical measurements and time-resolved scanning Kerr microscopy (TRSKM). It will be shown that the dynamic behaviour of vortices and anti vortices changes when the distances between the NCs within the devices is changed.

Title from first page of PDF document. Includes bibliographical references (p. 62-63).

Orientadores: Flávio Garcia, Kleber Roberto Pirota / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-24T22:02:36Z (GMT). No. of bitstreams: 1 Dias_CarlosSatoBaraldi_D.pdf: 19717682 bytes, checksum: 212154446eec9e3ee26eee15692adfec (MD5) Previous issue date: 2014 / Resumo: Durante este trabalho, estudamos a viabilidade de uma nova classe de partículas magnéticas otimizadas para a hipertermia magnética e que denominamos VIP (Vortex Iron oxide Particle). Estas partículas são compostas de óxidos de ferro (magnetita ou maghemita) e assumem a forma de nanoanéis ou nanotubos. Tal morfologia confere as partículas um comportamento magnético peculiar, fruto de seu estado magnético denominado vórtice. Graças a este estado magnético, estas partículas possuem uma resposta para hipertermia superior, se comparadas aos SPIONs (Superparamagnetic Iron Oxide Nanoparticles) tradicionalmente usados para este fim, atendendo os pré-requisitos para aplicações biomédicas (baixas toxicidade e remanência). Os experimentos realizados no trabalho se focaram em duas linhas principais. Na primeira, avaliamos o desempenho das VIPs para hipertermia magnética em um ambiente in vitro. Neste experimento, avaliamos tanto a citotoxicidade das partículas quanto os mecanismos que promovem a morte celular. Estes resultados mostraram que as VIPs não são citotóxicas. Além disso, observamos também que o tamanho e a forma das partículas permitem que estas sejam internalizadas, promovendo um processo de hipertermia magnética muito eficiente. De fato, o experimento de hipertermia in vitro mostrou que as VIPs são capazes de alcançar um alto grau de seletividade, matando principalmente as células que internalizam as VIPs minimizando o aquecimento do meio celular e reduzindo assim o dano às células vizinhas. Na segunda linha, trabalhamos no desenvolvimento de uma VIP recoberta com material antiferromagnético (AFM), cujo desempenho para hipertermia magnética seria superior ao da própria VIP testada nos experimentos in vitro. Desenvolvemos um extenso estudo teórico fundamental, baseado em simulações micromagnéticas, que permitiram prever muitos dos fenômenos que seriam observados experimentalmente. No entanto os experimentos para a síntese da VIP@AFM não foram completamente finalizados, impossibilitando maiores conclusões sobre o sistema simulado. Sendo assim, acreditamos que os resultados alcançados no projeto promoveram o desenvolvimento de partículas magnéticas otimizadas para hipertermia magnética. Tanto por mostrar sua viabilidade, quando testados in vitro, quanto por promover o conceito da VIP@AFM como um próximo passo para o aprimoramento desta classe de partículas / Abstract: In this work, we studied the feasibility of a new class of magnetic particles named as Vortex Iron oxide Particle or VIP and specifically designed for magnetic hyperthermia. Those iron oxide (magnetite or magnetite) particles have a ring shape morphology that grants them a very specific magnetic configuration know as vortex state. This magnetic configuration would grant a superior hyperthermia response when compared to the traditional Superparamagnetic Iron Oxide Particle (SPION) without compromising biomedical requirements as low cytotoxicity and lack of magnetic remanence. The experiments presented on this work explored two main paths. The first one evaluated the VIP performance in vitro. We were able to assess both the cytotoxicity of the particle as to observe the killing mechanism. The results showed that the VIPs are not cytotoxic and that the size and shape of the particle may promote the internalization of those particles, resulting on a very efficient magnetic hyperthermia. With the in vivo experiment, we notice a high degree of selectivity, thanks to the cell internalization, that preserved the surrounding cells. In the second research path, we further developed the concept of a VIP, by creating a core-shell structure where the VIP would be coated by an antiferromagnetic (AFM) material and improving the magnetic hyperthermia response. However, the experiments for the development of the VIP@AFM were not completed. Although we were able to complete a study based on micromagnetic simulations, the experiments intending to fabricate the VIP@AFM were not fully successful, preventing any further conclusion about this system. In any case, we believe that the results achieved on this project represent an important contribution to the development of magnetic hyperthermia specific particles / Doutorado / Física / Doutor em Ciências

The work in this PhD thesis covers two strands of x-ray experiments: firstly, the characterisation of large arrays of dense structures by means of x-ray scattering, and, secondly, the investigation of hybrid anisotropy square structures with x-ray microscopy. The ability to accurately characterise large arrays of nanoscale magnetic structures is a key requirement for both scientific understanding and technological advance such as bit patterned recording media (BPM). In this work small angle x-ray scattering (SAXS) was investigated as a characterisation technique for large arrays of patterned structures. Dense arrays of magnetic nanostructures were prepared on x-ray transparent membranes and measured. The SAXS data was then modelled to obtain structure parameters such as the mean structure diameter, the diameter distribution and the mean position variance with statistical significance. Arrays (500 x 500 μm2) of nominally uniform nanostructures with centre-to-centre distances between 250 nm-50 nm were structurally characterised and compared to structure diameters obtained by optical scanning electron microscopy measurements. The mean structure diameter was found to be between 39 nm-15nm and agree within the errors with the diameter obtained from SEM measurements. This work provides accurate data on the distribution (variance) of nanostructure sizes which is key for modelling these arrays for applicationin BPM. In the second part of the work, the static and dynamic properties of patterned hybrid anisotropy square structures ([Co/Pd]-Py) were investigated by using time-resolved scanning transmission X-ray microscopy (STXM). In these patterned structures the magnetisation in the layers change both in magnitude and direction and gives rise to interesting new domain configurations. The reciprocal interaction between magnetic vortices in the Py layer and locally circular stripe domains in the Co/Pd was investigated and a mutual domain imprint between the layer was observed. In dynamic excitation experiments the precession of the vortex core is studied and showed good agreement with micromagnetic simulations made by Hrkac and Bryan. As demonstrated patterned hybrid anisotropy square structures have promising magnetic properties with potential applications in data storage (vortex switching) or spintronics (vortex oscillators).

L’objectif principal de cette thèse vise la compréhension et la maitrise des mécanismes physiques menant à l’excitation du mode gyrotropique de vortex magnétique par transfert de spin, et en particulier l’origine des sources de bruit affectant sa dynamique. Ce travail est effectué dans la perspective de parvenir à l’amélioration des propriétés radiofréquences de ces dispositifs appelés Spin Transfer Oscillator.En effet, ces oscillateurs ont l’avantage d’être de taille submicronique (quelques dizaines à quelques centaines de nm), d’être compatibles avec les technologies CMOS et d’être résistants aux radiations. De plus, les mécanismes d’aimantation mis en jeu leur assurent une forte dépendance de la fréquence avec le courant, i.e. une bonne accordabilité, ainsi qu’une réponse dynamique rapide i.e. une agilité élevée. Cependant, différentes questions restent en suspens quant à la possibilité d’améliorer leurs conditions d’oscillations, leur puissance et la cohérence de leurs oscillations.Un premier aspect de mon travail de thèse a été d’étudier l’influence des fluctuations thermiques sur la dynamique entretenue du mode gyrotropique du cœur de vortex. Un des résultats a été de montrer que le bruit de phase du mode gyrotropique résulte majoritairement de fluctuations de phase issues directement des fluctuations thermiques auxquelles s’ajoutent des fluctuations d’amplitude converties en fluctuations de phase. Grâce à un modèle analytique, nous avons pu mettre en évidence le rôle important joué par les non-linéarités des forces agissant sur le vortex. De plus, nous avons pu mesurer les paramètres caractéristiques de l’oscillateur, à savoir, la rapidité à changer sa fréquence mais aussi le facteur de couplage amplitude-phase.La seconde étape de mes travaux a consisté à améliorer les conditions d’obtention de signal rf. Un résultat majeur de ce travail a été l’obtention d’un signal rf puissant en absence de champ magnétique. Les puissances mesurées sont de quelques centaines de µW correspondant à des largeurs de raie faibles allant de quelques centaines de kHz à quelques MHz. Cette spécificité est rendue possible pour une structure complexe de l’oscillateur où la couche magnétique qui polarise en spin le courant a une aimantation perpendiculaire et est différente de la couche de référence pour la magnétorésistance.La troisième étape a été d’optimiser le rendement de l’oscillateur. Un des résultats marquants est que nous avons pu mesurer une puissance rf émise record s’élevant à 3.6 µW, encore jamais obtenue à température ambiante pour les oscillateurs à transfert de spin à base de vortex. Ces fortes puissances résultent du développement de nouvelles jonctions à base de FeB effectués par le groupe de S. Yuasa (AIST, Japon) pour lesquelles l’amélioration de la qualité de la jonction, nous a permis d’obtenir une magnétorésistance atteignant 125% .La faible taille de l’oscillateur a donc un coût qui se paye en termes de bruit de phase. Une solution qui permettrait de résoudre cette limitation et d’améliorer la cohérence des oscillations est la synchronisation mutuelle de plusieurs oscillateurs à transfert de spin au travers des courants rf émis par chacun. Ainsi, la dernière étape de ma thèse a été d’étudier le comportement du mode gyrotropique lorsqu’il est soumis à un courant alternatif. Un résultat important a été de montrer, grâce à une étude expérimentale appuyée sur un modèle analytique, le rôle crucial des non-linéarités et des symétries des forces de synchronisation du mode excité.Ces différents travaux nous ont fournis les outils pour mieux comprendre la dynamique du vortex magnétique et nous ont amené à mettre en place un banc de mesure original pour lequel l’oscillateur se synchronise sur lui-même. En fonction du retard avec lequel le signal émis par l’oscillateur est réinjecté, nous avons pu montrer pour la première fois que la fréquence, la puissance mais aussi la largeur de raie des oscillations peuvent être modulées. / The main goal of this thesis is the understanding of the physical mechanisms and the subsequent control of the properties at the origin of the spin transfer induced magnetic vortex gyrotropic motion in confined systems. In particular the origin of the noise affecting the dynamics has been investigated. This work has been performed with a view to improving the radiofrequency (rf) properties of the so-called Spin Transfer Oscillator (STO).The advantages of such oscillators are their sub-micron size (from few tens to hundreds of nanometres), their compatibility with CMOS technologies and their radiation hardness. Moreover, the magnetization dynamics involved permit a large tunability of frequency as a function of the applied current and a high agility i.e. a fast dynamical response. Nevertheless, several open questions exist regarding the possible optimization of the sustained oscillation conditions and the improvement of the STO power and spectral coherence.The first aspect of my work was to investigate the influence of thermal fluctuations over the sustained vortex core gyrotropic motion. One of the key results of my thesis was to show that the phase noise results from direct phase fluctuations from thermal fluctuations plus amplitude fluctuations converted to phase noise. With an analytical model, we were able to highlight the major role played by the non-linearities of the forces acting on the vortex core. In addition, we were able to measure the characteristic parameters of the oscillator, namely, the speed of frequency response to perturbations as well as the phase-amplitude coupling coefficient.The second important part of my work has been to improve the conditions for obtaining an rf signal. An important result of this work was the measurement of a powerful rf signal in the absence of a magnetic field. The measured powers are a few hundred milliwatts and correspond to low linewidths, ranging from a few hundred kilohertz to a few megahertz. The zero field behaviour was made possible due to the complex structure of the oscillator where the magnetic layer which polarizes the spin current has a perpendicular magnetization, in contrast to the in-plane polarized reference layer.The third step was to optimize the performance of the oscillator. One of the striking results of this thesis is that we measured a record rf output power, up to 3.6 µW, the largest obtained at room temperature for vortex based STOs so far. This high output power results from the development of new FeB based junctions made by the group of Pr. S. Yuasa (AIST, Japan), where the improvement of the quality of the junction allowed us to obtain a magnetoresistance up to 125%.The small size of the oscillator has a cost that is paid in terms of the phase noise. One possible solution that would solve this limitation and enhance the coherence of the oscillations is via mutual synchronization of several STOs through rf currents emitted by each oscillator. Thus, the last stage of my thesis was to study the behaviour of gyrotropic motion when subjected to an alternating current. An important result was to show, through an experimental study in conjunction with an analytical model, the crucial role of the non-linearities and symmetries of the synchronization forces.These various studies have provided us the tools to better understand the dynamics of magnetic vortex and led us to develop an original tester for which the oscillator synchronizes itself with its own rf signal. Depending on the delay at which the oscillator is fed back, we showed for the first time that the frequency, the power and also the linewidth of the oscillations can be modulated.

The diploma thesis deals with the switching of spin vortices in magnetic nanodisks. First, the basic concepts of (micro)magnetism are defined and existing theoretical and experimental achievements in the field of switching of the two basic characteristics (chirality and polarity) of magnetic vortex are summarized. Then the principle of dynamic switching of magnetic vortex chirality using in-plane magnetic field pulse with a well defined amplitude and duration is presented. There is no need to use a certain shape of nanodisks or asymmetry in magnetic field distribution. Nanostructures were prepared by the multi-step electron beam lithography and ion beam sputtering. Individual steps of sample preparation and optimization for the magnetization dynamics measurements are described. Finally, the experimental measurements of the dynamic switching of chirality on prepared samples obtained by transmission x-ray microscopy at the synchrotron Advanced Light Source at Berkeley, USA are presented and discussed.

The diploma thesis is concerned with switching of vortex circulation in magnetic nanodisks. The results of micromagnetic simulations of hysteresis loops of individual disks with different degrees of asymmetry are presented. The influence of geometric asymmetry of the disk on the shape of the hysteresis loop is discussed as well as switching of vortex circulation in asymmetric nanodisks by external in-plane magnetic field. Simulations of pairs of magnetostatically coupled nanodisks were carried out for different interdisk distances and degrees of asymmetry. By analysing the results of the simulations, the effects of magnetostatic coupling and the asymmetry on resultant circulation of individual vortices were compared and the range of magnetostatic interaction between nanodisks of given dimensions and asymmetry was estimated. Experimental techniques used for fabrication and measurement of the samples are briefly summarized.