Within the exciting current trend to explore novel low-dimensional systems, the possibility to inject pure spin currents in graphene and other two-dimensional crystals has attracted considerable attention in the past few years. The theoretical prediction of large spin relaxation times and experimentally observed mesoscopic-scale spin diffusion lengths places graphene as a promising base system for future spintronics devices. This is due to the unique characteristics intrinsic to the two-dimensional lattice of carbon atoms forming graphene, such as the lack of nuclear spins and weak spin-orbit coupling of the charge carriers. Interestingly for some spintronic applications, the latter can be chemically and physically engineered, with large induced spin-orbit couplings found in functionalized graphene sheets. Understanding spin injection, spin current and spin dynamics in graphene is of a great interest, both from the fundamental and applied points of view. This thesis presents an experimental study of dynamical generation of spin currents in macroscopic graphene sheets by means of spin pumping from the precessing magnetization of an adjacent ferromagnet. The spin pumping characteristics are studied by means of ferromagnetic resonance (FMR) measurements in Permalloy/graphene (Py/Gr) bilayers. Changes in the FMR linewidth induced by the presence of graphene (when compared to studies with only Py films) correspond to an increase in the Gilbert damping in the ferromagnetic layer (proportional to the FMR linewidth) and interpreted as a consequence of spin pumping at the Py/Gr interface driven by the Py magnetization dynamics (i.e., magnetic induced by the microwave stimulus). FMR experiments are performed on different FM/Gr interfaces, completing a set of studies designed to systematically identify and eliminate damping enhancement arising from processes other than spin pumping. Remarkably, a substantial enhancement of the Gilbert damping observed in Py/Gr strips with graphene protruding a few micrometers from the strip sides is univocally associated to spin pumping at the quasi-onedimensional interface between the Py strip edges and graphene. This increase in the FMR linewidth compares with observations in other bilayer systems, in where thick (thicker than the spin diffusion length) layers of heavy metals with strong spin-orbit coupling are employed as the non-magnetic layer, indicating that spin relaxation in chemically grown graphene must be greatly enhanced in order to account for the losses of angular momentum lost by the ferromagnet. The fundamental implications of the results presented in this thesis point to a non-trivial nature of the spin pumping mechanism owing to the two-dimensionality of the non-magnetic layer (i.e., graphene). In addition, a spintronics device designed to interconvert charge and spin currents has been designed. A high-frequency microwave irradiation lock-in modulation technique is employed to detect the small electrical voltages generated by the inverse spin Hall effect (ISHE). As a proof of principle, a successful spin-charge interconversion in Py/Pt-based devices is experimentally demonstrated in this thesis. The challenges associated with the spin-charge interconversion in twodimensional devices are discussed and systematically addressed, and a potential device geometry for measuring the ISHE in Py/Gr-based systems is provided.

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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.

[Truncated abstract] Although exchange bias was discovered more than four decades ago, a satisfactory understanding of every instance of exchange bias observed in experiment has not yet emerged. Understanding exchange bias is complicated by many factors. For example, details of the antiferromagnet interface structure set up during the initial field cooling, thermal activation processes in the ferromagnet and antiferromagnet, and domain formation and domain wall movement in the antiferromagnet are all important in determining features associated with exchange bias. Two exchange bias systems are investigated in this thesis. One is a disordered system: a single layer Co/CoO film with random interfaces prepared by a reactive RF sputtering technique. The other is a ‘model’ system of Fe/KFeF 3 bilayers with compensated interfaces prepared by molecular beam epitaxy (MBE). The central theme of this work is to understand exchange bias and other related magnetic properties in these two very different systems. The Co/CoO exchange bias system studied here is different in structure from conventional exchange bias systems such as bilayer and multilayer systems where interfaces between ferromagnet and antiferromagnet are reasonably well defined. In this Co/CoO system, the Co and CoO is in the form of particles distributed randomly in a sputtered film. The interfaces between the Co and CoO are randomly distributed and may not be continuous over a large length scale. More importantly, the interface area is dependent on the shape and size of the particles and on their distribution. Many unique magnetic properties are related to the random interface in this system. For example, exchange bias and coercivity obtained at low temperatures are very large due to the large interface area between Co and CoO particles. The interface area can be controlled by changing the Co/CoO mass ratio in the film. Unlike in bilayer systems, film thickness in this single layer Co/CoO system turns out not to be critical for exchange bias and coercivity. The independence of film thickness may be technically important. More interestingly, because the interface is random, exchange bias can be setup by field cooling in any direction. Both training and magnetic viscosity effects were studied and provided evidence of thermal activation processes in this Co/CoO system. Training is explained as formation of a domain wall in the CoO with motion limited locally due to limited continuity of Synthesis and Characterization of Magnetic Thin Films - Exchange Bias Systems interfaces between the Co and CoO. Specific magnetization measurements over time were made and studied using viscosity theory. The magnetic viscosity was found to be strongly temperature dependent. There is a broad distribution of blocking temperatures which might be due to a broad distribution of Co particle sizes

Dans cette thèse, on analyse la possibilité de changer un courant de charge dans des jonctions supraconductrices par une manipulation des propriétés de spin en utilisant la résonance magnétique. On considère deux jonctions différentes: Premièrement, une jonction Josephson non-conventionnelle entre un supraconducteur conventionel de type s et un supraconducteur non-conventionel de type px. Deuxièmement, une jonction entre un demi-métal et un supraconducteur conventionel. La jonction spx contient deux états liés d'Andreev qui sont 2pi-periodiques. Ils donnent lieu à une aimentation spontanée à l'équilibre. Ceci ouvre la possibilité de manipuler l'occupation des niveaux d'Andreev en utilisant un champ magnétique dépendant du temps. On demontre que ce champ induit des oscillations de Rabi cohérentes entre différents états de spin de la jonction. Ces oscillations se manifestent comme des résonances dans la relation courant-phase de la jonction. Pour un champ polarisé circulairement, on trouve une règle de sélection de spin qui autorise des oscillations de Rabi seulement dans un certain interval de phases dans la relation courant-phase permettant une éventuelle détection du spin. De plus, le champ induit des transitions non-cohérentes qui nécessitent la présence d'une quasiparticule dans le continuum d'états. Ces transitions agissent comme processus de recharge et d'ionization pour les niveaux d'Andreev. Pour un champ polarisé circulairement, ces processus induits par le champ ne donnent pas lieu à un mécanisme de relaxation pour les oscillations de Rabi à cause des contraintes en spin et en énergie. Pour un champ polarisé linéairement, il n'y a pas de règle de selection de spin et la largeur des résonances de Rabi dans la relation courant-phase est déterminée par les processus d'ionization induits par le champs. Dans la jonction entre le demi-métal et le supraconducteur conventionel, il n'y a pas de courant pour une aimentation statique, puisque la polarization parfaite en spin du demi-métal interdit les processus de réflexion d'Andreev à l'interface. On demontre que pour une géométrie de point contact, un courant d'Andreev passe, si le demi-métal est soumis à la résonance ferromagnétique. La précession de la direction de l'aimentation dans le demi-métal donne lieu au mécanisme de spin-flip nécessaire. Le courant est forcé par la précession de la direction de l'aimentation qui crée une situation hors équilibre pour les porteurs de charge. De plus, dans un matériau ferromagnétique avec une densité de porteurs minoritaires non-nulle, le courant est réduit et disparaît si les densités majoritaires et minoritaires sont égales. On considère, par ailleurs, une géométrie d'interface étendue, plus réaliste. Pour une jonction ballistique, le courant est augmenté par rapport à la géometrie de point contact, en raison du nombre plus élevé de canaux. De plus, on demontre que le désordre est le plus important dans le matériau ferromagnétique. Le courant d'Andreev à travers la jonction désordonnée est beaucoup plus grand que le courant à travers la jonction ballistique dans la même géométrie. / In this thesis we investigate the possibility to change the charge current in superconducting junctions by manipulating the spin properties using magnetic resonance. We consider two different junctions: First, an unconventional Josephson junction between a conventional s-wave superconductor and an unconventional px-wave superconductor and second a half-metal/conventional superconductor junction. The spx junctions hosts two spin-polarized Andreev bound states, which are 2pi-periodic, giving rise to a spontaneous magnetization in equilibrium. This opens the possibility to manipulate the occupations of the Andreev levels using a time-dependent magnetic field. We show that the field induces coherent Rabi oscillations between different spin states of the junction that appear as resonances in the current-phase relation. For a cicularly polarized magnetic field, we find a spin selection rule, giving Rabi oscillations only in a certain range of superconducting phase differences, which provides a spin detection scheme. In contrary, for a linear polarization, there is no spin constraint on the Rabi oscillations. The field also induces non-coherent transitions including continuum states that act as refill and ionization processes for the Andreev levels. For a circularly polarized field, these field-induced processes do not provide a decay mechanism for Rabi oscillations, due to spin and energy constraints. For a linear polarization, the width of the Rabi resonances in the current-phase relation is determined by the field-induced ionization processes. In the half-metal/conventional superconductor junction no Andreev current may flow for a static magnetization direction, since the perfect spin polarization of the half-metal forbids Andreev reflection processes at the interface. We show that an Andreev current flows, if the half-metal is subject to ferromagnetic resonance. The precessing magnetization direction in the half-metal provides the necessary spin-flip mechanism. The current is driven by the precession of the magnetization direction that creates a non-equilibrium situation for the charge carriers. We also show for a point contact geometry that in a ferromagnet with non-zero minority carrier concentration the current is reduced and vanishes at equal minority and majority carrier concentrations. Additionally, we consider a more realistic, extended interface geometry. For a ballistic junction, the current is enhanced compared to a point contact geometry due to the larger number of transport channels. Furthermore, we show that disorder is most important in the ferromagnet. The Andreev current through the disordered junction is much larger than the current through a ballistic junction in the same geometry.

Made available in DSpace on 2014-07-29T15:07:11Z (GMT). No. of bitstreams: 1 Dissertacao_Marcos fisica.pdf: 953271 bytes, checksum: 27094fd5d8fb314f0df53abdef0bdb38 (MD5) Previous issue date: 2009-04-15 / The Ferromagnetic Resonance (FMR) technique at the X-band (~ 9.70 GHz) and Q-band (~ 34.0 GHz) was used to study the magnetic properties of NiO/Co, NiO/Py and Py/FeMn bilayers, and Co/NiO/Co trilayers with different thicknesses, deposited by dc magnetron sputtering technique in the presence of a 400 Oe magnetic field. The FMR experiments were all done at room temperature using a high sensitivity Bruker ESP-300 spectrometer, with static scanning field and usual modulation and phase sensitive detection techniques. The angular dependence of the inplane resonance field allows the measurement of the exchange bias field for some NiO/Py bilayers and the uniaxial anisotropy of the Co layer of NiO/Py bilayers and Co/NiO/Co trilayers. The value of the exchange bias field of the NiO/Py bilayers varies from 29 Oe to 123 Oe while the uniaxial anisotropy field of the Co layer, varies from 5 to 45 Oe. For Co/NiO/Co trilayers, the Co in-plane uniaxial anisotropy field varies from 34 to 216 Oe. The out-of-plane angular dependence of the resonance field allows the measurement of the effective magnetization and the g-factor, which depend not only on the specific parameters of the samples, such as thickness of the ferromagnetic layer, but also on the deposition conditions. The results obtained were compared with known values in the literature and show the high sensitivity of the ferromagnetic resonance technique in the study of the magnetic anisotropy and the exchange bias phenomena. / A técnica de ressonância ferromagnética (FMR) foi utilizada para estudar propriedades magnéticas de bicamadas NiO/Co, NiO/Py e Py/FeMn (Py = Ni81Fe19) e tricamadas Co/NiO/Co, com diferentes espessuras das camadas ferromagnética e antiferromagnética, depositadas pela técnica de dc magnetron sputtering na presença de campo magnético de 400 Oe. Os experimentos de FMR foram todos realizados em temperatura ambiente usando um espectrômetro Bruker ESP- 300 de alta sensibilidade, com varredura do campo estático e técnicas padrões de modulação e detecção sensível à fase, em Banda-X (~ 9.70 GHz) e Banda-Q (~ 34.0 GHz). A dependência angular do campo de ressonância res H no plano do filme permitiu medir o campo de exchange bias observado em algumas bicamadas NiO/Py. Para as demais bicamadas e as tricamadas Co/NiO/Co foi possível medir a anisotropia uniaxial no plano da camada de Co. Os valores dos campos de exchange bias medidos para as bicamadas variaram de 29 Oe a 123 Oe, enquanto que os campos de anisotropia uniaxial variaram de 5 a 45 Oe. Para as tricamadas Co/NiO/Co, os campos de anisotropia uniaxial no plano apresentaram grande variação de 34 a 216 Oe. A dependência angular do campo de ressonância fora-do-plano do filme permitiu obter os valores da magnetização efetiva e do fator-g, dependentes não somente de parâmetros específicos das amostras, como a espessura da camada ferromagnética, mas também das condições de deposição. Os resultados obtidos foram comparados com valores conhecidos na literatura e mostram a alta sensibilidade da técnica de ressonância ferromagnética para o estudo da anisotropia magnética e do fenômeno de exchange bias

The present work addresses in-depth magnetic films with magnonic surface patterning of variable size. Two different kinds of such structures referred to as surface-modulated magnonic crystals were investigated: Ion-irradiated magnonic crystals and structurally etched magnonic crystals. To achieve that, two different experimental approaches were pursued. On the one hand, the magnetic moment at the surface of lithographically patterned permalloy (Ni80Fe20) films was periodically reduced by means of ion irradiation. On the other hand, structural trenches were introduced at the surface of a pre-patterned thin film by sequential ion milling. The goal is the acquisition of a fundamental understanding of the behavior of spin-wave modes in the transition from a continuous magnetic thin film to a full magnonic crystal, i.e. separated periodic magnetic structures. In the framework of this thesis, the spin-wave eigen-modes of such magnonic crystals were mainly investigated spectroscopically by means of ferromagnetic resonance. Thereby, the “Two-magnon scattering perturbation theory” and the “plane-wave method” were employed as the theoretical methodologies to understand the complex dynamics of such systems. The first is a reliable method to calculate the dynamic response of surface-modulated magnonic crystals where the modulation is of a perturbation character, i.e. small compared to the film thickness. The latter is a quasi-analytical approach to calculate the dynamic eigen-modes of magnonic crystals consisting of different components with significantly varying properties. Moreover, numerical methods were employed to get further insight into the spin dynamics of these structures. In such systems, the spin-wave behavior follows the well-known dispersion relation of flat magnetic thin films as long as the surface-modulation is small compared to the film thickness. In this work, it was shown that this circumstance can be exploited for a parameter-free determination of the exchange constant A, which is not experimentally accessible for magnetic thin films in a straightforward manner. However, once the modulation height is of significant magnitude, the dynamics of surface-modulated magnonic crystals become substantially more complex. A straightforward understanding of such kind of system is hampered by the complex interplay of different effects. On the one hand, the internal demagnetizing field reveals an alternating character and depends itself on the modulation height and the field angle. On the other hand, the dynamic eigen-modes are hybridized, i.e., they reveal different characteristics in different regions of the magnonic crystal and, in addition, they couple to each other. Here, the approach is particularly favorable to investigate the spin dynamics of surface-modulated magnonic crystals by systematically altering the modulation height of the same sample. This is mainly due to two reasons. First, the two edge cases, namely the thin film and the full magnonic crystal, are already well understood and, second, other magnetic and structural parameters remain constant. With the help of the measurement results and the simulations, the quasi-analytical theory was validated. Subsequently, the mode profiles were calculated by theory and simulation in order to analyze the mode character in the transition from a thin film to a full magnonic crystal. Two kinds of dynamic eigen-modes were identified, namely hybridized modes and localized modes. For both types, simple formulae were derived describing their characteristic dynamic behavior. Besides, transition rules were found connecting the mode number n of film modes with the mode number m of modes in the full magnonic crystal. In order to correlate the symmetry and magnitude of the demagnetizing field with the spin-wave eigen-modes, the internal fields of a strongly surface-modulated magnonic crystal were reconstructed by electron holography measurements. By reemploying the measurement results for micromagnetic simulations, the dynamics of the whole system could be reproduced. This strategy allowed for a better understanding of the link between the demagnetizing field and the spin-wave mode characteristics. Based on these results, a simplified model for the analytical description of the inplane angular dependence was found. The acquired understanding of such systems led to the elaboration of specific applications, such as the spin-wave channelization. It should be noted that the coupling of uniform to non-uniform spin-wave phenomena, which is an intrinsic property of these structures, holds out the prospect of several applications in the future.

Popis magnetodynamických vlastností nanomagnetů a nanostrukturovaných magnetických materiálů vyžaduje metody vhodné pro zkoumání typické časové odezvy těchto systémů, tj. v řádu nanosekund a méně. Nedostatek technik, vhodných právě pro charakterizaci v časové doméně, je spojen s možnostmi současné elektroniky. Další možný přístup, jak popsat vlastnosti nanomagnetů, je charakterizace ve frekvenční doméně v pásmu GHz. Nejrozšířenější technikou charakterizace ve frekvenční doméně je měření feromagnetické rezonance (FMR). Ze spekter FMR lze získat cenné informace o systému: parametr tlumení, saturační magnetizace atd. Metoda, kterou využíváme k detekci excitací spinových vln, má za cíl zjednodušení charakterizace. Využíváme termoelektrickou detekci spinových vln v magnetických drátech prostřednictvím anomálního Nernstova jevu. Metoda je založena na disipaci tepla uvnitř magnetické vrstvy v důsledku útlumu spinových vln, a proto dochází k vytvoření teplotního gradientu směrem k substrátu (kolmo k povrchu). To vede k vytvoření elektrického pole kolmého jak na teplotní gradient, tak na směr magnetizace. Napětí je obvykle v řádu V, proto může být měřeno obvyklým laboratorním vybavením. Navzdory své jednoduchosti poskytuje tato metoda velmi zajímavé výsledky a může být použita pro charakterizaci magnonických vlnovodů, magnonických metamateriálů, emitorů spinových vln a dalších zařízení, pracujících se spinovými vlnami.

This work presents a detailed investigation of magnetization dynamics in synthetic antiferromagnets (SAFs), which has been studied both experimentally, using electrically-detected ferromagnetic resonance (ED-FMR) and vector-network analyzer-based ferromagnetic resonance (VNA-FMR), and theoretically. Two modes, one with in-phase and one with 180° out-of-phase precessing magnetizations of the layers, are identified in all applied field regimes, namely, a low-field antiferromagnetically coupled regime (when magnetizations of the layers have opposite directions), a spin-flop regime at intermediate field values (when magnetizations are non-collinear) and a high-field saturation regime (when both magnetazations are collinear to each other and the external magnetic field direction). The qualitative theoretical description, found to be in good agreement with the experimental data, is given using a system of coupled Landau-Lifshitz equations. In this work, for the first time to our knowledge, it is shown that for SAFs with different magnetic moments of the individual layers, dynamic, and not static, Zeeman and interlayer exchange coupling energies are solely resposible for the frequency-field dependence in the antiferromagnetically coupled regime. The changes in the dynamical energy terms lead to the changes in the dynamical components of the precessing magnetizations. As the external magnetic field is varied, the amplitude of the components vary continiously in different ways for different modes, dropping to zero in the spin-flop regime, which is reflected in the amplitudes of ED-FMR and VNA-FMR. This effect appears only in SAFs with different magnetic moments of the layers and is related to the different increase in the Zeeman energy of the layers as the externally applied field increases. The variation of the dynamical components leads to the modulation of an exchange of spin-angular momentum between the layers (so called spin-pumping effect).This is directly reflected in the measured linewidths for the modes in both ED-FMR and VNA-FMR. As was shown before in the literature and is also confirmed here, spin-pumping leads to a constant difference in linewidths between two modes for SAFs in the saturated case. Here, we extend the previous findings to non-saturated regimes. We show that in non-saturated states, the linewidth difference does not remain constant and varries with the external magnetic field. This linewidth difference changes can be qualitatively explained using the modulation of the dynamical magnetization components. In order to directly model the spin-pumping effects, additional terms are introduced in the coupled Landau-Lifshitz equations, related to the intrinsic damping in the magnetic layers and spin-pumping induced effects. The calculations of linewidth dependences using the extended model are not in full agreement with the experiments, suggesting that additional effects must be added to the model (for example, potential domain formations). Additionly, due to the dependence of the effect on the Zeeman energy asymmetry between the layers of the SAF, SAFs with different ratio of thicknesses are studied. Although the trends described above are common to all the samples, no significant enhancement of spin-pumping effects are observed. Modeling shows that, although the evolution of the dynamical magnetization components does depend on the magnetic moment ratio between the layers, the difference between maximum and minimum values remains almost constant and does not alter the observed linewidth-field dependence.:1 Fundamentals 1 1.1 Magnetic moment 1 1.2 Magnetic energy contributions 6 1.2.1 Zeeman energy 6 1.2.2 Demagnetization energy 7 1.2.3 Magnetic anisotropy 9 1.2.4 Direct Exchange Energy 13 1.2.5 Indirect Exchange. Interlayer Exchange Coupling Energy 15 1.3 Magnetoresistance 17 1.3.1 Ordinary Magnetoresistance 17 1.3.2 Anisotropic Magnetoresistance 20 1.3.3 Giant Magnetoresistance 22 1.4 Magnetization dynamics 25 1.4.1 Classical motivation 25 1.4.2 Quantum mechanical justification 27 1.5 Spin-currents and Spin-pumping 29 2 Experimental methods 33 2.1 Vibrating Sample Magnetometry 33 2.2 Ferromagnetic resonance 34 2.2.1 Cavity-FMR 35 2.2.2 VNA-FMR 36 2.3 Electrically detected ferromagnetic resonance 39 3 Synthetic antiferromagnets. Theoretical model 47 3.1 Static model 47 3.2 Dynamical model 54 4 Sample fabrication and characterization 58 4.1 Fabrication 58 4.2 Static characterization 62 5 Magnetization dynamics in asymmetric SAFs 68 5.1 Dynamical measurements on Py(3nm)/ Ru(0.85 nm)/Py(9 nm) 68 5.2 Theoretical explanation 71 5.3 Dynamics in SAFs with varying asymmetry of the layers 80 6 High-frequency spin-pumping in SAFs 86 6.1 Spin-pumping in magnetic trilayers 86 6.2 Spin-pumping in SAFs 88 6.3 Dependence on the asymmetry between the layers 94 7 Conclusions and outlook 99 A General description of the trilayer system without damping and spin-pumping contributions 102 A.1 Effective fields 103 A.2 Final equations 107 B Mathematica program used for the theoretical modeling 108 B.1 Matrix Elements 108 B.2 Frequency-Modes 109 B.3 Dynamical components and corresponding dynamical energies 112 B.4 Equilibrium condition 114 B.5 Dynamical Energies 118 B.6 Dynamical Trajectories 121 Bibliography 125