Nanoscale materials behave differently than their bulk counterparts due, in part, to the reduced length scales and the increased surface to core atom ratio. As the length scales decrease, the surface atoms become increasingly important as they make up a larger percentage of the total number of atoms. These surface atoms have magnetic properties that differ from the core atoms due to a surface anisotropy that alters the interparticle, intraparticle, and exchange interactions. In this work, we have synthesized three different nanoscale systems that will allow us to explore the physics of the different interactions. Cu/gamma-Fe2O3 core/shell nanoparticles were chosen because the gamma-Fe2O3 cores have vacancies in their B-sites, broken coordination at the surface, and experience superexchange interactions. As a comparison, multiphase undoped and V-doped SiO2/FeCo nanoparticles were chosen as these nanoparticles do not suffer from vacancies or surface disorder and experience both direct exchange interactions from the nanoparticle core and superexchange interactions between the FeCo core and the metal silicate interfacial phase. Finally, Fe nanocrystallites were grown in a Cu matrix as they present no vacancies or surface disorder, and they are single phase. We observed that the interfacial phases that form in these core/shell and nanocrystallite/matrix nanoscale systems alters significantly the physics of the magnetism. The overall magnetic properties, the elemental magnetism, and the atomic magnetism were all observed to be altered by this interfacial phase, along with the interparticle and intraparticle interactions. In addition, the thickness of this interfacial phase, and thus the strength of its affect, was controlled by controlling the thickness of the shells or the amount of intermixing in the case of the nanostructured thin film. / February 2016

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<p>Magnetic properties of selected compounds and superlattices have been experimentally studied using SQUID (superconducting quantum interference device) and VSM (vibrating sample magnetometer) magnetometry, neutron diffraction and Mössbauer spectroscopy measurements combined with theoretical <i>ab initio</i> calculations. </p><p>The magnetic compounds (Fe_1-xM_x)₃P, M=Co or Mn have been studied extensively. It was found that Co can substitute Fe up to <i>x</i>=0.37. Increasing the Co content leads to a reduction of the Curie temperature and the magnetic moment per metal atom. Mn can substitute Fe up to<i> x</i>=0.25 while Fe can be substituted into Mn₃P to 1-<i>x</i>=0.33. On the iron rich side, the drop in Curie temperature and magnetic moment when increasing the Mn content is more rapid than for Co substitution. On the manganese rich side an antiferromagnetic arrangement with small magnetic moments was found. </p><p>The interlayer exchange coupling and the magnetocrystalline anisotropy energy of Fe/V superlattices were studied. The coupling strength was found to vary with the thickness of the iron layers. To describe the in-plane four-fold anisotropy, the inclusion of surface terms proved necessary. </p><p>The in-plane four fold anisotropy was also studied in a series of Fe/Co superlattices, where the thickness of the Co layers was kept thin so that the bcc structure could be stabilized. Only for samples with a large amount of iron, the easy axis was found to be [100]. The easy axis of bulk bcc Co was therefor suggested to be [111]. </p>

Magnetic properties of selected compounds and superlattices have been experimentally studied using SQUID (superconducting quantum interference device) and VSM (vibrating sample magnetometer) magnetometry, neutron diffraction and Mössbauer spectroscopy measurements combined with theoretical ab initio calculations. The magnetic compounds (Fe1-xMx)3P, M=Co or Mn have been studied extensively. It was found that Co can substitute Fe up to x=0.37. Increasing the Co content leads to a reduction of the Curie temperature and the magnetic moment per metal atom. Mn can substitute Fe up to x=0.25 while Fe can be substituted into Mn3P to 1-x=0.33. On the iron rich side, the drop in Curie temperature and magnetic moment when increasing the Mn content is more rapid than for Co substitution. On the manganese rich side an antiferromagnetic arrangement with small magnetic moments was found. The interlayer exchange coupling and the magnetocrystalline anisotropy energy of Fe/V superlattices were studied. The coupling strength was found to vary with the thickness of the iron layers. To describe the in-plane four-fold anisotropy, the inclusion of surface terms proved necessary. The in-plane four fold anisotropy was also studied in a series of Fe/Co superlattices, where the thickness of the Co layers was kept thin so that the bcc structure could be stabilized. Only for samples with a large amount of iron, the easy axis was found to be [100]. The easy axis of bulk bcc Co was therefor suggested to be [111].

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

Le but de cette thèse est l'étude des propriétés de jonctions tunnel magnétiques à aimantation perpendiculaire, en utilisant l'anisotropie perpendiculaire présente à l'interface entre un métal magnétique et un oxyde. En théorie, dans le cas des applications mémoires, les jonctions tunnel perpendiculaires devraient nécessiter moins d'énergie (courant) pour l'écriture par courant polarisé en spin. Mais la fabrication de telles structures représente un défi et une tâche difficile puisque les propriétés de transport (TMR) et d'anisotropie imposent des contraintes sur les matériaux utilisées en limitant la fenêtre de travail, notamment en ce qui concerne l'épaisseur des couches magnétiques. Pour atteindre cet objectif nous avons tout d'abord étudié les propriétés de ces structures comme l'anisotropie de l'interface métal magnétique-oxyde, le transport tunnel et le couplage entre les couches magnétiques à travers la barrière isolante. L'amplitude de l'anisotropie d'interface entre un métal magnétique et un oxyde dépend de l'épaisseur des couches magnétiques, de la température de recuit et la concentration de l'oxygène à l'interface. Différentes structures ont été réalisées afin de choisir la structure la mieux adaptée pour les applications mémoires MRAM. Une corrélation entre la TMR et l'anisotropie a été observée permettant de valider l'origine de l'anisotropie perpendiculaire : la formation de liaisons métal magnétique-oxygène. Un couplage antiferromagnétique à été aussi observé entre les couches magnétiques à anisotropie perpendiculaire à travers l'oxyde. Une étude détaillée sur le couplage a été faite en fonction de la température de recuit et de l'épaisseur des couches magnétiques pour mieux comprendre l'origine du couplage et une possible relation avec l'amplitude de l'anisotropie perpendiculaire. Finalement des jonctions perpendiculaires ont été nano-lithographiées et des mesures de commutation d'aimantation par transfert de spin sur des piliers nanométriques ont été réalisées avec de faibles courants critiques. / The aim of this thesis is the study of magnetic tunnel junctions with perpendicularly magnetized electrodes (pMTJ), using perpendicular magnetic anisotropy (PMA) arising from the magnetic metal/oxide interfaces. For magnetic memories applications, it was predicted in theory that perpendicular junctions should need less energy (current) for spin transfer torque (STT) writing applications. However, the engineering of such structures is a real challenge and a difficult task since simultaneous transport (TMR) and PMA properties impose constraints on materials being used and also limit the working window of the device, especially in terms of magnetic layer thickness. In order to reach our goal we first studied different properties of these structures, such as the origin of PMA from the metal/oxide interface, tunnel transport and interlayer exchange coupling phenomena. The PMA at magnetic metal/oxide interface was showed to strongly depend on different parameters like annealing temperature, oxygen concentration, layer thickness etc. Several pMTJ structures were tested in order to choose the best one for MRAM memories applications. A correlation between TMR and PMA was observed and confirms the PMA origin from the magnetic metal-oxygen bond formation at the interface. Furthermore, antiferromagnetic interlayer exchange coupling was observed in our structures in the presence of out of plane anisotropy. A detailed study was made as a function of annealing temperature and layers thickness, in order to understand the origin of this coupling and its possible relationship to the anisotropy strength. Finally the STT-pMTJ concept was validated and low critical currents were observed on submicronic dots prepared by electron beam lithography.

Ce travail de thèse portait sur la synthèse et l'étude de complexes magnétiques redox-actifs comme prototypes pour la conception d'aimants moléculaires à haute température. L'activité redox est assurée par le ligand pontant, qui peut moduler et parfois améliorer significativement les propriétés magnétiques. Après un chapitre d'introduction présentant les derniers développements dans le domaine des matériaux magnétiques moléculaires, un accent particulier est mis sur l'importance d'avoir un fort couplage d'échange magnétique J entre les porteurs de spin. Une étude bibliographique présentant deux approches émergentes pour augmenter J dans les composés polynucléaires est également présentée et discutée. Le chapitre 2 présente les synthèses et caractérisations de complexes dinucléaires [M2(tphz)(tpy)2](PF6)n (M = Co(II) ou Ni(II); n = 4, 3, 2, tphz = tétrapyridophénazine, tpy = terpyridine) construits à partir de ligands pontant (tphz) et bloquant (tpy) fortement coordinants et redox-actifs. Les études approfondies de ces composés montrent que le ligand pontant redox-actif peut être utilisé comme un outil de choix pour promouvoir une délocalisation des spins, de forts couplages magnétiques, ainsi que de la commutabilité. L’analyse des résultats obtenus permet également de mieux comprendre les paramètres clés pour l’élaboration de systèmes fortement couplés magnétiquement. Dans le prolongement de ce travail visant à sélectionner les meilleurs composants pour la conception rationnelle d'aimants moléculaires à haute température, le chapitre 3 décrit une nouvelle série de complexes mononucléaires [Cr(III)(tphz)(tpy)](CF3SO3)n (n = 3, 2, 1). Les complexes mono- et doublement réduits présentent des interactions magnétiques remarquablement fortes entre les ions métalliques et les ligands radicalaires, et pourraient servir d'unités magnétiques intéressantes pour la conception d'aimants de plus hautes nucléarités. / The thesis work aims at the synthesis and study of redox-active magnetic molecules as prototypes towards the design of molecule-based magnets with high operating temperature, a prerequisite for technological applications. The redox activity is provided by the bridging ligand, which could tune and sometimes enhance significantly the magnetic properties of the resulting molecular architectures. After an introduction chapter presenting the latest developments in the field of molecule-based magnetic materials, special emphasis is given on the importance of having large magnetic exchange coupling J between the spin carriers to reach high operating temperature. This is supported by a bibliographic study concerning two emerging approach to enhance J values in polynuclear compounds. Chapter 2 presents the syntheses and characterizations of dinuclear M(II) complexes [M2(tphz)(tpy)2](PF6)n (M = Co or Ni; n = 4, 3, 2, tphz = tetrapyridophenazine) built by using strongly complexing, redox-active bridging ligand (tphz), and terpyridine (tpy) as capping ligands. The extensive studies on these compounds show that the redox-active bridging ligand can be used as a tool to promote spin delocalization, high spin complexes and magnetic multi-switchability. Importantly the work reveals the key parameters towards building strongly magnetically coupled systems. As a continuation research of finding the best magnetic components for the rational design of high temperature molecule-based magnets, Chapter 3 describes a new series of [Cr(III)(tphz)(tpy)](CF3SO3)n (n = 3, 2, 1) mononuclear complexes. Both the mono and doubly-reduced complexes show remarkable magnetic interactions between metal center and radical ligands, which could further act as interesting magnetic units for the design of higher nuclearities magnets.