L'amélioration des techniques de dépôts et l’évolution de la compréhension de la physique de la matière condensée a conduit à la découverte de phénomènes nouveaux en électronique de spin (spintronique). En particulier, le retournement de l’aimantation par couple de transfert de spin et couple spin-orbite, ainsi que le développement de dispositifs basés sur la propagation d’ondes de spin ont fait de l’amortissement magnétique de Gilbert un paramètre central pour les futures technologies de stockage et de traitement de l’information. Dans cette étude, la prédiction de valeurs très faibles d’amortissement dans les alliages d’Heusler demi métaux magnétiques Co2MnZ est expérimentalement observée et directement corrélée à la structure électronique sous-jacente. En effet, en substituant l’élément Z dans des couches minces monocristallines de haute qualité de Co2MnZ (Z= Al, Si, Ga, Ge, Sn, Sb) faites par épitaxie par jet moléculaire, les propriétés électroniques telles que le gap de spin minoritaire, la position du niveau de Fermi et la polarisation en spin peuvent être accordées et leurs conséquences sur la dynamique de l’aimantation sont analysées. Les résultats expérimentaux nous permettent de comprendre la relation existante entre la structure électronique mesurée et la valeur d’amortissement magnétique, ainsi que de les comparer aux calculs ab initio. Les valeurs d’amortissement entre 4.1 x10-4 et 9 x10-4 pour Co2MnSi, Co2MnGe, Co2MnSn et Co2MnSb sont les plus petites valeurs jamais reportées pour des couches conductrices et constituent une preuve expérimentale qui confirme les prédictions théoriques sur ces alliages d’Heusler demi métaux magnétiques. Ensuite, la relation entre l’amortissement magnétique de Gilbert et le temps de désaimantation ultra-rapide induit par pulse laser dans la série d’alliages quaternaires Co2MnSixAl1-x à polarisation en spin variable est étudiée. Cette partie vise à vérifier des modèles théoriques qui essaient d’unifier ces deux quantités vivant sur des échelles de temps différentes. Finalement, les propriétés structurales et magnétiques de super réseaux Mn3Ga/Co2YZ sont étudiées dans le but de combiner un amortissement de Gilbert très faible, un gap de spin minoritaire ainsi que l’aimantation perpendiculaire aux plans des couches, une caractéristique indispensable pour des dispositifs à faible consommation d’énergie. / Improvements in thin film elaboration methods and a deeper understanding of condensed matter physics have led to new exciting phenomena in spin electronics (spintronics). In particular, magnetization reversal by spin-orbit and spin-transfer torque as well as the development of spin waves based devices have placed the Gilbert magnetic damping coefficient as a key parameter for future data storage and information processing technologies. The prediction of ultralow magnetic damping in Co2MnZ Heusler half-metal magnets is explored in this study and the damping response is shown to be linked to the underlying electronic structure. By substitution of the Z element in high quality Co2MnZ (Z=Al, Si, Ga, Ge, Sn and Sb) epitaxial thin films grown by molecular beam epitaxy, electronic properties such as the minority-spin band gap, Fermi energy position in the band gap, and spin polarization can be tuned and the consequences for magnetization dynamics analyzed. Experimental results allow us to directly explore the interplay of spin polarization, spin gap and Fermi energy position, with the magnetic damping obtained in these films (together with predictions from ab initio calculations). The ultralow magnetic damping coefficients measured in the range from 4.1 x10-4 to 9 x10-4 for Co2MnSi, Co2MnGe, Co2MnSn and Co2MnSb are the lowest values ever reported in conductive layers and offer a clear experimental demonstration of theoretical predictions on half metal magnetic Heusler compounds. Then, the relation between the Gilbert damping and the ultrafast demagnetization time in quaternary Co2MnSixAl1-x compounds with a tunable spin polarization is analyzed. This way, it is possible to confront theoretical models unifying those two quantities that live in different timescales. Finally, structural and magnetic properties of Mn3Ga/Co2YZ Heusler superlattices are investigated in order to combine ultralow Gilbert damping coefficient, minority spin band gap and perpendicularly magnetized heterostructures, another requirement for low energy consumption devices. Through the present work, we aim to prove that Heusler compounds provide an excellent playground to study fundamental magnetism and offer a pathway for future materials design.

<div>The response time of magnetization switching in current spintronic devices is limited to nanosecond timescales due to the precessional motion of the magnetization during reversal. To overcome this limit two routes of investigation leading to novel recording and logic devices are considered in this thesis: 1) Magnetic tunnel junction structures where the recording and reference layers are replaced by synthetic ferrimagnets and switching is induced by spin transfer torque and 2) Hybrid magneto-photonic devices where switching is induced by plasmon-enhanced all-optical switching. To circumvent limitations of the materials and magnetic properties of CoFeB, the most utilized alloy in spintronics, hcp-CoCrPt, a material that exhibits superior perpendicular anisotropy and thermal stability, is chosen as the ferromagnetic electrode in this work. Whereas actual devices based on the two schemes aforementioned are still in the process of being fabricated, through collaborative work with our international collaborators, this thesis describes fundamental magnetic and structural characterization needed for the realization of said ultrafast switching devices. The magnetic switching behavior of CoCrPt-Ru-CoCrPt synthetic ferrimagnets with perpendicular magnetic anisotropy have been studied in the temperature range from 2K to 300K. It was found that two sets of magnetic transitions occur in the CoCrPt-Ru-CoCrPt ferrimagnet systems studied. The first set exhibits three magnetization states in the 50K – 370K range, whereas the second involves only two states in the 2K and 50K range. The magnetic hysteresis curves of the synthetic ferrimagnet are assessed using an energy diagram technique which accurately describes the competition between interlayer exchange coupling energy, Zeeman energy, and anisotropy energy in the system. This energy diagram analysis is then used to predict the changes in the magnetic hysteresis curves of the synthetic ferrimagnet from 200K to 370K. This represents the potential operation temperature extrema that a synthetic ferrimagnet could be expected to operate at, were it to be utilized as a free layer in a memory or sensor spintronic device in the device configuration described in this dissertation.</div><div>Circularly polarized fs laser pulses generate large opto-magnetic fields in magnetic materials, through the inverse Faraday effect. These fields are attributed to be largely responsible for achieving ultrafast all-optical magnetization switching (AOS). All experimental demonstrations of AOS thus far have been realized on thin films over micron-sized irradiated regions. To achieve magnetization switching speeds in the ps and potentially fs time regimes, this work proposes the use of surface plasmon resonances at the interface of hybrid magneto-photonic heterostructures. In addition to the ability of plasmon resonances to confine light in the nm scale, the resonant excitation can largely enhance induced opto-magnetic fields in perpendicular magnetic anisotropy materials. This requires strong spin-photon coupling between the plasmonic and the magnetic materials, which thus requires the minimization of seed layers used for growth of the magnetic layer. This work reports on the development of ultrathin (1 nm thick) interlayers to control the growth orientation of hcp-Co alloys grown on the refractory plasmonic material, TiN, to align the magnetic axis out-of-plane. CoCrPtTa seed layers down to 1 nm were developed to seed the growth of CoCrPt, and the dependence of the quality of the CoCrPt is investigated as Ta composition is varied in the seed layer. Whereas bismuth iron garnet (BIG) meets the magneto-optical requirements for a hybrid magneto-photonic material, its magnetic and structural properties are highly sensitive to the Bi:Fe ratio and must be grown epitaxially on single crystalline substrates. Therefore, in this work we have investigated alternative materials that offer superior magnetic properties and are amenable to growth on inexpensive substrates. Opto-magnetic field enhancements up to 2.6x in Co-ferrite magneto-photonic heterostructures have been obtained via finite element analysis modelling. Alternative materials for plasmon-enhanced all-optical switching such as Co/Pd multilayers have also been investigated. Successful growth of Co/Pd multilayers on TiN using ultrathin Ti interlayers has been achieved. </div><div><br></div>

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Les mémoires MRAM (Magnetic Random Access Memory) sont l’une des technologies émergentes visant à devenir un dispositif de mémoire «universelle» applicable à une grande variété d’applications. La combinaison du couple de spin-orbite (SOT) résultant de l’effet Hall de spin (SHE) et de l’interaction de Dzyaloshinskii – Moriya (DMI) aux interfaces entre un métal lourd et une couche ferromagnétique s’est révélée être un mécanisme efficace pour induire une propagation de parois magnétiques chirales à des faibles densité de courant. Les dispositifs à parois magnétiques devraient constituer la prochaine génération de supports d’information en raison de leur potentiel pour des densités de stockage très élevées. Cependant, une limitation cruciale est la présence de défauts structuraux qui piègent les parois magnétiques et induisent des courants de seuil élevés ainsi que des effets stochastiques importants. L’origine du piégeage résulte de la présence de défauts structuraux aux interfaces entre la couche magnétique ultra-mince et les autres couches (isolants et/ou métaux lourds) qui induisent une distribution spatiale des propriétés magnétiques comme l’anisotropie magnétique perpendiculaire (PMA) ou le DMI. Comprendre l’influence de la structure des interfaces sur la propagation de parois et sur le DMI en particulier est cruciale pour la conception de futurs dispositifs basse consommation. C’est dans ce contexte très novateur que mon doctorat s’est focalisé sur la manipulation de la structure des interfaces dans des couches ultra-minces à anisotropie magnétique perpendiculaire. Des structures de CoFeB-MgO ont été utilisées afin de mieux comprendre l'impact de la structure des interfaces sur l’anisotropie, le DMI, la propagation de parois et les phénomènes de SOT. L’approche innovante que nous avons utilisée est basée sur l’irradiation par des ions légers pour contrôler le degré de mélange aux interfaces. Sous l’effet du mélange induit par l’irradiation, nous avons observé dans des structures de W-CoFeB-MgO une forte augmentation de la vitesse de parois dans le régime de creep, compatible avec une réduction de la densité des centres de piégeage. Nous avons aussi démontré que l'anisotropie de l'interface Ki et le DMI mesuré par propagation asymétrique de parois se comportent de la même façon en fonction du mélange aux interfaces. Finalement, nous avons fabriqué des barres de Hall afin de mesurer la commutation de l’aimantation induite par SOT. Le centre des croix de Hall a été irradié afin de diminuer localement l’anisotropie. Nous avons observé une réduction de 60% de la densité de courant critique après l’irradiation correspondant au retournement des croix de Hall irradiés par propagation de parois. Notre étude fournit de nouvelles pistes concernant le développement de mémoires magnétiques à faible consommation, de dispositifs logiques et neuromorphiques. / Magnetic Random Access Memory (MRAM), as one of the emerging technologies, aims to be a “universal” memory device for a wide variety of applications. The combination of the spin orbit torque (SOT) resulting from the spin Hall effect (SHE) and the Dzyaloshinskii–Moriya interaction (DMI) at interfaces between heavy metals and ferromagnetic layers has been demonstrated to be a powerful mean to drive efficiently domain-wall (DW) motion, which are expected to be the promising next generation of information carriers owing to ultra-low driving currents and ultra fast DW motion. However, the crucial limitation of SOT induced domain wall motion results from the presence of pinning defects that can induce large threshold currents and stochastic behaviors. Such pinning defects are strongly related to structural inhomogeneities at the interfaces between the ultra-thin ferromagnetic layer and the other materials (insulator and/or heavy metals) that induce a spatial distribution of magnetic properties such as perpendicular magnetic anisotropy (PMA) or DMI. Therefore, understanding the role of the interface structure on DW motion and DMI is crucial for the design of future low power devices.It is under this innovative context that my Ph.D. research has focused on the manipulation of interface structure in ultra-thin magnetic films with perpendicular magnetic anisotropy. CoFeB-MgO structures have been used in order to understand the impact of interface structure on anisotropy, DMI, domain wall motion and SOT phenomena. The innovative approach we have used in this PhD research is based on light ion irradiation to control the degree of intermixing at interfaces. In W-CoFeB-MgO structures with high DMI, we have observed a large increase of the DW velocity in the creep regime upon He⁺ irradiation, which is attributed to the reduction of pinning centres induced by interface intermixing. Asymmetric in-plane field-driven domain expansion experiments show that the DMI value is slightly reduced upon irradiation, and a direct relationship between DMI and interface anisotropy is demonstrated. Using local irradiated Hall bars in SOT devices, we further demonstrate that the current density for SOT induced magnetization switching through DW motion can be significantly reduced by irradiation. Our finding provides novel insights into the development of low power spintronic-memory, logic as well as neuromorphic devices.

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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.

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