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A Lagrangian formalism is used to find steady-state solution of the Landau–Lifshitz–Gilbert–Slonczewski equation corresponding to the linear autonomous dynamics of a magnetic auto-oscillatory system subject to the action of a spin-polarized electric current. In such a system, two concurrent dissipative mechanisms, arising from the positive intrinsic dissipation and the negative current-induced one, take place simultaneously and make the excitation of a steady precessional motion of the magnetization vector conceivable. The proposed formulation leads to the definition of a complex generalized non-Hermitian Eigenvalue problem, both in the case of a macrospin model and in the more general case of an ensemble of magnetic particles interacting each other through magnetostatic and exchange interactions. This method allows to identify the spin-wave normal modes which become unstable in the presence of the two competing dissipative contributions and provides an accurate estimation of the value of the excitation threshold current.

International audience; Time-resolved scanning transmission x-ray microscopy measurements were performed to study the current-induced magnetization switching mechanism in nanopillars exhibiting strong perpendicular magnetic anisotropy. This technique provides both short-time (70 ps) and high-spatial (25 nm) resolutions. Direct imaging of the magnetization demonstrates that, after an incubation time of ∼1.3 ns, a 100 × 300 nm 2 ellipsoidal device switches in ∼1 ns via a central domain nucleation and opposite propagation of two domain walls toward the edges. High domain-wall velocities on the order of 100 m/s are measured. Micromagnetic simulations are shown to be in good agreement with experimental results and provide insight into magnetization dynamics during the incubation and reversal periods. Spin-polarized current-induced magnetization switching (CIMS) has now been reported in many experimental works involving a wide variety of geometries including point contacts, nanopillars (spin valves or tunnel junctions), and nanowires with or without notches. 1 These systems are extensively studied, in part, because they hold the potential for applications in spin-transfer magnetic random access memory. 2 Interest in materials with perpendicular magnetic anisotropy (PMA) has grown considerably as a pathway for lowering the critical current required to switch the magnetization while maintaining thermal stability as compared with in-plane systems. 3,4 This interest has, to a large degree, stemmed from calculations of the switching behavior using a macrospin approximation. 5

Electrical transport characteristics of structures consisting of normal metals and ferromagnetic materials depend strongly on the magneti- zation direction of the ferromagnets. Thus, different spin polarizations can lead to different resistance values of such structures. The absorp- tion of spin polarized electrons in a ferromagnetic material (spin transfer torque) by domain/domain walls leads to magnetization switching or domain wall movement. This can be achieved by driving a current perpendicular to the plane of the ferromagnet (CPP) or in the plane (CIP). In this experiment we investigate the magnetization behavior of ferromagnetic nanopillars located between two lateral spin injectors in the CIP configuration. Using Scanning Transmission X-ray Mircoscopy (STXM) these studies will give more insights in the switching behav- ior and dynamics. Technological applications can mostly be found in memory structures, where the magnetization can be stored and read out.

The charge and spin diffusion equations taking into account spin-flip and spin-transfer torque were numerically solved using a finite element method in complex non-collinear geometry with strongly inhomogeneous current flow. As an illustration, spin-dependent transport through a non-magnetic nanoconstriction separating two magnetic layers was investigated. Unexpected results such as vortices of spin-currents in the vicinity of the nanoconstriction were obtained. The angular variations of magnetoresistance and spin-transfer torque are strongly influenced by the structure geometry., Comment: 11 pages, 5 figures

This thesis deals with MRAMs, new non volatile memories using spintronics original properties. The goal of this work has mainly been to demonstrate that a new MRAM concept was possible to break through limitations due to previous generations (TA-MRAM and STT-RAM). Their respective advantages have been combinated, that is to say thermal stability for the TA-MRAM and writing without any magnetic field for the STT-RAM. A first demonstration of this new STT-TA-MRAM has been given at the beginning of this study, followed by improvements on the structure with significantly better electrical results. As a consequence, the spin transfer torque switching of the storage layer has been a key point of our investigation. Moreover, an innovative experimental setup has been tested to measure the real time magnetization reversal in order to understand better the writing physics. To finish, the tunnel barrier lifetime has been explored, showing that breakdown is slower than expected for short pulses.; Cette thèse s'inscrit dans la thématique des MRAM, nouvelles mémoires non volatiles utilisant des propriétés originales de l'électronique de spin. Le but de ce travail a été principalement de démontrer qu'un nouveau concept de MRAM était possible afin de passer outre les limitations imposées par chacune des générations déjà existantes (TA-MRAM et STT-RAM). Pour cela, leurs avantages respectifs ont été combinés, à savoir la stabilité thermique pour la TA-MRAM et l'écriture sans champ magnétique pour la STT-RAM. C'est ainsi qu'a été donnée au cours de cette étude la première démonstration de STT-TA-MRAM ainsi qu'une optimisation de ses propriétés grâce une structure améliorée. Le retournement de la couche de stockage par couple de transfert de spin a donc été au centre de ces recherches. Un montage expérimental innovant a également permis d'observer le retournement de l'aimantation en temps réel dans le but de mieux comprendre la physique de l'écriture. Enfin, la problématique de la durée de vie des barrières tunnel a été abordée montrant que celles-ci claquent plus lentement que prévu pour les impulsions courtes.

International audience; An experimental technique has been developed to characterize spin-transfer driven magnetization dynamics. It was tested on a nanopillar spin valve with perpendicular anisotropy by measuring the nanopillar voltage under ac injected current ͑dV / dI͒, and ac magnetic field ͑dV / dH͒. Both the amplitude and the sign of the signals are different which reveals the different influences of the current and the field on the magnetization dynamics. Comparison between experiments and macrospin simulation shows that dV / dH measurements reveal the presence of a " canted state " demonstrating that dV / dH and dV / dI measurements are complementary techniques to probe magnetic states and their dynamics. It is now well established that magnetization switching or precession can be induced by spin polarized injected current due to the spin transfer torque ͑STT͒ effect. 1,2 This topic has been extensively studied because of the interesting physics and the potential applications for spin transfer magnetic random access memories and high-frequency oscillators.

In this thesis magnetic domain walls in laterally confined ferromagnetic elements are studied, as well as the influence of a spin polarized current on domain wall motion (spin transfer torque effect). One important aspect is the impact of finite temperatures on domain walls. Computer simulations based on a classical extended Heisenberg approach are performed. Furthermore, a novel micromagnetic macrospin model for finite temperatures (Landau-Lifshitz-Bloch equation) is employed.In magnetic nanostructures constrictions are a useful tool to position and manipulate domain walls. In this thesis we investigate the spin structure of domain walls in constrictions down to widths of 20nm at zero Kelvin. Depending on the details of the geometry, two types of domain walls, symmetric and asymmetric walls, are observed. In order to analyze the spin structures an opening angle of the domain wall is introduced and an average domain wall width is calculated.Within the framework of the Landau-Lifshitz-Bloch equation derived by Garanin domain walls in Permalloy nanorings are investigated at finite temperatures. Vortex domain walls can be found in more narrow rings than in comparison to results of micromagnetic simulations at zero Kelvin. This trend is also observed in existing experiments performed at room temperature.Apart from the influence of geometry on the domain wall structure current-induced domain wall motion is investigated. Rather than conventional micromagnetic methods, we use an extended classical Heisenberg spin model approach, which is well suited to study domain walls at elevated temperatures. We compute the behavior of domain walls in a one dimensional chain for the cases, that currents are injected using adiabatic and non-adiabatic spin torque terms. It is found that the influence of thermal fluctuations is pronounced close to the critical effective spin current, respectively close to the Walker threshold. The behavior observed depends strongly on the value of the nonadiabatic prefactor. For the adiabatic case as well as the nonadiabatic cases with nonadiabatic prefactors smaller than the Gilbert damping constant the domain wall velocities increase with increasing temperature. In contrast the domain wall velocities decrease with increasing temperature for cases with nonadiabatic prefactors larger than the Gilbert damping constant.Furthermore current-induced domain wall motion is studied at finite temperatures using micromagnetic simulations. For this purpose we extend the Landau-Lifshitz-Bloch equation of motion by adding an adiabatic and nonadiabatic spin torque term. Here, the spin torque terms are now temperature dependent. We investigate analytically as well as numerically domain wall motion at various temperatures for the adiabatic and nonadiabatic cases. The Walker threshold as well as the domain wall velocities show a strong temperature dependence. Furthermore a different behavior is found for the temperature-dependent Walker threshold and domain wall velocities assuming the Gilbert form of damping or the Landau-Lifshitz form.

Magnetization dynamics in nano-contact spin torque oscillators (STOs) is investigated from an experimental and theoretical point of view. The fundamentals of magnetization dynamics due to spin transfer torque are given. A custom-made high frequency (up to 46 GHz) in large magnetic fields (up to 2.2 T) microwave characterization setup has been built for the purpose and described in this thesis. A unique feature of this setup is the capability of applying magnetic fields at any direction θe out of the sample plane, and with high precision. This is particularly important, because the (average) out-of-plane angle of the STO free magnetic layer has fundamental impact on spin wave generation and STO operation. By observing the spin wave spectral emission as a function of θe, we find that at angles θe below a certain critical angle θcr, two distinct spin wave modes can be excited: a propagating mode, and a localized mode of solitonic character (so called spin wave bullet). The experimental frequency, current threshold and frequency tuneability with current of the two modes can be described qualitatively by analytical models and quantitatively by numerical simulations. We are also able to understand the importance, so far underestimated, of the Oersted field in the dynamics of nano-contact STOs. In particular, we show that the Oersted field strongly affects the current tuneability of the propagating mode at subcritical angles, and it is also the fundamental cause of the mode hopping observed in the time-domain. This mode hopping has been observed both experimentally using a state-of-the-art real-time oscilloscope and corroborated by micromagnetic simulations. Micromagnetic simulations also reveal details of the spatial distribution of the spin wave excitations. By investigating the emitted power as a function of θe, we observed two characteristic behaviors for the two spin wave modes: a monotonic increase of the power for increasing out-of-plane angles in the case of the propagating mode; an increase towards a maximum power followed by a drop of it at the critical angle for the localized mode. Both behaviors are reproduced by micromagnetic simulations. The agreement with the simulations offers also a way to better understand the precession dynamics, since the emitted power is strongly connected to the angular variation of the giant magnetoresistance signal. We also find that the injection locking of spin wave modes with a microwave source has a strong dependence on θe, and reaches a maximum locking strength at perpendicular angles. We are able to describe these results in the theoretical framework of non-linear spin wave dynamics. QC 20101130

Die vorherrschenden Magnetisierungskonfigurationen in Mikrometer kleinen weichmagnetischen Drähten und Scheiben sind Domänenwände und Vortices. Diese Arbeit beschäftigt sich mit der Manipulation solcher Konfigurationen mithilfe spinpolarisierter Ströme. Für die Untersuchung der lithografisch hergestellten Strukturen werden Röntgen-Photoemissions-Elektronen-Mikroskope verwendet. Unter Ausnutzung des zirkularen Dichroismus kann die Magnetisierung in der Probe hoch aufgelöst abgebildet werden. Die Experimente teilen sich in zwei Gruppen: Untersuchungen der strominduzierten Domänenwandbewegung in magnetischen Drähten und Untersuchungen zu strominduzierten Vortexkernverschiebungen in magnetischen Scheiben. Erstere beschäftigen sich unter anderen mit systematischen Studien zu den kritischen Stromdichten, die erforderlich sind, um Domänenwände zu verschieben. Des Weiteren werden die strominduzierten Umwandlungen des Domänenwandtypes sowie die Abhängigkeit der Domänenwandgeschwindigkeit von der Stromdichte untersucht. Diese Studien beziehen sich auf das Material Permalloy (Ni80Fe20). Die Ergebnisse erlauben Rückschlüsse bezüglisch des nicht-adiabatischen Spintransfer Terms. Dieser Beitrag und der mit ihm assoziierte nicht-adiabatische Parameter wird gegenwärtig kontrovers diskutiert. Die Untersuchungen zeigen unter anderem, dass der nicht-adiabatische Parameter größer als die Dämpfungskonstante ist. Die Studien zur strominduzierten Domänenwandverschiebung werden vervollständigt durch Untersuchungen an anderen Materialien, wie zum Beispiel bei der Multilagenschicht Pt/CoFeB/Pt, in dem die Magnetisierung aus der Probenebene hinaus zeigt. In diesem System wird der dominante Einfluss des Oersted Feldes auf die magnetischen Domänenkonfiguration nachgewiesen. Dieser ermöglicht alternativ zu dem Spintransfer eine gezielte Manipulation der Spinkonfiguation über das Oersted Feld. Die Untersuchungen der strominduzierten Vortexkernverschiebung in magnetischen Scheiben basieren auf einer neuenMessmethode, die im Rahmen der Doktorarbeit entwickelt wurde. Diese erlaubt es, während der Strominjektion Bilder der magnetischen Konfiguration aufzunehmen. Unter Ausnutzung der speziellen Vortextopologie gelingt so erstmals eine Trennung der verschiedenen Effekte, über die der Strom auf die Spinstruktur wirkt. Dadurch lassen sich die relativen Stärken des adiabatischen und nicht-adiabatischen Spin-Transfer Terms sowie des Oersted Feldes bestimmen. Insbesondere gelingt auf diese Weise eine Bestimmung des nicht-adiabatischen Parameters zu 0.15. Die experimentellen Ergebnisse werden ergänzt durch analytische Rechnungen sowie mikromagnetische Simulationen, die einen besseren Vergleich von Theorie und experimentellen Ergebnissen gestatten und somit zu einem besseren Verständnis beitragen. Die im Verlauf dieser Arbeit erhaltenen Ergebnisse zeigen deutlich die wichtige Rolle des nicht-adiabatischen Term für den Spintransfer zwischen injizierten Strom und lokaler Magnetisierung.

International audience; This chapter presents a review on spin transfer torque in magnetic tunnel junctions. In the first part, we propose an overview of experimental and theoretical studies addressing current-induced magnetization excitations in magnetic tunnel junctions. The most significant results are presented and the main observable characteristics are discussed. A description of the mechanism of spin transfer in ferromagnets is finally proposed. In the second part, a quantum description of spin transport in magnetic tunnel junctions with amorphous barrier is developed. The role of spin-dependent reflections as well as electron incidence and spin-filtering by the barrier are described. We show that these mechanisms give rise to specific properties of spin transfer in tunnel junctions, very different from the case of metallic spin-valves. In the third part, the theoretical observable features of spin transfer in magnetic tunnel junctions are derived and the validity of these results is discussed and compared to recent experiments. To conclude this chapter, we study the mechanism of spin transfer in half-metallic tunnel junctions, expected to mimic MgO-based magnetic tunnel junctions.

The nature of spin wave modes excited by spin-polarized direct current in a spin-torque auto-oscillator based on a magnetic nanocontact was studied by a micromagnetic simulation in the case when the external bias magnetic field was rotated from the in-plane to perpendicular-to-plane orientation. In qualitative agreement with the weakly-nonlinear analytical theory it was found, that at a certain critical angle, an abrupt switching from the self-localized nonlinear "bullet" mode to a propagating quasi-linear Slonczewski mode takes place, and is accompanied by an upward jump in generated microwave frequency. It was, also, found that the analytical theory overestimates the magnitude of a critical magnetization angle, corresponding to the mode switching, and that the magnitude of the frequency jump caused by the mode switching is inversely proportional to the nanocontact radius.

This chapter presents a review on spin transfer torque in magnetic tunnel junctions. In the first part, we propose an overview of experimental and theoretical studies addressing current-induced magnetization excitations in magnetic tunnel junctions. The most significant results are presented and the main observable characteristics are discussed. A description of the mechanism of spin transfer in ferromagnets is finally proposed. In the second part, a quantum description of spin transport in magnetic tunnel junctions with amorphous barrier is developed. The role of spin-dependent reflections as well as electron incidence and spin-filtering by the barrier are described. We show that these mechanisms give rise to specific properties of spin transfer in tunnel junctions, very different from the case of metallic spin-valves. In the third part, the theoretical observable features of spin transfer in magnetic tunnel junctions are derived and the validity of these results is discussed and compared to recent experiments. To conclude this chapter, we study the mechanism of spin transfer in half-metallic tunnel junctions, expected to mimic MgO-based magnetic tunnel junctions.

We have investigated the pinning of laterally confined head-to-head domain walls at constrictions. The experiments provide insight into the static properties of pinned domain walls. We found that position, extent, and pinning strength can be reliably determined by transport measurements. The position of the domain walls inside the pinning potential has been determined from the angular dependence of the depinning field, and we found that transverse walls are pinned symmetrically inside the notch whereas vortex walls are pinned on either side, but repelled from the centre of the notch. Both domain wall types can be stable in different regions of the same sample, if one tries to lift the vortex wall over the barrier separating the two individual potential wells, the wall is eventually transformed into a transverse wall which stays pinned inside the notch [KVW+04]. Depinning measurements constitute a solid base for understanding the potential landscape around a constriction and are sufficiently understood to be used as a tool to gain further insight into dynamic processes.The results of our combined current- and field- induced domain wall propagation experiments at different temperatures provide insight into the interplay between the adiabatic and non-adiabatic spin-transfer torque terms and thermal effects. We could demonstrate the importance of thermal activations for the current- and field-induced case: while for purely field-induced domain wall propagation the field threshold is always lowered for increasing temperatures, the current threshold shows a non-monotonous dependence on the temperature.Even though we could confirm purely current driven domain wall propagation, we found an intrinsic dependency of the spin-torque efficiency on the temperature which has the opposite trend as expected from the simple theory, pointing to the need of further theoretical studies. Having determined width and depth of the pinning potential and the interaction with currents, we have performed domain wall excitation experiments with AC currents and showed that domain walls can be described as quasiparticles with an effective mass, oscillating in the pinning potential. We have presented a novel physical effect, the homodyne rectification of AC currents by oscillating domain walls, which we explained by micromagnetic simulations and a harmonic oscillator model. The results obtained using homodyne rectification have been confirmed by a second independent method, the depinning spectroscopy. It was found that the homodyne detection is much faster and additionally offers a higher frequency resolution than the depinning spectroscopy.The homodyne detection method has the additional advantage that the parameters power, temperature and external field can be varied and thus their influence on the domain wall resonance and on the underlying potential well shape can be studied. We presented a fast and reliable method based on the homodyne detection to determine the polarity of vortex cores, demonstrating an inversion of the vortex core polarity using out-of-plane magnetic fields and resonant microwave pulses. The homodyne detection technique was used to directly measure the local profile of a symmetric pinning potential, and an experiment has been proposed to completely chart an asymmetric pinning potential.

Magnetic thin films and patterned nanostructures have been studied with respect to their magnetic properties using SQUID-magnetometry, magnetic force microscopy, electrical measurements, and micromagnetic calculations. Properties of vortex domain walls, trapped in Permalloy nanowires with artificial constrictions, were investigated experimentally and by numerical calculations. In particular, the geometrical extent and strength of the pinning potential were evaluated. In these wires, long-range vortex domain wall displacement induced by spin polarized alternating currents was obtained numerically at reduced threshold current densities as compared with the direct current case. Due to the asymmetry of the energy potential, the long-range displacement direction is determined by the vortex chirality. Strained FeCo/Pt superlattices with strong perpendicular anisotropy were investigated experimentally. The strain was controlled by varying the thickness of each alternating layer with monolayer precision and was found to have a dominating effect on the total anisotropy. Epitaxial films of the diluted magnetic semiconductor (Ga,Mn)As were studied with focus on how the ferromagnetic transition temperature could be controlled by post-growth annealing. The ferromagnetic transition temperature was enhanced by approximately 85% for a Mn-doping concentration of 6% under certain conditions. A method to manipulate micrometer sized magnetic particles on patterned arrays of elliptical Permalloy microstructures was studied. Controlled motion and separation of the magnetic particles were obtained using applied rotating magnetic fields. The domain structure of the elliptical elements was studied numerically.

In this thesis, the spin structure of domain walls in confined magnetic elements was determined and the behaviour of domain walls on the application of external magnetic fields and current pulse injection was observed. For this, magnetic elements of various shapes, materials, and substrates were prepared which were each dedicated to the purpose of the experiment. Different fabrication techniques were described, including several patterning processes and contacting of magnetic elements for current-pulse experiments. The spin structure of transverse walls in constrictions down to 30nm in permalloy wavy lines was measured using electron holography. It is known that the domain wall spin structure influences both the magnetoresistance and spin transfer torque effects. For the transverse walls, three different types were determined and the domain wall width was found to decrease faster than linearly with decreasing constriction width. The magnetic fields needed to depin the domain walls from such constrictions were measured using magnetoresistance measurements and were directly related to the domain wall spin structure. While an approach was made to image the spin structure of patterned thin films of the halfmetal CrO2 coupled to a permalloy thin film using photoemission electron microscopy (PEEM), a complicated coupling mechanism between the two layers was observed. The spin structure of an array of crossed wires corresponding to an array of holes (antidots) in a cobalt thin film was investigated to understand the switching behaviour on the application of an external magnetic field. It was found that switching occurs by the nucleation and propagation of domain chains, and the moving chain ends can be pinned when they meet the ends of perpendicular domain chains or are blocked due to the formation of a 360° domain wall when they approach a perpendicular domain chain. The current-induced domain wall motion in contacted permalloy wires on membrane samples was studied using electron holography. Effects due to the spin torque were separated from heating effects on the domain wall due to the current. A variety of effects such as domain wall transformations, domain wall jumping between two pinning sites, or structural changes of the magnetic material crystallites due to the combined influence of current pulses and heating was observed. A set of indicators was derived to distinguish current-induced domain wall motion due to the spin torque from the heating effects.

Current-induced magnetic domain wall motion at zero magnetic field is observed in the permalloy layer of a spin-valve-based nanostripe using photoemission electron microscopy. The domain wall movement is hampered by pinning sites, but in between them high domain wall velocities (exceeding 150 m/s) are obtained for current densities well below $10^{12} \unit{A/m^2}$, suggesting that these trilayer systems are promising for applications in domain wall devices in case of well controlled pinning positions. Vertical spin currents in these structures provide a potential explanation for the increase in domain wall velocity at low current densities., Comment: Published version, Applied Physics Express 2, 023003 (2009) http://dx.doi.org/10.1143/APEX.2.023003

We report micromagnetic simulations of current-driven magnetization reversal of the free-layer nanomagnet in ${\mathrm{Ir}}_{20}{\mathrm{Mn}}_{80}∕{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕\mathrm{Cu}∕{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ nanoscale spin valves for large current pulses and compare the results to experiments. Our simulations demonstrate that the mechanism of current-driven magnetization reversal in these samples can depend strongly on the equilibrium angle between the magnetizations of the pinned layer and the free layer of the spin valve. In the case of collinear equilibrium magnetizations, the reversal proceeds via nucleation of a vortex. However, even small misalignments of the magnetizations of the free and the pinned layers, on the order of 5\ifmmode^\circ\else\textdegree\fi{}--10\ifmmode^\circ\else\textdegree\fi{}, can result in magnetization reversal by a macrospinlike coherent precessional rotation. This result is in good agreement with recent experimental observations.

We present results on current-induced domain wall motion in Co/Cu/CoFeB and Co/Cu/NiFe trilayered stripes. We find that the threshold current densities in our CoFeB spin valve stripes are around 10^6 A/cm^2 at zero magnetic field i.e. about one order of magnitude smaller than in our NiFe spin valve stripes (that is about two orders of magnitude smaller than in single NiFe stripes). The motion of the domain wall can be reversible or irreversible depending on the experimental conditions. In the irreversible regime, we observe switching of the spin valve by irreversible domain wall displacement in the free layer. The domain wall displacement is field-assisted when the applied field acts in the same direction as the spin transfer torque. The switching current density decreases as the applied field increases. This feature can have interesting applications in spintronics devices such as magnetoresistive random access memories. When the applied field torque is opposed to the spin transfer one, and above a threshold field, we observe a reversible displacement of the domain wall (corresponding to a peak in the differential resistance measurements). This can be ascribed to the onset of domain wall fluctuations, which is confirmed by micromagnetic simulations, taking into account the non adiabaticity in the spin transfer model. To understand the origin of the reversible domain wall behavior, we have performed time dependent resistance measurements that show evidences of domain wall fluctuations dominated by telegraph noise. The domain wall fluctuations occur between two well-defined states, corresponding to domain wall pinning centers distant of 400 nm. The dwell times are measured in the two states as a function of both the magnetic field and the current. The main features of the fluctuations are probably linked to the energy form of the pinning potentials.

A spin-polarized current is able to excite magnetization dynamics in nanomagnets. A detailed theoretical study of dynamics in low and high field regimes in AlO and MgO magnetic tunnel junctions (MTJs) is presented, considering the maximum value of the applied current which comes from the breakdown voltage of the tunnel barrier. In low field regime, dynamics with a well-defined peak in frequency is observed. In high field regime, AlO MTJ presents the same behavior, while the magnetization in the MgO MTJ shows chaotic motion with a noisy spectrum. Lastly, an effect of the presence of a pinhole in the tunnel barrier is discussed.

The subjects of the thesis are theoretical first principles calculations of X-ray absorption (XA) spectra and current induced spin transfer torque. XA spectra calculated from atomic multiplet theory and from band structure calculations, based on density functional theory for La0.7Sr0.3MnO3 have been compared to experiment. The comparison shows that the effect of the core hole created in the XA process must be considered in the calculation. The theory by Mahan, Nozières and De Dominicis (MND) of dynamical core hole screening is generalised to multiband systems and implemented in first principle calculations. Calculations of the XA spectrum of graphite, including dynamical core hole screening, are shown to better reproduce the relative intensity of the peaks in the experimental spectrum compared to static calculations based on the local density of state of a core excited atom. In combination with experiments the developed method to calculate XA spectra is used to investigate the electronic structure of mixed valent Yb, hydrogen storage in carbon nanotubes and the structure of liquid water. Moreover, a method to calculate the current induced spin transfer torque in materials with a helical spin density wave from first principles has been developed. The method is applied to rare earth metals and it is shown that a current along the axis of spin rotation induces a torque which gives rise to a rotation of the magnetisation direction.

A free electron description of spin-dependent tranport in magnetic tunnel junctions with non collinear magnetizations is presented. We investigate the origin of transverse spin density in tunnelling transport and the quantum interferences which give rise to oscillatory torques on the local magnetization. Spin transfer torque is also analyzed and an important bias asymmetry is found as well as a damped oscillatory behaviour. Furthermore, we investigate the influence of the s-d exchange coupling on torque in particular in the case of half-metallic MTJ in which the spin transfer torque is due to interfacial spin-dependent reflections.

We report micromagnetic simulation of spin transfer torque (STT) switching of a thin elliptical disc made of full-Heusler Co 2 FeAl 0.5 Si 0.5 alloy. The STT results in switching the direction of initial magnetization of the disc to a new state based on the spin-polarization factor ( ƞ ) of the material and magnitude of current density ( J ) applied. The value of J required for magnetization switching need to be reduced in the order of 10 6 A/cm 2 . In our simulation we obtained 3 × 10 5 A/cm 2 as the critical current density ( J c ) required for complete magnetization switching of the disc. We analyzed the effect of ƞ on magnetization switching time by reducing the value of ƞ by 0.10 from the actual value of 0.76. The decrement in ƞ results in the increment of time taken to switch the direction of magnetization. The change in switching time for variable disc thickness was also studied. This simulation result holds a key factor in the study of STT switching in spin-valve nanopillar.

The term "spin caloritronics" defines a novel branch of spintronics that focuses on the interplay between electron spins with heat currents. In the frame of this research area, this thesis is aimed at investigating the effect of a heat current on magnetization dynamics in two different typologies of systems and materials: magnetic insulators and metallic nanostructures. In the first case we conduct studies on yttrium iron garnet (YIG) samples subjected to a temperature gradient. The irreversible thermodynamics of a continuous medium with magnetic dipoles predicts that a thermal gradient across a YIG slab, in the presence of magnetization waves, produces a magnetic field that is the magnetic analog of the well known Seebeck effect. This thermally induced field can influence the time evolution of the magnetization, in such a way that it is possible to modulate the relaxation of the precession when applying a heat current. We found evidence for such a magnetic Seebeck effect (MSE) by conducting transmission measurements in a thin slab of YIG subjected to an in-plane temperature gradient. We showed how the MSE can modulate the magnetic damping depending on the direction of the propagating magnetostatic modes with respect to the orientation of the temperature gradient. In the second part of the thesis we focus our investigation on metallic nanostructures subjected to a heat current. In a metal, the three-current model (current of entropy, of spin up and spin down electrons) predicts that a heat current induces a spin current which will then influence the magnetization dynamics like a charge-driven spin current would. Hence, we explore what has been called Thermal Spin Torque in electrodeposited Co / Cu / Co asymmetric spin valves placed in the middle of copper nanowires. These samples are fabricated by conventional electrodeposition technique in porous polycarbonate membranes using an original method that allows high frequency electrical measurements. We used a modulated laser to investigate the effect of a temperature gradient. We observed a heat-driven spin torque by measuring electrically the quasi-static magnetic response of a spin valve when subjected to the heat current, generated by two laser diodes heating the electrical contact at one end or the other of the nanowire. Analysing the variation in the resistance induced by a heat-driven spin torque, represented by peaks occurring in correspondence with the GMR transition, we found that a temperature difference of the order of 5 K is sufficient to produce sizeable torque in spin valves.

This paper reviews the state of the art in spin-torque and spin-Hall-effect-driven nano-oscillators. After a brief introduction to the underlying physics, the authors discuss different implementations of these oscillators, their functional properties in terms of frequency range, output power, phase noise, and modulation rates, and their inherent propensity for mutual synchronization. Finally, the potential for these oscillators in a wide range of applications, from microwave signal sources and detectors to neuromorphic computation elements, is discussed together with the specific electronic circuitry that has so far been designed to harness this potential. QC 20161111

We theoretically predict that vanadium-based Janus dichalcogenide monolayers constitute an ideal platform for spin-orbit-torque memories. Using first principles calculations, we demonstrate that magnetic exchange and magnetic anisotropy energies are higher for heavier chalcogen atoms, while the broken inversion symmetry in the Janus form leads to the emergence of Rashba-like spin-orbit coupling. The spin-orbit torque efficiency is evaluated using optimized quantum transport methodology and found to be comparable to heavy nonmagnetic metals. The coexistence of magnetism and spin-orbit coupling in such materials with tunable Fermi-level opens new possibilities for monitoring magnetization dynamics in the perspective of non-volatile magnetic random access memories., 5 pages, 4 figures

In this thesis we make use of the Spin Transfer Torque effect from a continuous microwave current to induce and study the spin dynamics of an individual sub-100 nm nanostructure. The idea that an electrical current can carry a spin angular momentum was introduced in the 1980 with the advent of Spintronics. In 1996, Slonczewski and Berger predicted that, when flowing through a metal ferromagnet, such a spin polarized current can exert a torque on the magnetization. This effect is known as Spin transfer Torque (STT) and implies the possibility to manipulate the magnetization by means of an electrical current. One of the characteristics of the STT effect is that it allows for the magnetization dynamics to be excited and detected electrically. This makes the STT a perfect tool for the investigation of spin dynamics at the nano-scale. Indeed, the intrinsic local character of transport measurements permits a single nanostructure to be addressed and studied. This thesis is devoted to the study of the STT effect induced by a microwave current (AC STT). Our experiment focuses on two main aspects: the intrinsic dynamics of the magnetization and the role of a microwave current in assisting the magnetization reversal. We perform our measurements on Co/Cu nanowires electrodeposited in nanoporous polycarbonate templates. The electrical contact to a nanostructure is realized thanks to a home-made sample-holder, without the use of any lithographic technique. In the first part of the thesis a continuous microwave current is used to both drive and probe the magnetization dynamics of a pseudo spin valve nanostructure. Our measurements show that the magnetization dynamics in such a structure is extremely complex and arises from the excitation of both magnetic layers in the pseudo spin valve. In the dynamical spectrum we identify the fundamental modes of both magnetic layers, as confirmed by macrospin simulations. Higher frequency modes are attributed to spatially non-uniform spin wave excitations, in agreement with that observed by others in arrays of nanoelements. These results validate our technique for making samples and contacting them for AC STT-induced dynamical studies. The second part of this work is aimed at testing the effect of AC STT on the static switching field of magnetic nanoelements. The magnetoresistive curve of a pseudo spin valve structure is recorded with and without an additional microwave current. Our results prove the effectiveness of AC STT in assisting the magnetization reversal. We measured a reduction of the switching field up to 80 mT by injecting a microwave current of about 100 µA. This experiment suggests that the range of sensitivity of magnetoresistive devices could be tuned by simply injecting a continuous microwave current. This point is particularly interesting for the prospect of technological applications such as magnetic sensors.