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In recent years, spin-orbit effects have been widely used to produce and detect spin currents in spintronic devices. The peculiar symmetry of the spin Hall effect allows creation of a spin accumulation at the interface between a metal with strong spin-orbit interaction and a magnetic insulator, which can lead to a net pure spin current flowing from the metal into the insulator. This spin current applies a torque on the magnetization, which can eventually be driven into steady motion. Tailoring this experiment on extended films has proven to be elusive, probably due to mode competition. This requires the reduction of both the thickness and lateral size to reach full damping compensation. Here we show clear evidence of coherent spin-orbit torque-induced auto-oscillation in micron-sized yttrium iron garnet discs of thickness 20 nm. Our results emphasize the key role of quasi-degenerate spin-wave modes, which increase the threshold current.

© 2015 IEEE. We perform a comparative study of spin-transfer-induced excitation of the gyrotropic motion of a vortex core with either uniform or vortex spin polarizers. The microwave output voltage associated with the vortex dynamics, detected in both cases, displays a strong reduction of phase fluctuations in the case of the vortex polarizer, with a decrease of the peak linewidth by one order of magnitude down to 200 kHz at zero field. A thorough study of radio frequency emission features for the different accessible vortex configurations shows that this improvement is related to the excitation of coupled vortex dynamics by spin-transfer torques.

© 2015, Nature Publishing Group. All rights reserved. Due to their nonlinear properties, spin transfer nano-oscillators can easily adapt their frequency to external stimuli. This makes them interesting model systems to study the effects of synchronization and brings some opportunities to improve their microwave characteristics in view of their applications in information and communication technologies and/or to design innovative computing architectures. So far, mutual synchronization of spin transfer nano-oscillators through propagating spinwaves and exchange coupling in a common magnetic layer has been demonstrated. Here we show that the dipolar interaction is also an efficient mechanism to synchronize neighbouring oscillators. We experimentally study a pair of vortex-based spin transfer nano-oscillators, in which mutual synchronization can be achieved despite a significant frequency mismatch between oscillators. Importantly, the coupling efficiency is controlled by the magnetic configuration of the vortices, as confirmed by an analytical model and micromagnetic simulations highlighting the physics at play in the synchronization process.

Copyright © EPLA, 2014. Damping processes, associated to magnetization dynamics, allow to generate spin currents from precessing ferromagnets. These can be transmitted into adjacent conducting layers through an interface exchange interaction with conduction electrons. It is in principle also possible to inject angular momentum into insulators but the relevant physical mechanisms are not yet identified. In order to test some ideas concerning pure spin transport through insulating oxides, the present paper reports on the behaviour of two materials with very different properties: NiO is an antiferromagnet whereas SiO2 is a non-magnetic light element insulator. While a sizeable flow of angular momentum is found to be able to propagate through nickel oxide, a SiO2 layer as thin as 2 nm completely blocks this transfer. This underlines some essential features required to conduct a spin current, including the presence of either magnetic order through which magnons can propagate, or compounds with large spin-orbit interactions where phonons can carry angular momentum.

We report on a spectroscopic study of the spin-wave eigen-modes in a circular spin-valve nano-pillar, perpendicularly magnetized along ẑ. Spectroscopy is performed by Magnetic Resonance Force Microscopy (MRFM). Distinct spectra are measured depending on whether the nano-pillar is excited by a uniform in-plane radio-frequency (RF) magnetic field or by an RF current flowing perpendicularly through the layers. These results are in agreement with micromagnetic simulations of the time decay response of the local magnetization to excitations with different azimuthal symmetries, (x̂ + iŷ)e-ilφ. This demonstrates that the azimuthal l-index is the discriminating parameter for the selection rules, as only l = 0 modes are excited by the RF magnetic field, whereas only l = +1 modes are excited by the RF current, owing to the orthoradial symmetry of the induced RF Oersted field. © Springer-Verlag Berlin Heidelberg 2013.

International audience; Using a magnetic resonance force microscope (MRFM), the power emitted by a spin transfer nano-oscillator consisting of a normally magnetized Py$|$Cu$|$Py circular nanopillar is measured both in the autonomous and forced regimes. From the power behavior in the subcritical region of the autonomous dynamics, one obtains a quantitative measurement of the threshold current and of the noise level. Their field dependence directly yields both the spin torque efficiency acting on the thin layer and the nature of the mode which first auto-oscillates: the lowest energy, spatially most uniform spin-wave mode. From the MRFM behavior in the forced dynamics, it is then demonstrated that in order to phase-lock this auto-oscillating mode, the external source must have the same spatial symmetry as the mode profile, \textit{i.e.}, a uniform microwave field must be used rather than a microwave current flowing through the nanopillar.

We review how a magnetic-resonance force microscope (MRFM) can be applied to perform ferromagnetic resonance spectroscopy of individual submicron-size samples. We restrict our attention to a thorough study of the spin-wave eigenmodes excited in Permalloy (Py) disks patterned out of the same 43.3-nm-thin film. The disks have a diameter of either 1.0 or 0.5 μm and are quasisaturated by a perpendicularly applied magnetic field. It is shown that quantitative spectroscopic information can be extracted from the MRFM measurements. In particular, the data are extensively compared with complementary approximate models of the dynamical susceptibility: (i) a two-dimensional analytical model, which assumes a homogeneous magnetization dynamics along the thickness, and ii) a full three-dimensional micromagnetic simulation, which assumes a homogeneous magnetization dynamics below a characteristic length scale c and approximates the cylindrical sample volume by a discretized representation with regular cubic mesh of lateral size c=3.9 nm. In our analysis, the distortions due to a breaking of the axial symmetry are taken into account; both models incorporating the possibility of a small misalignment between the applied field and the normal of the disks. © 2008 The American Physical Society.

Spin wave spectra of perpendicularly magnetized disks with trilayers consisting of a 100~nm permalloy (Py) layer sandwiched by two Cu layers of 30~nm, are measured individually with a Magnetic Resonance Force Microscope (MRFM). It is demonstrated by 3D micromagnetic simulations that in disks having sub-micron size diameters, the lowest energy spin wave mode of the saturated state is not spatially uniform but rather is localized at the center of the Py/Cu interface in the region of a minimum demagnetizing field.

Using a magnetic resonance force microscope, we have performed ferromagnetic resonance spectroscopy of the parametric magnons created by a four-wave process. This is achieved by measuring the differential response to a small amplitude modulation superimposed to a large constant excitation power that drives the dynamics of the uniform mode in the saturation regime. By sweeping the applied field, we observe an abrupt readjustment of the total number of magnons each time the excitation coincides with a parametric mode. This gives rise to ultranarrow peaks whose linewidth is lower than 5× 10-6 of the applied field. © 2007 The American Physical Society.

The aim of this thesis is to understand the influence of a DC current flowing through a spin-valve device on its magnetization dynamics. An original spectrometer has been developed in order to measure the FMR signal of an individual nanostructure. Inspired by near field techniques, it is using a mechanical detection. The excitation spectrum of an individual Permalloy nano-disk has first been measured. This study has allowed us to quantitatively understand spectral modifications induced by finite-size effects. Then, hybrid systems made of metallic magnetic multilayers have been studied. In the measured samples, spin transfer effects have been observed though Oersted field effects are dominating. The experimental setup allowed us as well to study the relaxation term of a ferromagnetic spins system in the high power regime. Our results show that the damping decreases as linear effects appear.; L'objectif de ce travail de thèse est de comprendre l'influence d'un courant continu traversant une vanne de spin sur la dynamique de son aimantation. Pour ce faire, un spectromètre original capable de mesurer la résonance ferromagnétique (RFM) dune nanostructure individuelle a été développé. Son caractère innovant est d'utiliser une détection mécanique, inspirée des techniques de microscopie en champ proche. Le spectre d'excitation RFM d'un nano-disque isolé de Permalloy a d'abord été mesuré. Cette étude a permis de comprendre quantitativement les modifications spectrales induites par les effets de taille finie. Ensuite, des systèmes hybrides composés de multicouches magnétiques métalliques ont été étudiés. Dans les échantillons mesurés, des effets de transfert de spin ont été observés, bien qu'ils soient dominés par ceux du champ d'oersted. Le dispositif expérimental a également permis de suivre la variation du terme de relaxation d'un système de spins ferromagnétique dans le régime haute puissance. Nos résultats montrent que l'amortissement diminue quand les effets non-linéaires entrent en jeu.

The aim of this thesis is to understand the influence of a DC current flowing through a spin-valve device on its magnetization dynamics. An original spectrometer has been developed in order to measure the FMR signal of an individual nanostructure. Inspired by near field techniques, it is using a mechanical detection. The excitation spectrum of an individual Permalloy nano-disk has first been measured. This study has allowed us to quantitatively understand spectral modifications induced by finite-size effects. Then, hybrid systems made of metallic magnetic multilayers have been studied. In the measured samples, spin transfer effects have been observed though Oersted field effects are dominating. The experimental setup allowed us as well to study the relaxation term of a ferromagnetic spins system in the high power regime. Our results show that the damping decreases as linear effects appear.; L'objectif de ce travail de thèse est de comprendre l'influence d'un courant continu traversant une vanne de spin sur la dynamique de son aimantation. Pour ce faire, un spectromètre original capable de mesurer la résonance ferromagnétique (RFM) dune nanostructure individuelle a été développé. Son caractère innovant est d'utiliser une détection mécanique, inspirée des techniques de microscopie en champ proche. Le spectre d'excitation RFM d'un nano-disque isolé de Permalloy a d'abord été mesuré. Cette étude a permis de comprendre quantitativement les modifications spectrales induites par les effets de taille finie. Ensuite, des systèmes hybrides composés de multicouches magnétiques métalliques ont été étudiés. Dans les échantillons mesurés, des effets de transfert de spin ont été observés, bien qu'ils soient dominés par ceux du champ d'oersted. Le dispositif expérimental a également permis de suivre la variation du terme de relaxation d'un système de spins ferromagnétique dans le régime haute puissance. Nos résultats montrent que l'amortissement diminue quand les effets non-linéaires entrent en jeu.