Direct Observation of Massless Domain Wall Dynamics in Nanostripes with Perpendicular Magnetic Anisotropy

Domain wall motion induced by nanosecond current pulses in nanostripes with perpendicular magnetic anisotropy (Pt/Co/AlO$_x$) is shown to exhibit negligible inertia. Time-resolved magnetic microscopy during current pulses reveals that the domain walls start moving, with a constant speed, as soon as the current reaches a constant amplitude, and no or little motion takes place after the end of the pulse. The very low 'mass' of these domain walls is attributed to the combination of their narrow width and high damping parameter $\alpha$. Such a small inertia should allow accurate control of domain wall motion, by tuning the duration and amplitude of the current pulses

High domain wall velocity at zero magnetic field induced by low current densities in spin-valve nanostripes

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.2.02300

Micro bobines à champ pulsé : applications aux champs forts et à la dynamique de renversement de l'aimantation à l'échelle de la nanoseconde par effet Kerr et Dichroïsme Circulaire Magnétique de rayons X

In this thesis we have developed three techniques for the dynamic measurement of magnetisation reversal on the nanosecond time scale using micro coils as a source of pulsed field. The dynamic behaviour of several thin film magnetic systems was studied. With the time resolved X-rays Magnetic Circular Dichroism (XMCD) technique developed at the ESRF, one obtains the chemical selectivity in species as well as a temporal resolution of 100 ps. That enabled us to carry out studies on magnetisation reversal and dynamic coupling in spin valve and tunnel junction type structures. Under certain conditions of pulsed field the magnetisation reversal dynamics of the free layer (FeNi) is about one nanosecond. In these structures, depending on the thickness of the nonmagnetic layer, the dynamic coupling between the free and the hard layers can be very different from the static one. The Kerr/Faraday dynamic technique was shown to be effective for the dynamic study of thin films with strong perpendicular anisotropy due to the possibility of obtaining high magnetic fields (up to 10 T) associated with a high sensitivity (~2x10(+8) µB). With the time resolved Kerr/Faraday imaging technique we have studied the magnetisation reversal of a magnetic garnet following a field pulse. The processes involved in the reversal are easily identified by the time resolved image of the magnetic domains. With the micro coils developed in collaboration with the LETI, we showed the possibility of obtaining very strong magnetic fields (up to 50 T) at a very low cost compared with conventional high field installations. For the continuation of this work we consider the development of a dynamic technique of imaging with high spatial resolution (tens of nm) and chemical selectivity based on a PEEM microscope (Photoemission Electron Microscope).Dans cette thèse nous avons développé trois techniques de mesures dynamiques du renversement de l'aimantation à l'échelle de la nanoseconde en utilisant des micro bobines comme source de champ pulsé. Le comportement dynamique de plusieurs systèmes magnétiques sous la forme de couches minces a été étudié. Avec la technique de Dichroïsme Circulaire Magnétique de Rayons X (XMCD) résolu en temps développée à l'ESRF on obtient la sélectivité en espèce chimique outre à une résolution temporelle de 100 ps. Cela nous a permis d'effectuer des études du renversement de l'aimantation et du couplage dynamique dans des structures du type vanne de spins et jonctions tunnel. Dans certaines conditions de champ pulsé le temps de renversement de l'aimantation de la couche libre (FeNi) est de l'ordre de la nanoseconde. Dans ces structures, suivant l'épaisseur de la couche non magnétique, le couplage dynamique entre les couches libre et dure peut être très différent de celui statique. La technique de mesure dynamique Kerr/Faraday s'est montrée assez efficace pour l'étude dynamique des couches minces à forte anisotropie perpendiculaire grâce aux possibilités de champs importants (jusqu'à 10 T) associées à une haute sensibilité (~2 x 10(+8) µB). Avec la technique d'imagerie Kerr/Faraday résolue en temps on a pu suivre le renversement d'un grenat magnétique suite à une impulsion de champ. Les processus du renversement sont facilement identifiés par la visualisation dynamique des domaines magnétiques. Avec les micro bobines développées en collaboration avec le LETI, nous avons pu montrer la possibilité d'obtenir des champs magnétiques très forts (jusque 50 T) à un très bas coût comparé aux installations conventionnelles. Pour la suite de ce travail nous envisageons le développement d'une technique d'imagerie dynamique avec haute résolution spatiale (dizaine de nm) et sélectivité chimique basée sur un microscope PEEM (Microscope à photoémission d'électrons)

Desenvolvimento de um sistema RP/RTCVD para deposição de filmes finos isolantes e metalicos

Orientador : Jacobus W. SwartDissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia EletricaResumo: A técnica de deposição de filmes finos por CVD (Chemical Vapor Deposition) é de grande interesse atual, especialmente na mlcroeletrônica. Este trabalho apresenta o desenvolvimento de um sistema RT/RPCVD (Rapid Thermal / Remote Plasma CVD) para deposição de filmes isolantes e metálicos, voltado para aplicações em microeletrônica. O sistema possui alta versatilidade, devido principalmente às características de aquecimento rápido e disponibilidade de plasma-remoto. A câmara de reação é isolada do meio ambiente através de uma antecâmara, o que torna o processo menos susceptível a contaminantes externos, bem como permite a automação da seqüência de processo, entre outras vantagens. Um estudo preliminar da deposição de Óxido de Silício foi efetuado para caracterização e validação do equipamento. Foi avaliada a dependência do processo com as variáveis temperatura, tempo, pressão, fluxo de reagentes e potência de plasma. As características dos filmes obtidos foram determinadas a partir de medidas de espessura, uniformidade, espectroscopia de Infra Vermelho, microscopia óptica e eletrônica. Taxas de deposição entre 10 nm/min e 1000 nm/min foram obtidas em determinadas condições de processo, enquanto que a uniformidade de deposição situou-se entre 2% e 5%. Á partir das análises por espectroscopia de Infra Vermelho, os filmes demonstraram estequiometria SiO1,7-1,9, e baixa incorporação de outros elementos mesmo em deposições auxiliadas por plasmaAbstract: Thin film deposition by means of Chemical Vapor Deposition (CVD) has been of a great interest, specially in microelectronics. This work shows the development of a RT/RPCVD (Rapid Thermal / Remote Plasma CVD) systems for thin film deposition of dielectrics and metais, for use on microelectronics appliations. It is a very fIexible system because of its capabilities of Remote Plasma and Rapid Thermal Processing. It's reaction chamber is isolated from environment through a load-Iock, making the process less susceptible to external contaminants and allowing automated loading, and other advantages. A preliminary study of Silicon Dioxide deposition was used to characterize and validate the system. The process dependency with temperature, time, pressure, fIow of reactants and plasma uniformity measurements, Infra power was evaluated. Film thickness and Red spectroscopy, optical and electronics microscopy were performed to characterize the films. Deposition rates between 10 nm/min and 1000 nm/min were achieved in selected process conditions, while the uniformity of the film was from 2% to 5%. Film stoiquiometry was Si 1,7-1,9 with low incorporation of strange elements, even under plasma assisted depositionsMestradoMestre em Engenharia Elétric

Preparation and characterisation of compositionally graded SmCo films

International audienc

Magnetic domain wall dynamics in the precessional regime: Influence of the Dzyaloshinskii-Moriya interaction

International audienceThe domain wall dynamics driven by an out of plane magnetic field was measured for a series of magnetic trilayers with different strengths of the interfacial Dzyaloshinskii-Moriya interaction (DMI). The features of the field-driven domain wall velocity curves strongly depend on the ratio of the field $H_{D}$ stabilizing chiral N\'{e}el walls to the demagnetizing field within the domain wall $H_{DW}$. The measured Walker velocity, which in systems with large DMI is maintained after the Walker field, giving rise to a velocity plateau up to the Slonczewski field $H_S$, can be related to the DMI strength. Yet, when $H_{D}$ and $H_{DW}$ have comparable values, a careful analysis needs to be done in order to evaluate the impact of the DMI on the domain wall velocity. By means of a one-dimensional model and 2D simulations, we extend this method and we clarify the interpretation of the experimental curves measured for samples where $H_{D}$ and $H_{DW}$ are comparable

Velocity asymmetry of Dzyaloshinskii domain walls in the creep and flow regimes

International audienceWe have carried out measurements of domain wall dynamics in a Pt/Co/GdOx(t) wedge sample with perpendicular magnetic anisotropy. When driven by an easy-axis field Hz in the presence of an in-plane field Hx, the domain wall expansion along x is anisotropic, as expected for samples presenting Dzyaloshinskii-Moriya interaction. In the creep regime, the sign and the value of the domain wall velocity asymmetry changes along the wedge. We show that in our samples the domain wall speed vs. Hx curves in the creep regime cannot be explained simply in terms of the variation of the domain wall energy with Hx, as suggested by previous works. For this reason the strength and the sign of the Dzyaloshinskii-Moriya interaction (DMI) cannot be extracted from these measurements. To obtain reliable information on the DMI strength using magnetic field-induced domain wall dynamics, measurements have been performed with high fields, bringing the DW close to the flow regime of propagation. In this case, the large values of DMI are coherent with those obtained from current-driven domain wall dynamics

Geometry optimization for application of radio frequency signals on diamond samples

This work describes the design and optimization of a printed circuit board containing a rectangular half-loop coil designed to apply radio frequency signals on micrometric diamond crystals. The structure was optimized to maximize the magnetic field perpendicular to the coil at 2,87 GHz. In the optimization process we have used transmission line theory, circuit level and 3D electromagnetic simulations. The structure was implemented and measured. The adopted methodology was validated by the good agreement between theory and experiments