Analysis of anisotropy crossover due to oxygen in Pt/Co/MOx trilayer

Extraordinary Hall effect and X-ray spectroscopy measurements have been performed on a series of Pt/Co/MOx trilayers (M=Al, Mg, Ta...) in order to investigate the role of oxidation in the onset of perpendicular magnetic anisotropy at the Co/MOx interface. It is observed that varying the oxidation time modifies the magnetic properties of the Co layer, inducing a magnetic anisotropy crossover from in-plane to out-of-plane. We focused on the influence of plasma oxidation on Pt/Co/AlOx perpendicular magnetic anisotropy. The interfacial electronic structure is analyzed via X-ray photoelectron spectroscopy measurements. It is shown that the maximum of out-of-plane magnetic anisotropy corresponds to the appearance of a significant density of Co-O bondings at the Co/AlOx interface

The contribution of x-ray specular reflectometry to the oxygen-induced magnetic properties in Pt/Co/AlOx

3 pagesInternational audienceTwo key parameters were analyzed in Si/SiO/Pt/Co/AlOx: the oxidation time of the Al layer resulting in AlOx, and the ex situ annealing temperatures varied in the 15 and 55 s and 20, 300, and 450 °C ranges, respectively. For intermediate annealing temperatures (∼300 °C), the quantitative analysis of specular reflectometry data shows that the progressive oxidation of layers by increasing the oxidation time goes along with an improvement of the homogeneity of the alumina layer. This outcome casts new light on the temperature dependence of magnetic properties of the samples. The remarkable temperature variation of the coercive field, extracted from extraordinary Hall effects in the 5-300 K range, is associated with structural change due to Co-oxygen bondings, which leads to strong pinning of Co spins in the low temperature regime

Investigation of metallic/oxide interfaces in Pt/Co/AlOx trilayers by hard x-ray reflectivity

International audienceX-ray reflectivity (XRR) is used to determine the oxidation front at the nanometer scale in sputtered perpendicular semi tunnel junctions, as the form Pt/Co/AlOx, by varying the oxidation time tOx of the capping layer. From XRR simulations, we show that the nature of the stack is gradually defined according to the value of tOx. For low tOx values (<40 s), a simple Pt/Co/Al/AlOx multilayer is appearing whereas a Pt/Co/CoO/AlOx architecture takes place for higher tOx. The oxygen-induced magnetic properties obtained by extraordinary Hall effects measurements are explained by the structural results. The increase of Co-O bondings with tOx is at the origin of the appearing of the perpendicular magnetic anisotropy (PMA)

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

Exploring the limits of soft x-ray magnetic holography: Imaging magnetization reversal of buried interfaces (invited)

The following article appeared in Journal of Applied Physics 109.7 (2011): 07D357 and may be found at http://scitation.aip.org/content/aip/journal/jap/109/7/10.1063/1.3567035Only a very few experimental techniques can address the microscopic magnetization reversal behavior of the different magnetic layers in a multilayered system with element selectivity. We present an element-selective study of ferromagnetic (FM) [Co/Pt]n multilayers with perpendicular anisotropy exchange-coupled to antiferromagnetic (AFM) FeMn and IrMn films performed with a new experimental set-up developed for both soft x-ray spectroscopy and holography imaging purposes. The spectroscopy analysis allows the quantification of the unpinned (pinned) uncompensated AFM moments, providing direct evidence of its parallel (antiparallel) alignment with respect to the FM moments. The holography experiments give a direct view of both FM and uncompensated AFM magnetic structures, showing that they replicate to each other during magnetization reversal. Remarkably, we show magnetic images for effective thicknesses as small as one monolayer. Our results provide new microscopic insights into the exchange coupling phenomena and explore the sensitivity limits of these techniques. Future trends are also discussed.We acknowledge technical support by the ESRF staff R. Barrett, R. Homs-Regojo, T. Trenit, and G. Retout. A. B. acknowledges support through a Ramo´n y Cajal contract from the Spanish MICINN. This work was supported in part by the Spanish MICINN through Projects CSD2007-00010, and MAT2010-21822 and by Comunidad de Madrid through Project S2009/MAT-1726.Comunidad de Madrid. S2009/MAT-1726/NANOBIOMAGNE

Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection

International audienceModern computing technology is based on writing, storing and retrieving information encoded as magnetic bits. Although the giant magnetoresistance effect has improved the electrical read out of memory elements, magnetic writing remains the object of major research efforts. Despite several reports of methods to reverse the polarity of nanosized magnets by means of local electric fields and currents, the simple reversal of a high-coercivity, single-layer ferromagnet remains a challenge. Materials with large coercivity and perpendicular magnetic anisotropy represent the mainstay of data storage media, owing to their ability to retain a stable magnetization state over long periods of time and their amenability to miniaturization. However, the same anisotropy properties that make a material attractive for storage also make it hard to write to. Here we demonstrate switching of a perpendicularly magnetized cobalt dot driven by in-plane current injection at room temperature. Our device is composed of a thin cobalt layer with strong perpendicular anisotropy and Rashba interaction induced by asymmetric platinum and AlOx interface layers. The effective switching field is orthogonal to the direction of the magnetization and to the Rashba field. The symmetry of the switching field is consistent with the spin accumulation induced by the Rashba interaction and the spin-dependent mobility observed in non-magnetic semiconductors as well as with the torque induced by the spin Hall effect in the platinum layer. Our measurements indicate that the switching efficiency increases with the magnetic anisotropy of the cobalt layer and the oxidation of the aluminium layer, which is uppermost, suggesting that the Rashba interaction has a key role in the reversal mechanism. To prove the potential of in-plane current switching for spintronic applications, we construct a reprogrammable magnetic switch that can be integrated into non-volatile memory and logic architectures. This device is simple, scalable and compatible with present-day magnetic recording technolog

Effets de taille finie sur les couplages magnétostatiques et l'anisotropie d'échange dans le domaine de l'enregistrement magnétique

Le sujet de cette thèse est l'étude des effets de taille finie sur les propriétés fondamentales de matériaux utilisés dans les dispositifs associés au stockage magnétique de l'information. Nous proposons l'approche multiniveaux comme alternative possible pour augmenter les densités de stockage des media planaires ou perpendiculaires, continus ou discrets. En fonctionnement, une couche doit être manipulée sans modifier l'état magnétique des autres couches. La première partie de ce mémoire est consacrée à l'étude des effets de taille finie sur les couplages magnétostatiques dans des systèmes composés de deux couches ferromagnétiques à base de cobalt/platine à anisotropie perpendiculaire, séparées par un métal non-magnétique. Pour des états désaimantés, nous avons mis en évidence la corrélation des configurations en domaines des couches, attribuée aux couplages inter-couches. Pour différentes impulsions de champs, la réplication des domaines nous a permis d'observer des états rémanents singuliers permettant de mieux comprendre le phénomène. Pour des réseaux de nanostructures, la réplication due aux couplages intra-nanostructure se manifeste par un décalage en champ du cycle d'hystérésis sur la couche de plus faible champ coercitif. Le seconde partie de ce mémoire traite des effets de taille finie sur les propriétés d'anisotropie d'échange pour des bicouches ferromagnétique/antiferromagnétique typiquement utilisées dans les têtes de lecture d'ordinateurs. Pour des systèmes à anisotropie planaire, nous avons montré que les nanostructures et les antiferromagnétiques minces sont plus sujets à l'activation thermique. Il en résulte une augmentation ou une réduction du champ d'échange pour les nanostructures par rapport aux couches continues en fonction des conditions d'épaisseur d'antiferromagnétique et de température. Ces résultats permettent de lever certaines contradictions apparentes de la littérature. Pour des couches continues à anisotropie perpendiculaire, l'orientation relative des spins du ferromagnétique et de l'antiferromagnétique influence de manière significative les propriétés d'anisotropie d'échange. L'impression des domaines du ferromagnétique dans l'antiferromagnétique, par traitements thermiques permet de générer des cycles d'hystérésis " doubles " et d'ajuster leurs propriétés magnétiques. Des premiers résultats prometteurs concernant les effets de taille finie sur les propriétés des systèmes à anisotropie perpendiculaire ont été obtenus.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Macroscopic probing of domain configurations in interacting bilayers with perpendicular magnetic anisotropy

International audienceMagnetostatic interactions in perpendicularly magnetized Co/ Pt bilayers lead to the formation of mirror domains in both soft and hard layers. We show that it is possible to take advantage of domain replication in order to probe the microscopic domain configuration of the hard layer through macroscopic hysteresis minor loop measurements on the soft layer. The minor curves consist of two loops, which magnetization amplitudes and field shifts can be quantitatively related to the domains sizes, shapes, and up/down relative proportion in the hard layer

MAGNETORESISTANCE ET TRANSFERT DE SPIN DANS LES JONCTIONS TUNNEL MAGNETIQUES

L'observation du renversement d'aimantation induit par courant polarisé (CIMS) dans les jonctions tunnel magnétiques (JTM) a ouvert de nouvelles perspectives d'applications pour l'électronique de spin, en particulier à travers les mémoires magnétiques (MRAM). Cette réalisation exige une bonne maîtrise de l'oxydation de la barrière tunnel mais aussi une connaissance approfondie du phénomène de transfert dans ces dispositifs. En premier lieu, une étude expérimentale de l'influence de l'oxygène sur les propriétés magnétiques d'une tricouche Pt/Co/MOx (MOx est un métal oxydé) est présentée. La modification d'anisotropie magnétique due aux atomes d'oxygène peut être utilisée pour contrôler l'oxydation des barrières tunnel. Le second aspect étudié est la détermination, théorique et expérimentale, des caractéristiques du transfert de spin dans les JTM. Ces caractéristiques sont d'abord discutées à travers un modèle d'électrons libres puis estimatées expérimentalement dans des JTM à travers la réalisation de diagrammes de phase statiques.The recent observation of current-induced magnetization switching (CIMS) in magnetic tunnel junctions (MTJs) has opened new possibilities of applications for spin electronics, especially with magnetic memories (MRAM). This achievement demands the accurate control of the barrier oxidation in order to obtain MTJs with good magnetic and electronic properties. Moreover, spin transfer torque is expected to show different characteristics in MTJ compared to metallic spin valves. In a first step, an experimental study of the influence of oxygen on the magnetic properties of a Pt/Co/MOx trilayers (MOx is an oxidized metal) is presented. These modifications may be used as a probe of the oxidation state of the tunnel barrier. In a second step, we study the specific properties of spin transfer torque in a MTJ. These properties are first described within a free electron model and then estimated experimentally in MgO-based MTJ through the measurement of static phase diagrams.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Macroscopic probing of domain configurations in interacting bilayers with perpendicular magnetic anisotropy

International audienceMagnetostatic interactions in perpendicularly magnetized Co/ Pt bilayers lead to the formation of mirror domains in both soft and hard layers. We show that it is possible to take advantage of domain replication in order to probe the microscopic domain configuration of the hard layer through macroscopic hysteresis minor loop measurements on the soft layer. The minor curves consist of two loops, which magnetization amplitudes and field shifts can be quantitatively related to the domains sizes, shapes, and up/down relative proportion in the hard layer