Universality of thermally assisted magnetic domain wall motion under spin torque

Thermally assisted motion of magnetic domain wall under spin torque is studied theoretically. It is shown that the wall velocity $v$ depends exponentially on the spin current, \Is, below the threshold value, in the same way as in a thermally activated motion driven by a force. A novel property of the spin torque driven case at low temperature is that the linear term in spin current is universal, i.e., \ln v \sim \frac{\pi\hbar}{2e}(\Is/\kB T). This behavior, which is independent of pinning and material constants, could be used to confirm experimentally the spin torque as the driving mechanism

Highly anisotropic magnetic domain wall behavior in-plane magnetic films

We have studied nucleation of magnetic domains and propagation of magnetic domain walls (DWs) induced by pulsed magnetic field in a ferromagnetic film with in-plane uniaxial anisotropy. Different from what have been seen up to now in out-of-plane anisotropy films, the nucleated domains have a rectangular shape in which a pair of the opposite sides are perfectly linear DWs, while the other pair present zigzags. This can be explained by magnetostatic optimization, knowing that the pulse field is applied parallel to the easy magnetization axis. The field induced propagation of these two DW types are very different. The linear ones follow a creep law identical to what is usually observed in out-of-plane films, when the velocity of zigzag DWs depends linearly on the applied field amplitude down to very low field. This most unusual feature can be explained by the shape of the DW, which makes it possible to go round the pinning defects. Thanks to that, it seems that propagation of zigzag walls agrees with the 1D model, and these results provide a first experimental evidence of the 1D model relevance in two dimensional ferromagnetic thin films. Let's note that it is the effective DW width parallel to DW propagation direction that matters in the 1D model formula, which is a relevant change when dealing with zigzag DWs.Comment: 20 pages, 13 figure

Magnetoresistive sensors based on the elasticity of domain walls

Magnetic sensors based on the magnetoresistance effects have a promising application prospect due to their excellent sensitivity and advantages in terms of the integration. However, competition between higher sensitivity and larger measuring range remains a problem. Here, we propose a novel mechanism for the design of magnetoresistive sensors: probing the perpendicular field by detecting the expansion of the elastic magnetic Domain Wall (DW) in the free layer of a spin valve or a magnetic tunnel junction. Performances of devices based on this mechanism, such as the sensitivity and the measuring range can be tuned by manipulating the geometry of the device, without changing the intrinsic properties of the material, thus promising a higher integration level and a better performance. The mechanism is theoretically explained based on the experimental results. Two examples are proposed and their functionality and performances are verified via micromagnetic simulation.Comment: 4 figures, 13 page

Diagnostic des systèmes techniques de transformation de l'igname en cossettes séchées au Bénin

Les pertes après-récolte des tubercules d'igname enregistrées dans les pays producteurs de l'Afrique de l'Ouest sont très importantes (40-50 % après 6 mois de stockage). Ces pertes sont dues à l'absence de moyens et méthodes de conservation appropriés. La transformation des tubercules en produits stables (cossettes, farine) est une solution à la conservation de l'igname frais. Cette technique permet, en outre, de réduire de plus de la moitié le poids de la matière à transporter. La méthode de fabrication des cossettes d'igname, connue depuis longtemps au Bénin et dans les pays voisins (Nigeria, Togo), permet de conserver le surplus des tubercules pour les utiliser pendant les périodes de soudure. Depuis plus d'une décennie, la farine de cossettes d'igname (Elubo) est passée dans les habitudes alimentaires des populations urbaines. L'importance de la demande actuelle nécessite que cette technique soit évaluée, améliorée et valorisée

Probing symmetries of quantum many-body systems through gap ratio statistics

The statistics of gap ratios between consecutive energy levels is a widely used tool, in particular in the context of many-body physics, to distinguish between chaotic and integrable systems, described respectively by Gaussian ensembles of random matrices and Poisson statistics. In this work we extend the study of the gap ratio distribution $P(r)$ to the case where discrete symmetries are present. This is important, since in certain situations it may be very impractical, or impossible, to split the model into symmetry sectors, let alone in cases where the symmetry is not known in the first place. Starting from the known expressions for surmises in the Gaussian ensembles, we derive analytical surmises for random matrices comprised of several independent blocks. We check our formulae against simulations from large random matrices, showing excellent agreement. We then present a large set of applications in many-body physics, ranging from quantum clock models and anyonic chains to periodically-driven spin systems. In all these models the existence of a (sometimes hidden) symmetry can be diagnosed through the study of the spectral gap ratios, and our approach furnishes an efficient way to characterize the number and size of independent symmetry subspaces. We finally discuss the relevance of our analysis for existing results in the literature, as well as point out possible future applications and extensions.Comment: 22 pages, 7 figures, Mathematica notebook as electronic supplementary file (enclosed in source file

Domain-wall motion induced by spin transfer torque delivered by helicity-dependent femtosecond laser

In magnetic wires with perpendicular anisotropy, moving domain with only current or only circularly polarized light requires a high power. Here, we propose to reduce it by using both short current pulses and femtosecond laser pulses simultaneously. The wires were made out of perpendicularly magnetized film of Pt/Co/Ni/Co/Pt. The displacement of the domain wall is found to be dependent on the laser helicity. Based on a quantitative analysis of the current-induced domain wall motion, the spin orbit torque contribution can be neglected when compared to the spin transfer torque contribution. The effective field of the spin transfer torque is extracted from the pulsed field domain wall measurements. Finally, our result can be described using the Fatuzzo-Labrune model and considering the effective field due to the polarized laser beam, the effective field due to spin transfer torque, and the Gaussian temperature distribution of the laser spot.Comment: 14 pages, 4 figure

Glacier flow monitoring by digital camera and space-borne SAR images

International audienceMost of the image processing techniques have been first proposed and developed on small size images and progressively applied to larger and larger data sets resulting from new sensors and application requirements. In geosciences, digital cameras and remote sensing images can be used to monitor glaciers and to measure their surface velocity by different techniques. However, the image size and the number of acquisitions to be processed to analyze time series become a critical issue to derive displacement fields by the conventional correlation technique. In this paper, an efficient correlation software is used to compute from optical images the motion of a serac fall and from Synthetic Aperture Radar (SAR) images the motion of Alpine glaciers. The optical images are acquired by a digital camera installed near the Argentière glacier (Chamonix, France) and the SAR images are acquired by the high resolution TerraSAR-X satellite over the Mont-Blanc area. The results illustrate the potential of this software to monitor the glacier flow with camera images acquired every 2 h and with the size of the TerraSAR-X scenes covering 30 × 50 km2