Highly asymmetric magnetic domain wall propagation due to coupling to a periodic pinning potential

Magneto-optical microscopy and magnetometry have been used to study 19 magnetization reversal in an ultrathin magnetically soft [Pt/Co]2 ferromagnetic film 20 coupled to an array of magnetically harder [Co/Pt]4 nanodots via a predominantly 21 dipolar interaction across a 3 nm Pt spacer. This interaction generates a spatially 22 periodic pinning potential for domain walls propagating through the continuous 23 magnetic film. When reversing the applied field with respect to the static nanodot 24 array magnetization orientation, strong asymmetries in the wall velocity and switching 25 fields are observed. Asymmetric switching fields mean that the hysteresis of the film is 26 characterized by a large bias field of dipolar origin which is linked to the wall velocity 27 asymmetry. This latter asymmetry, though large at low fields, vanishes at high fields 28 where the domains become round and compact. A field-polarity-controlled transition 29 from dendritic to compact faceted domain structures is also seen at low field and a 30 model is proposed to interpret the transition

Magnetically textured y-Fe2O3 nanoparticles in a silica gel matrix: structural and magnetic properties

International audienceThis paper is devoted to magnetic and structural properties of anisotropic g -Fe2O3 superparamagnetic particles dispersed in a transparent xerogel matrix. The effect of frozen anisotropy axes and magnetic texture, induced by a magnetic field applied during the solidification of the matrix on the in-field magnetization process, is studied by alternating gradient force magnetometry and first and second order magneto-optical effects. The changes of magnetization curves with respect to the ferrofluid solution at the same particle concentration are interpreted on the basis of an existing statistical approach extended to systems with particle size distribution, which has to be taken into account for real samples. A very good agreement between the experiment and theory was achieved for a log-normal distribution of diameters which well resembles that deduced from electron microscopy observations in different imaging modes. This structural analysis states the parameter values used in calculations and confirms the relevance of basic assumptions of the model for the specimens studied. The experimental results and the related theoretical discussion should be of use to understand magnetic properties of other magnetically textured superparamagnetic system

Electric field phase diagram of thiourea determined by optical birefringence

The phase diagram of thiourea : SC(ND2)2 in the ( E, T) plane has been determined by linear birefringence measurements. To our knowledge, it is the first time that such diagram has been determined in a system which undergoes transitions to an incommensurate phase. The transition line consists of three parts : a first-order line TH(E) separating the ferroelectric phase from the incommensurate phase which terminates at a maximum field EM = 2 285 ± 10 V/mm with TM = 210 ± 0.2 K, where the birefringence jump is zero, a second-order line Tλ(E) separating the paraelectric phase from the incommensurate one which terminates at a point Etr ≃ 2 050 V/mm , Ttr ≃ 212.5 K; between (E M, TM) and (Etr, Ttr ) the transition line looks to be weakly first-order.Le diagramme de phase de la thiourée SC(ND2)2 dans le plan (E, T) a été déterminé par des mesures de biréfringence linéaire. A notre connaissance, c'est la première fois qu'un tel diagramme est déterminé dans un système qui présente des transitions vers une phase incommensurable. La ligne de transition se divise en trois parties : une ligne de premier ordre TH(E), séparant les phases ferroélectrique et incommensurable, qui se termine à un champ maximum EM = 2 285 ± 10 V/mm avec TM = 210 ± 0,2 K où le saut de biréfringence s'annule; une ligne de second ordre T λ(E) séparant les phases paraélectrique et incommensurable, qui se termine à un point Etr ≃ 2 050 V/mm, T tr ≃ 212,5 K ; entre (EM, TM) et (Etr, Ttr), la ligne de transition parait être faiblement de premier ordre

Observation of a new memory effect in a modulated structure

We report the first observation of a new memory effect in a modulated structure (deuterated thiourea). We present a qualitative explanation of this effect in terms of mobile defects, or impurities, interacting with the modulation order parameter. We show that this effect can be used to obtain the lines of constant modulation wavevector in the E, T plane of the phase diagram, by thermodynamic methods. We suggest that a similar periodic modulation of defect concentration occurs in CDW systems, like NbSe 3, and accounts for the non linearity induced by the electric field. (CDW : Charge Density Wave.)Nous décrivons la première observation d'un nouvel effet de mémoire dans une structure modulée (thiourée deutériée). Nous présentons une explication qualitative de cet effet basée sur l'interaction de défauts ou d'impuretés mobiles avec le paramètre d'ordre de la modulation. Nous montrons que cet effet peut être utilisé pour obtenir les lignes d'égal vecteur d'onde de modulation dans le plan E, T du diagramme de phase par des méthodes thermodynamiques. Nous suggérons que les systèmes à ODC comme NbSe3 présentent eux aussi une modulation périodique de concentration de défauts, ce qui rendrait compte des effets non linéaires en champ électrique. (ODC : Onde de Densité de Charge.

Gestion informatisée des projets de coopération internationale GIPCI

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Optical birefringence in the incommensurate phase of {N(CH3) 4}2ZnCl4

The optical birefringence of { N(CH3)4 }2ZnCl 4 has been measured. Below T0, the exponent of its temperature dependence is identified to 2 - α - Φ = 0.87 ± 0.02. The lock-in transition is observed, exhibiting an hysteresis of about 0.25°. These data are compared with the predictions of Mashiyama's theory.La biréfringence du tétraméthyl tétrachlorozincate d'ammonium {N(CH 3)4 }2ZnCl4 a été mesurée entre 35 °C et 0 °C. Au-dessous de To, elle obéit à une loi de puissance en fonction de la température, dont l'exposant 0,87 ± 0,02 est identifié à 2 - α - Φ. La transition de blocage présente une hystérésis de l'ordre de 0,25°. Ces données sont comparées aux prédictions de la théorie de Mashiyama

Theory of the memory effect in thiourea. Defect density waves in modulated systems

A simple Landau-Ginzburg theory is shown to account qualitatively and semi-quantitatively for the memory effect anomalies of the susceptibility and the birefringence in thiourea. The importance of the gradient amplitude coupling energy in thiourea is emphasized. The possibility of creating a defect-induced locked incommensurate phase around any arbitrary temperature within the modulated phase is demonstrated. Such a locked phase will appear as a consequence of the condensation of a periodic defect concentration if extrinsic mobile impurities are allowed to interact for a sufficient time with a static modulation. We briefly discuss some aspects of such defect density waves in the physics of modulated structures.Une approche Landau-Ginzburg simple rend compte qualitativement et semi-quantitativement des anomalies de susceptibilité et de biréfringence liées à l'effet mémoire. Le couplage gradient amplitude joue, dans la thiourée, un rôle déterminant. Nous démontrons la possibilité de créer, autour d'un point arbitraire dans la partie modulée du diagramme de phase, une phase incommensurable bloquée induite par défauts... Ceci arrive grâce à l'apparition d'une Onde de Densité de Défauts lorsque ceux-ci, mobiles, ont pu interagir pendant un temps suffisant avec une modulation statique. Nous discutons brièvement les conséquences pour la physique des systèmes modulés

First paleomagnetic results from Mio-Pliocene series of the hellenic sedimentary ARC

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Scientific co-operation between INRA eastern and central european countries

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