No abstract

Hysteresis does not vanish in materials essentially free from defects and domain wall pinning. It arises from the existence of many geometrically different domain structures separated by intrinsic energy barriers. Although generally metastable, these states can nevertheless be explored by suitable excursion in the manifold of domain structures, along which irreducible irreversibility is present. This idea is illustrated by experiments performed on garnet layers with uniaxial anisotropy (bubble material). Cooling the layer from T>Tc in a field H normal to it results in various states: parallel stripes (H=0), bubbles of increasing diameters and mixtures of bubbles and stripes (H<52 Oe), and mazes of convoluted stripes with defects (forks, dead ends). Cycling the applied field at constant temperature also causes evolution of the structure. For example, starting from parallel stripes the structure evolves toward a maze with intermediate steps corresponding to stripe folding, and then there is the onset of defects (forks, dead ends). We classify these various structures in a kind of phase diagram, depending on the following parameters: current field H, maximum field Hmax previously applied, and memory temperature Tm at which the domain structure is nucleated. The relations and irreversible paths between various structures are precised. We also discuss mechanisms for topological evolution linked with changes in the equlibrium period of the structure. Irreversibility arises from asymmetric processes of domain collapse and nucleation. The first stages in the processes leading from bubbles to stripes, or from parallel stripes to mazes, can be analyzed respectively in terms of elliptical and folding instabilities, the latter being similar to the undulation instability of smectic A liquid crystals.

No abstract

No abstract

No abstract

No abstract

2014 L'étude des amplificateurs galvanométriques linéaires ou intégrateurs, considérés comme systèmes asservis, montre que leur fonction de transfert peut se mettre sous la forme d'un produit de deux termes : l'un étant le terme recherché, l'autre un terme perturbateur correspondant à un filtre passe-bas. Il est possible de corriger l'influence de ce dernier en utilisant des boucles de retour convenables. L'examen des critères de stabilité confirme que de telles corrections sont possibles et met en évidence leur limite. Pour les parfaire, on peut mettre en série avec le système corrigé un filtre ayant une fonction de transfert inverse de celle du filtre perturbateur.Les résultats expérimentaux obtenus confirment le calcul. Ils montrent l'intérêt des amplificateurs galvanométriques linéaires dans le domaine des gains élevés. En effet, après correction, on passe sans affaiblissement et à moins de 20° de déphasage : 1000 Hz au gain 104, 700 Hz au gain 105 et 400 Hz au gain 106. En ce qui concerne l'intégrateur très bas niveau, sa stabilité est exceptionnelle. Nous avons pu obtenir des dérives intrinsèques correspondant à des tensions moyennes ramenées à l'entrée de quelques dizaines de picovolts (1 picovolt = 10-12 V) pendant des durées voisines d'une heure. Même dans le cas où l'on exige la plus grande stabilité (galvanomètre lent) la bande passante de l'intégrateur reste très acceptable : moins de 20° de déphasage à 1 100 Hz et un temps de montée de 100 03BCs.Cette étude montre qu'il est possible de corriger l'essentiel des défauts des amplificateurs galvanométriques sans en altérer les qualités.Abstract. -The dynamical behaviour of galvanometric amplifiers is derived from servomechanism theory. It is shown that their transfer function has the form of the product of two terms, one of them being the ideal function and the other being the transfer function of a low-pass filter. It is possible to correct for the influence of this term by using suitable feedback loops. The theoretical stability conditions show that such a correction is possible to a certain amount. A further correction is performed by a tunable active network which almost cancels the effect of the lowpass filter.The amplifier described in this paper features a good frequency response at high gains : its bandwidth values are 1 000 CPS, 700 CPS and 400 CPS at gains 104, 105 and 106, with a phase lag less than 20°. The drift of the long-term integrator proved to be extremely low : during recordings up to 54 minutes, the input error voltage was less than a few 10-11 volt. Even with a slow (sensitive) galvanometer the integrator has a good frequency response : phase lag less than 20° at 1 100 CPS, rise time 100 03BCs.This work shows that it is possible to improve to a great amount the behaviour of galvanometric amplifiers.

No abstract