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The hysteresis of acoustic parameters of smectic-C in rotating magnetic field was observed for the h t time. The obtained experimental results are compared with the theory predictions based on the bistability of the orientational structure of smectic-C in a magnetic field.Compared with other smectics the smectic-C phase (Sc) is of special interest because its orientational structure can be altered (rotation of the local C-director or the projection of the director n on the layer plane) easily enough under the action of a moderate magnetic field

2014 Nous analysons l'anisotropie de vitesse et d'absorption ultrasonore en fonction de la température et de la fréquence. Nous déterminons les paramètres caractérisant la dépendance angulaire de la vitesse et de l'absorption ultrasonore dans la phase nématique NPOB orientée au moyen du champ magnétique. L'analyse des résultats a été réalisée dans le cadre d'une théorie hydrodynamique généralisée tenant compte de deux processus relaxationnels. La théorie relaxationnelle rend compte des données expérimentales au voisinage de la transition de phase nématique-smectique A.Abstract. 2014 The temperature-frequency dependences of both velocity and attenuation coefficient anisotropy, as well as their angular dependence coefficients in the nematic phase of nitrophenyl-octyl-oxybenzoate oriented by magnetic field were investigated. The analysis of experimental results was carried out in the framework of generalized hydrodynamic theory with two relaxation processes. It was shown that in the vicinity of the N-A phase transition the experimental results are described by the relaxation theory. J. Physique 46 (1985) 583-588 AVRIL 1985, Classification Physics Abstracts 61. 30C