The phase transition properties of a ferroelectric superlattice described by the Ising model in a transverse field have been investigated using the effective-field theory with a probability distribution technique that accounts for the self-spin correlation functions. An independent interface plane has been assumed and the parameters that imitate BaTiO3/SrTiO3 structure have been used in our calculation. The Curie temperature, polarization, and dielectric constant have been obtained. The most striking feature is that the dielectric constant presents a maximum at room temperature. This kind of feature has been observed in a recent experimental measurement.

No abstract

No abstract

No abstract

2014 Nous avons étudié plusieurs échantillons de poudre recuite de Ho1,2Mo6S8 pur ou dopé. Nous avons observé une courbe de première aimantation à l'extérieur du cycle d'hystérésis, parfois une susceptibilité alternative diamagnétique dans l'état ferromagnétique, une grande magnétorésistance positive qui n'est pas d'origine magnétique, un cycle d'hystérésis de cette magnétorésistance, une variation de la résistance avec le courant et un trainage simultané de l'aimantation et de la résistance. Nous pensons que ces phénomènes peuvent être dus à la supraconductivité de parois de domaines qui exerce une influence sur la résistivité et le processus d'aimantation. Abstract. 2014 We have performed various measurements on annealed powder samples derived from Ho1.2Mo6S8. We have observed a first magnetization curve outside the hysteresis cycle, sometimes a diamagnetic a.c. susceptibility in the ferromagnetic state, a large positive magnetoresistance which is not of magnetic origin, an hysteresis cycle of the magnetoresistance, a variation of the resistance with the current, simultaneous after-effects on the magnetization and on the resistance. We believe that these results can be related to some superconductivity inside domain walls and to the influence of that state on the resistivity and on the process of magnetization.