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We show that the misfit strain due to the film-substrate lattice mismatch strongly increases the value of the quadratic magnetoelectric coupling. The coupling, size effects, and misfit strains cause strong changes in the phase diagrams of ferroic films at zero external magnetic and electric fields. The antiferromagnetic to ferromagnetic or ferrimagnetic phase transitions for compressive or tensile misfit strains open the way for the tailoring of magnetic and electric properties of ferroic films leading to advanced applications. [ABSTRACT FROM AUTHOR]

The direct conversion of electrical energy to mechanical work by a material is relevant to a number of applications. This is illustrated by ferroelectric ‘relaxors’ such as Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT; refs 5, 6): these materials exhibit a giant electromechanical (piezoelectric) response that is finding use in ultrasonic and medical applications, as well as in telecommunications. The origins of this effect are, however, still unclear. Here we show that the giant electromechanical response in PMN-PT (and potentially other ferroelectric relaxors) is the manifestation of critical points that define a line in the phase diagram of this system. Specifically, in the electric-field–temperature–composition phase diagram of PMN-PT (the composition being varied by changing the PT concentration), a first-order paraelectric–ferroelectric phase transition terminates in a line of critical points where the piezoelectric coefficient is maximum. Above this line, supercritical evolution is observed. On approaching the critical point, both the energy cost and the electric field necessary to induce ferroelectric polarization rotations decrease significantly, thus explaining the giant electromechanical response of these relaxors. [ABSTRACT FROM AUTHOR]

The central problem in the physics of relaxors is the nature of the polar nanoclusters. Whereas relaxors are homogeneous at high enough temperatures, polar nanoregions immersed in a neutral matrix are formed below a certain temperature T b . This should lead to a two component system. Here we present direct microscopic evidence for the two component nature of relaxors. We show that the chemical shift perturbed 207 Pb NMR spectra of these systems consist of an isotropic component corresponding to a spherical glassy matrix which does not respond to an applied electric field, and an anisotropic component, corresponding to frozen out polar nanoclusters which order in a strong enough electric field, forming a ferroelectric phase. This is as well reflected in the dynamic properties where the relaxation time distribution function starts to become asymmetric with decreasing temperature and a second maximum—which is never seen in dipolar glasses and is obviously due to polar clusters—appears on further cooling. We also show that the basic difference between dipolar glasses and relaxors is the fact that polar nanoclusters can be oriented in a strong enough electric field and a ferroelectric phase can be induced. This is not the case in dipolar glasses where the response is due to single dipoles which can not be ordered by applied electric fields. [ABSTRACT FROM AUTHOR]

Recent developments in the understanding of order-disorder and displacive type transitions are presented. It is shown that classical order-disorder type transitions like those in squaric acid and KDP contain in addition to the order disorder also a displacive component in the phase transition mechanism. Similarly classical displacive ferroelectrics like BaTiO3 which have been traditionally described by the condensation of the soft mode contain an order-disorder component as the Ti ion is dynamically disordered between different off-center sites already in the cubic phase. The comparison of the experimental and theoretical electric field gradient tensors at the Ti sites in the various phases of BaTiO3 allows for a quantitative determination of the mixed character of this phase transition. [ABSTRACT FROM AUTHOR]

The motionally averaged electric field gradient tensors at the Ti-sites have been evaluated within the 8-site Ti disorder model for the various phases of BaTiO3. The results have been compared with the experimental data and with the observation of a non-zero Ti quadrupole coupling in the cubic phase. The pure 8-site “order-disorder” model does not agree with the experiment. A good agreement is however obtained if in addition to the dynamic Ti disorder the “displacive” soft mode induced deformation of the unit cell shape is taken into account. The presence of the soft mode and the Ti disorder leads to a special type of phase transition with combined “displacive” and “order-disorder” character. [ABSTRACT FROM AUTHOR]

We have measured the d.c. electric field dependence of the birefringence and conoscopic images for the smectic C* phase of a partially racemized ferroelectric chiral smectic liquid crystal CE-8. The experiments were performed using 50mu m thick homeotropic cells with lateral electrodes which created a d.c. electric field parallel to the smectic layers. The observed field induced birefringence shows a characteristic step-like behaviour which is due to the stepby-step unwinding of the helical structure in a sample with finite dimensions along the helical axis. In conoscopic observations we observe that these steps are associated with moving disclination lines that traverse the sample in the direction of the smectic layers. The observed electric field dependence of the birefringence is discussed in terms of the soliton-like unwinding of helical smectic structures and compared with the predictions of the Landau theory. A qualitatively good agreement is obtained. [ABSTRACT FROM AUTHOR]