The low-field dielectric response of the relaxor-ferroelectric (1−x)PbFe2/3W1/3O3–xPbTiO3 ceramics with various x, was investigated. The permittivity data were analyzed with empirical laws that describe the diffuse phase transitions in relaxors. A change of the character of the phase transition was found with increasing x, from a total diffuse, characteristic of relaxors, to a sharp one, typical of ferroelectrics. The deviations from the Curie–Weiss law of the dielectric constant data in the paraelectric phase were used to calculate a local order parameter within a modified-Landau theory for relaxors. The nonzero values of the local order parameter far above the Curie region indicate the thermal stability of the polar nanoregions in the relaxor state. The temperature dependence of the local order parameter clearly shows the evolution of the system from a short range ordered to a long range ordered ferroelectric, with increasing the PbTiO3 addition.

International audiencePhotoinduced phase transformations [1,2] occur when a laser pulse impacts a material, thereby transforming its electronic and/or structural orders, consequently directing the functionalities [3,4,5,6,7]. The transient nature of photoinduced states has thus far severely limited the application scope. It is of paramount importance to explore whether structural feedback during the solid deformation has capacity to amplify and stabilize photoinduced transformations. Contrary to coherent optical phonons long under scrutiny [8,9,10] , coherently propagating cell deformations over acoustic timescale [11,12,13,14] have not been explored to similar degree, particularly in light of cooperative elastic interactions. Herein we demonstrate experimentally and theoretically a self-amplified responsiveness in a spin-crossover material [15] during its delayed volume expansion. The cooperative response at material scale prevails above a threshold excitation, significantly extending the lifetime of photoinduced states. Such elastically-driven cooperativity triggered by a light pulse offers a new efficient route to the generation and stabilization of photoinduced phases in many volume-changing materials

The magnetic characterization technique of hysteretic materials based on the measurement of the first-order reversal curves (FORC) is one of the most appealing methods recently introduced in hundreds of new laboratories, but due to the complexity of the FORC data analysis, it is not always properly used. This method originated in identification procedures for the classical Preisach model and consequently often the FORC distribution is interpreted as a slightly distorted Preisach distribution. In this paper, we discuss this idea from two points of view derived from the basic assumptions used in the Preisach model. One is that the interaction field is equivalent with a shift of the rectangular hysteron along the applied field axis without changing the intrinsic coercivity. The other is the direct use of switching fields as coordinates, in fact, the ones defining the Preisach plane. We discuss the compatibility between the experimental FORC distribution and the Preisach model developed on the interaction field hypothesis. As a "toy model," we are using a system of ferromagnetic nanowires, explaining from the physical point of view the complex FORC diagrams as they are obtained in experiments. This explanation gives a fundament for the correct interpretation of the FORC diagram in order to get "Preisach type" information about the system, mainly about the distributions of coercive and interaction fields within the sample. These results are relevant for many ferromagnetic systems and give a valuable guide for understanding the FORC technique and its fundamental link with the Preisach model. V

The First Order Reversal Curve ͑FORC͒ diagrams of interacting single-domain ferromagnetic particle systems have been found experimentally to contain negative regions. In this paper, we use micromagnetic and phenomenological ͑Preisach-type͒ models to help explain the occurrence of these negative regions. In Preisach-type modeling, the position of the negative region is correlated with the sign of the mean-field interactions. In micromagnetic modeling, the position of the negative region is correlated with the spatial arrangement of the particles in the model.

The relaxation in a spin transition compound is modeled on the basis of molecules interacting by the way of connecting springs and situated in a bidimensional open boundary hexagonal lattice. The switch of individual molecules is randomly checked using a standard Monte Carlo procedure. The switching probability depends on the energy gap between the two states in the absence of interactions and on the elongations of the nearest springs. The main characteristics of the experimental relaxation curves are reproduced and clustering and nucleation phenomena are detected