Relaxor ferroelectricity and colossal magnetocapacitive coupling in ferromagnetic CdCr2S4

Multiferroic materials, which reveal magnetic and electric order, are in the focus of recent solid state research. Especially the simultaneous occurrence of ferroelectricity and ferromagnetism, combined with an intimate coupling of magnetization and polarization via magneto-capacitive effects, could pave the way for a new generation of electronic devices. Here we present measurements on a simple cubic spinel with unusual properties: It shows ferromagnetic order and simultaneously relaxor ferroelectricity, i.e. a ferroelectric cluster state, reached by a smeared-out phase transition, both with sizable ordering temperatures and moments. Close to the ferromagnetic ordering temperature the magneto-capacitive coupling, characterized by a variation of the dielectric constant in an external magnetic field, reaches colossal values of nearly 500%. We attribute the relaxor properties to geometric frustration, which is well known for magnetic moments, but here is found to impede long-range order of the structural degrees of freedom.Comment: 4 pages, 3 figure

Calculation of the Resonant Frequencies in the Vicinity of the \upalpha -\upbeta α - β Transition in Quartz

The transition (847 K) in quartz is investigated using the temperature dependence of the resonant frequency and of the lattice constant which are correlated by means of the mode Gruneisen parameter. By determining the mode Gruneisen parameter, the resonant frequencies are predicted in the vicinity of the transition in quartz. This calculation is extended to low temperatures (down to about , and the resonant frequencies are predicted at low temperatures using the lattice parameter data for quartz. Predictions indicate that the method of calculating the resonant frequencies using the lattice parameter data is satisfactory, and it can be applied to some other crystalline systems

Third and fifth harmonic responses in viscous liquids

International audienceWe review the works devoted to third and fifth harmonic susceptibilities in glasses, namely χ (3) 3 and χ (5) 5. We explain why these nonlinear responses are especially well adapted to test whether or not some amorphous correlations develop upon cooling. We show that the experimental frequency and temperature dependences of χ (3) 3 and of χ (5) 5 have anomalous features, since their behavior is qualitatively different to that of an ideal gas, which is the high-temperature limit of a fluid. Most of the works have interpreted this anomalous behavior as reflecting the growth, upon cooling, of amorphously ordered domains, as predicted by the general framework of Bouchaud and Biroli (BB). We explain why most—if not all—of the challenging interpretations can be recast in a way which is consistent with that of Bouchaud and Biroli. Finally, the comparison of the anomalous features of χ (5) 5 and of χ (3) 3 shows that the amorphously ordered domains are compact, i.e., the fractal dimension d f is close to the dimension d of space. This suggests that the glass transition of molecular liquids corresponds to a new universality class of critical phenomena