Dynamics of relaxor ferroelectrics

We study a dynamic model of relaxor ferroelectrics based on the spherical random-bond---random-field model and the Langevin equations of motion. The solution to these equations is obtained in the long-time limit where the system reaches an equilibrium state in the presence of random local electric fields. The complex dynamic linear and third-order nonlinear susceptibilities $\chi_1(\omega)$ and $\chi_3(\omega)$, respectively, are calculated as functions of frequency and temperature. In analogy with the static case, the dynamic model predicts a narrow frequency dependent peak in $\chi_3(T,\omega)$, which mimics a transition into a glass-like state.Comment: 15 pages, Revtex plus 5 eps figure

O egzistenciji rezonantne nuklearne magnetske relaksakcije pomoću spin-rotacionih međudjelovanja u čvrstom tijelu

The temperature and frequency dependences of the nuclear spin-lattice relaxation times have been measured for a series of nearly spherical molecules both in the solid and in the liquid state, using a pulse technique. The same anomalous behaviour which was found before in SF6, SeF6 and TeF6 is now shown to occur in MoF6, WF8 , DF8 and solid white phosphorus in a certain temperature range, The results seem to indicate that spin-rotational interactions do operate in a relatively large number of molecular solids and liquids and open the possibility of studying correlation functions for the angular velocity of molecules in the solid by NMR techniques.Sa impulsnim nuklearnim magnetnim rezonantnim spektrometrom izmerene su temperaturne i frekventne zavisnosti longitudinalnih relaksacionih vremena spin-rešetka u nizu gotov sfero simetričih molekula kako u čvrstoj tako i u tečnoj fazi. Na osnovu rezultata može se zaključiti da je spin-rotaciona interakcija dominantan relaksacioni mehanizam u srazmerno velikom broju molekula kako u čvrstom tako i u tečnom stanju, što daje mogućnost izučavanja korelacionih funkcija za uglovnu brzinu molekula metodama NMR-a

O egzistenciji rezonantne nuklearne magnetske relaksakcije pomoću spin-rotacionih međudjelovanja u čvrstom tijelu

The temperature and frequency dependences of the nuclear spin-lattice relaxation times have been measured for a series of nearly spherical molecules both in the solid and in the liquid state, using a pulse technique. The same anomalous behaviour which was found before in SF6, SeF6 and TeF6 is now shown to occur in MoF6, WF8 , DF8 and solid white phosphorus in a certain temperature range, The results seem to indicate that spin-rotational interactions do operate in a relatively large number of molecular solids and liquids and open the possibility of studying correlation functions for the angular velocity of molecules in the solid by NMR techniques.Sa impulsnim nuklearnim magnetnim rezonantnim spektrometrom izmerene su temperaturne i frekventne zavisnosti longitudinalnih relaksacionih vremena spin-rešetka u nizu gotov sfero simetričih molekula kako u čvrstoj tako i u tečnoj fazi. Na osnovu rezultata može se zaključiti da je spin-rotaciona interakcija dominantan relaksacioni mehanizam u srazmerno velikom broju molekula kako u čvrstom tako i u tečnom stanju, što daje mogućnost izučavanja korelacionih funkcija za uglovnu brzinu molekula metodama NMR-a

Size and doping effects on the coercive field of ferroelectric nanoparticles

A microscopic model for describing ferroelectric nanoparticles is proposed which allows us to calculate the polarization as a function of an external electric field, the temperature, the defect concentration and the particle size. The interaction of the constituents of the material, arranged in layers, depends on both the coupling strength at the surface and that of defect shells in addition to the bulk values. The analysis is based on an Ising model in a transverse field, modified in such a manner to study the influence of size and doping effects on the hysteresis loop of the nanoparticles. Using a Green function technique in real space we find the coercive field, the remanent polarization and the critical temperature which differ significantly from the bulk behavior. Depending on the varying coupling strength due to the kind of doping ions and the surface configuration, the coercive field and the remanent polarization can either increase or decrease in comparison to the bulk behavior. The theoretical results are compared with a variety of different experimental data.Comment: 16 pages, 7 figure

Epitaxial growth and structural characterization of Pb(Fe1/2Nb1/2)O3 thin films

We have grown lead iron niobate thin films with composition Pb(Fe1/2Nb1/2)O3 (PFN) on (0 0 1) SrTiO3 substrates by pulsed laser deposition. The influence of the deposition conditions on the phase purity was studied. Due to similar thermodynamic stability spaces, a pyrochlore phase often coexists with the PFN perovskite phase. By optimizing the kinetic parameters, we succeeded in identifying a deposition window which resulted in epitaxial perovskite-phase PFN thin films with no identifiable trace of impurity phases appearing in the X-ray diffractograms. PFN films having thicknesses between 20 and 200 nm were smooth and epitaxially oriented with the substrate and as demonstrated by RHEED streaks which were aligned with the substrate axes. X-ray diffraction showed that the films were completely c-axis oriented and of excellent crystalline quality with low mosaicity (X-ray rocking curve FWHM<0.09). The surface roughness of thin films was also investigated by atomic force microscopy. The root-mean-square roughness varies between 0.9 nm for 50-nm-thick films to 16 nm for 100-nm-thick films. We also observe a correlation between grain size, surface roughness and film thickness.Comment: 13 Pages, 6 figures. To be published in J. Mag. Mag Mater. proceedings of EMRS200