Baodong Qu

Curie temperature of a ferroelectric superlattice described by the transverse Ising model

Abstract The Curie temperature of a ferroelectric superlattice formed from two alterating materials was studied using the transverse Ising model in the mean-field theory. The dependence of the Curie temperature on the interface coupling strength J AB and the layer-number of the two slabs in one period were obtained. Assuming the bulk Curie temperature of slab B is higher than that of slab A, the Curie temperature of the superlattice increases with the increases in J AB , but decreases with increasing the layer- number of slab A. While fixing the layer-number of slab A, there exists a critical interface coupling strength J c ; as the layer- number of slab B increases, the Curie temperature increases for J AB J c but decreases for J AB > J c . The temperature dependence of the spin average in the two slabs we also calculated.

Oriented PbTiO3 films from metalorganic precursors

PbTiO3 films on various substrates were prepared by a metalorganic decomposition process using a multilayer spinning technique. Films on LaAlO3 and MgO substrates were preferentially c‐axis oriented, and those on SrTiO3 have a completely c‐axis orientation. The mechanism leading to the orientation was discussed. Contrary to the c‐axis orientation, a‐axis orientation can be induced on otherwise random films if they were annealed under pressure.

Polarization and Dielectric Susceptibility of Ferroelectric Superlattice

The temperature dependence of dielectric susceptibility as well as the spontaneous polarization of a ferroelectric superlattice were discussed based on the Ising model in a transverse field. When the slab thicknesses are larger, the temperature dependence of the spontaneous polarization shows a steplike structure. The dielectric susceptibility becomes infinite at the Curie temperature T c and shows finite peaks corresponding to the rapid decrease in the spontaneous polarization near the bulk Curie point of each slab. The height of the finite peaks decreases with the decrease in the slab thicknesses. When they become so thin that the polarization profiles are predominantly controlled by the interface, the finite peaks in susceptibility as well as the steps in polarization disappear.

Strain analysis of PbTiO3 fine particles and films prepared by sol-gel method

Abstract The micro-strain as well as the grain size in PbTiO3 fine particles and films was determined by x-ray diffraction. For isolated particles, the micro-strain supposedly resulting from non-equilibrium defects decreases monotonically with the increase in sintering temperature. The micro-strain in films is higher than that in isolated particles because there are many defects accumulated at the grain boundary. As the sintering temperature increases the micro-strain in films first decreases and then increases. The decrease in micro-strain at lower sintering temperatures can be attributed to the thermally induced defect-elimination, while the increase in micro-strain at higher sintering temperatures is supposed due to the extension of the transition region at the surface and interface.

Surface analysis of PbTiO3 films prepared by the sol‐gel method

The surface of PbTiO3 films prepared by the sol‐gel method was analyzed by the use of x‐ray photoelectron spectra combined with secondary‐ion mass spectra and infrared absorption spectra. The results suggested that traces of organic materials still existed at least in the surface layer of the films. The outmost surface was partly covered by adsorbed oxygen. The surface layer of the film deviated from stoichiometry because of the shortage of Ti; the thickness of it was estimated about 400 A.

Ferroelectric superlattice with first-order phase transition

Abstract The Mean field expressions derived from the transverse Ising model are presented to describe the first-order ferroelectric phase transitions. The theory is used to discuss the static properties of ferroelectric superlatticcs formed by inserting periodically a monolayer of material B into material A; the bulk properties of material B are assumed paraelectric. Both the Curie temperature and the critical value of the spin average are dependent sensitively on the period of the superlattice when the period is small. For the case of large period, the Curie temperature is reduced slight compared with the bulk value of material A; while the spin average is reduced remarkably.

Ion-beam-assisted deposition of ferroelectric PbTiO3 films

Ferroelectric PbTiO3 films were deposited by ion‐beam‐assisted deposition (O2+Ar 75–150 eV). The effects of ion bombardment on the Pb/Ti ratio and the structures of the film are discussed. For a given target‐substrate distance and substrate temperature, the Pb/Ti ratio decreased with increasing bombarding beam energy. Compared with the films deposited without ion bombardment, the deposition rate was increased under ion bombardment, which is attributed to an increase in the surface reaction rate. The crystal grains are larger for films deposited under ion bombardment, which implies that ion bombardment enhances the surface mobility of adatoms and hence the growth kinetics of the growing films. Dielectric and ferroelectric properties of the as‐deposited films are also reported.

A new type of first-order phase transition in ferroelectric thin films

Phase transition behaviour in ferroelectric thin films is studied via the calculation of free energy for the case when the bulk phase transition is second order. If the spontaneous polarization is reduced near the surface, the phase transition in ferroelectric thin films will become of first order. As well as shifts in the Curie point to a lower temperature, the peak height of susceptibility at the critical point decreases with decrease in the film thickness. When the spontaneous polarization is enhanced near the surface, the phase transition behaviour of spontaneous polarization shows a tail-like structure.

Microstructure and composition analysis of PbTiO3 films

PbTiO3 films prepared by metalorganic decomposition process were analyzed by X-ray diffraction, TEM and Auger electron spectroscopy. The film on SrTiO3 substrate is highly c-axis oriented: the grains are columnar with their longer axes perpendicular to the film surface. The grain size observed from TEM. The Auger energies of Pb, Ti and O on the film surface are higher than those from within the film. The surface layer of the film is non-stoichiometric, with excessive Pb and insufficient O and Ti. The thicknesses of the surface layer and the film-substrate interface layer increase with firing temperature.

Ultra-thin ferroelectric films described by the transverse Ising model

Abstract The properties of ultra-thin ferroelectric films were studied by transverse Ising model. The Curie temperature and ground state profile were obtained for different film thickness, surface layer number and surface interaction. The critical surface interaction for multi-surface-layer is about 1.078, which is different from 1.25 for one-surface-layer film.