[en] In this paper, we present the atomic and magnetic structures accompanying the change, from axial to planar, of the magnetic anisotropy in BaFe_12-2xIr_xCo_xO₁₉ using magnetic measurements and single-crystal neutron diffraction. The magnetic measurements allowed us to determine the characteristic anisotropy field H_θ and anisotropy constants k_i for the different stages. Our structure refinements show that the change of magnetic anisotropy in BaFe_12-2xIr_xCo_xO₁₉ leads, on a microscopic level, to a collective canting of the moments from 0 to 90 deg. through ferrimagnetic structures, which is in contrast to what has been observed up to now for other substituted single-crystal hexaferrites. The peculiar change of the magnetic anisotropy in BaFe_12-2xIr_xCo_xO₁₉ is correlated to the substitution of Fe³⁺ by Ir⁴⁺ on the 4e bipyramidal site

[en] The results of CT studies of 105 aneurysms were reviewed (69 axial and 36 spiral). The quality of reconstruction was compared. The following features of aneurysms were taken into consideration: the range of dilatation and location in respect to surrounding tissues. The spiral technique turned out to be superior. MPR in spiral CT enabled visualization of morphology of aneurysm and its anatomical relations. (author)

[en] In recent years a number of imaging techniques to determine the optical properties of materials, either in reflection or in transmission, have been developed. Here the use of an imaging version of the so-called rotating-polarizer method in the study of phase transformations in crystals is demonstrated. This method creates false-coloured images representing the light transmission I₀, the extinction angle φ (orientation of the optical indicatrix) and vertical stroke sin δ vertical stroke, a function of the retardation resulting from the birefringence (and a measure of the magnitude of optical anisotropy). When combined with a computer-controlled heating stage, this method provides an opportunity to create separate moving images of orientation and magnitude of optical anisotropy, showing the dynamics of twinning and domain-wall behaviour during temperature changes. It is believed that this is the first time that quantitative imaging of changes in birefringence has been used in this way to describe phase transitions. Two-phase transitions in a crystal of Na_0.5Bi_0.5TiO₃ (NBT) are presented as examples of the use of the system. (orig.)

[en] We report on a study of pressure-dependent structural properties of LaAlO₃, which is considered to be a model material of the class of 'tilted' perovskites. Our high-pressure investigation by means of Raman spectroscopy and synchrotron radiation shows that LaAlO₃ presents structural instabilities under pressure and undergoes a soft-mode-driven rhombohedral-to-cubic phase transition in the vicinity of 14 GPa, which can be analysed in the framework of a Landau model. The structural changes are described in terms of a collective restoring of the initially tilted AlO₆ octahedra towards the ideal cubic perovskite structure without tilting. Up to 40 GPa, the pressure dependence of the volume of the pseudo-cubic phase is described by a third-order Birch-Murnaghan equation of state with parameters V₀=54.57(4) A³, K_T=190(5) GPa and K'=7.2(4). (author)

[en] We report a temperature-dependent Raman scattering investigation of DyScO₃ and GdScO₃ single crystals from room temperature up to 1200 deg. C. With increasing temperature, all modes decrease monotonically in wavenumber without anomaly, which attests to the absence of a structural phase transition. The high temperature spectral signature and extrapolation of band positions to higher temperatures suggest a decreasing orthorhombic distortion towards the ideal cubic structure. Our study indicates that this orthorhombic-to-cubic phase transition is close to or higher than the melting point of both rare-earth scandates (≅2100 deg. C), which might exclude the possibility of the experimental observation of such a phase transition before melting. The temperature-dependent shift of Raman phonons is also discussed in the context of thermal expansion.

[en] We report an investigation of DyScO₃ and GdScO₃ single crystals by Raman scattering in various scattering configurations and at various wavelengths. The Raman spectra are well defined and the reported spectral signature together with the mode assignment set the basis for the use of Raman scattering for the investigation of rare earth scandates. The observed positions of Raman modes for DyScO₃ are for most bands in reasonable agreement with recent theoretical ab initio predictions of the vibrational spectrum for the same material. Further to the phonon signature, a luminescence signal is observed for both scandates. While the luminescence is weak for DyScO₃, it is very intense for GdScO₃ when using a 488 or 514 nm excitation line, which in turn inhibits full analysis of the phonon spectrum. We show that a meaningful phonon Raman analysis of GdScO₃ samples can be done by using a 633 nm excitation.

[en] The development of thin films, in the context of ongoing reduction in the size of electronic systems, poses challenging questions for the materials sciences of multifunctional nanostructures. These include the limits of size reduction, integration of heterogeneous functions, and system characterization or process control at an atomic scale. We present here different studies devoted to perovskite oxide materials (or materials with derived structure), where in specific directions of the crystal structure the atomic organization decreases down to a few nanometers, thus building nanostructures. In these materials, very original physical phenomena are observed in multilayers or superlattices, nanowires (NWs) or nanodots, mainly because strain, surfaces, and interfaces play here a predominant role and can tune the physical properties. Metal-organic chemical vapor deposition (MOCVD) routes have been used for the synthesis of oxide materials. We first introduce the basic rules governing the choice of metal-organic precursors for the MOCVD reaction. Next we discuss the principles of the pulsed injection MOCVD system. A laser-assisted MOCVD system, designed to the direct growth of 2D and 3D photonic structures, will also be described. Selected case studies will finally be presented, illustrating the powerful development of different oxide nanostructures based on dielectric, ferroelectric, or superconducting oxides, manganites, and nickelates, as well as first results related to the growth of ZnO NWs. (authors)

[en] Ferroic domain walls are currently investigated by several state-of-the art techniques in order to get a better understanding of their distinct, functional properties. Here, principal component analysis (PCA) of Raman maps is used to study ferroelectric domain walls (DWs) in LiNbO₃ and ferroelastic DWs in NdGaO₃. It is shown that PCA allows us to quickly and reliably identify small Raman peak variations at ferroelectric DWs and that the value of a peak shift can be deduced—accurately and without a priori—from a first order Taylor expansion of the spectra. The ability of PCA to separate the contribution of ferroelastic domains and DWs to Raman spectra is emphasized. More generally, our results provide a novel route for the statistical analysis of any property mapped across a DW. (paper)

[en] High pressure measurements of the ferroelastic phase transition of SrTiO₃ (Guennou et al 2010 Phys. Rev. B 81 054115) showed a linear pressure dependence of the transition temperature between the cubic and tetragonal phase. Furthermore, the pressure induced transition becomes second order while the temperature dependent transition is near a tricritical point. The phase transition mechanism is characterized by the elongation and tilt of the TiO₆ octahedra in the tetragonal phase, which leads to strongly nonlinear couplings between the structural order parameter, the volume strain and the applied pressure. The phase diagram is derived from the Clausius-Clapeyron relationship and is directly related to a pressure dependent Landau potential. The nonlinearities of the pressure dependent strains lead to an increase of the fourth order Landau coefficient with increasing pressure and, hence, to a tricritical-second order crossover. This behaviour is reminiscent of the doping related crossover in isostructural KMnF₃.

[en] This review stands in the larger framework of functional materials by focussing on heterostructures of rare-earth nickelates, described by the chemical formula RNiO₃ where R is a trivalent rare-earth R  =  La, Pr, Nd, Sm, …, Lu. Nickelates are characterized by a rich phase diagram of structural and physical properties and serve as a benchmark for the physics of phase transitions in correlated oxides where electron–lattice coupling plays a key role. Much of the recent interest in nickelates concerns heterostructures, that is single layers of thin film, multilayers or superlattices, with the general objective of modulating their physical properties through strain control, confinement or interface effects. We will discuss the extensive studies on nickelate heterostructures as well as outline different approaches to tuning and controlling their physical properties and, finally, review application concepts for future devices. (review)