Lattice excitations in NdFeO3 through polarized optical spectroscopies.

The possibility of inducing new polar and/or magnetic transient states through the pumping of optical phonons towards the non-linear regime has renewed the scientific interest in orthoferrites. Nonetheless, to perform these studies it is fundamental to have a deep knowledge of the lattice excitations at equilibrium conditions. In this work, we present a complete characterization of the optically-active zone-center phonons in NdFeO3 single crystals at room temperature by means of polarized Raman and infrared spectroscopies. The study is complemented with polarized infrared spectroscopy at 4 K and unpolarized Raman scattering at 10 K. The predicted polar phonons were successfully observed together with some of the crystal-field excitations. First-principles simulations further allow the eigenmode and symmetry assignments of the optical phonons. The calculated atomic motions of each mode are of significant interest, as they are common for all orthoferrites and to most of the large family of orthorhombic Pbnm perovskites.

Reference values for the 1-minute sit-to-stand and 5 times sit-to-stand tests to assess functional capacity: a cross-sectional study.

OBJECTIVES: To establish age-specific and sex-specific reference values and equations for the 1-minute sit-to-stand (1MSTS) and 5 times sit-to-stand (5TSTS) tests for Portuguese adults. DESIGN: Cross-sectional study. Descriptive statistics were explored to compute reference values and reference equations were established with a forward stepwise multiple regression. SETTING: Community. PARTICIPANTS: 546 adult volunteers without disabilities [age range 18 to 95 years; 58% female] were recruited. MAIN OUTCOME MEASURES: Data on age, sex, height, weight, body mass index (BMI), smoking status and physical activity were collected using a structured questionnaire developed specifically for this study. PROCEDURE: Participants performed three repetitions of the 1MSTS and 5TSTS and the best repetition was used for analysis. RESULTS: Normative values were provided by sex for each age decade. Reference equations were: 1MSTS= 61.53 - (0.34 x age) - (3.57 x sex) - (0.33 x BMI), r2 = 26%; and 5TSTS= 3.89 + (0.10 x age) - (0.96 x physical activity), r2 = 27%. CONCLUSIONS: The proposed reference values and equations will help to interpret the results of functional capacity obtained from healthy or diseased adult populations. CONTRIBUTION OF THE PAPER.

Magnetostructural coupling in RFeO3 (R = Nd, Tb, Eu and Gd).

We investigate the interplay of magnetization and lattice vibrations in rare-earth orthoferrites RFeO3, with a specific focus on non-symmetry-breaking anomalies. To do so, we study the magnetization, magnon excitations and lattice dynamics as a function of temperature in NdFeO3, TbFeO3, EuFeO3 and GdFeO3. The magnetization shows distinct temperature anomalous behavior for all investigated rare-earth orthoferrites, even in the compounds with no phase transitions occurring at those temperatures. Through spin-phonon coupling, these magnetic changes are mirrored by the FeO6 rotation mode for all the studied RFeO3, revealing a common magnetostructural effect associated with the octahedra rotations. The R3+ oscillation modes evidence a Fe3+/R3+ spins cross-talk for the NdFeO3 and TbFeO3 cases. Our work sheds light into the common magnetostructural coupling in rare-earth orthoferrites, and the important role of magnetic anisotropy and spin-orbit coupling strength of the R-Fe interactions on the spin-reorientation transition at high temperatures.

Strain relaxation dynamics of multiferroic orthorhombic manganites.

Resonant ultrasound spectroscopy has been used to characterise strain coupling and relaxation behavior associated with magnetic/magnetoelectric phase transitions in GdMnO3, TbMnO3and TbMn0.98Fe0.02O3through their influence on elastic/anelastic properties. Acoustic attenuation ahead of the paramagnetic to colinear-sinusoidal incommensurate antiferromagnetic transition at ∼41 K correlates with anomalies in dielectric properties and is interpreted in terms of Debye-like freezing processes. A loss peak at ∼150 K is related to a steep increase in electrical conductivity with a polaron mechanism. The activation energy,Ea, of ≳0.04 eV from a loss peak at ∼80 K is consistent with the existence of a well-defined temperature interval in which the paramagnetic structure is stabilised by local, dynamic correlations of electric and magnetic polarisation that couple with strain and have relaxation times in the vicinity of ∼10-6s. Comparison with previously published data for Sm0.6Y0.4MnO3confirms that this pattern may be typical for multiferroic orthorhombicRMnO3perovskites (R= Gd, Tb, Dy). A frequency-dependent loss peak near 10 K observed for TbMnO3and TbMn0.98Fe0.02O3, but not for GdMnO3, yieldedEa⩾ ∼0.002 eV and is interpreted as freezing of some magnetoelastic component of the cycloid structure. Small anomalies in elastic properties associated with the incommensurate and cycloidal magnetic transitions confirm results from thermal expansion data that the magnetic order parameters have weak but significant coupling with strain. Even at strain magnitudes of ∼0.1-1‰, polaron-like strain effects are clearly important in defining the development and evolution of magnetoelectric properties in these materials. Strains associated with the cubic-orthorhombic transition due to the combined Jahn-Teller/octahedral tilting transition in the vicinity of 1500 K are 2-3 orders of magnitude greater. It is inevitable that ferroelastic twin walls due to this transition would have significantly different magnetoelectric properties from homogeneous domains due to magnetoelastic coupling with steep strain gradients.

Strain-Engineered Tetragonal Phase and Ferroelectricity in GdMnO3 Thin Films Grown on SrTiO3 (001).

A previously unreported tetragonal phase has been discovered in a epitaxially strained GdMnO3 thin films deposited on (001)-oriented SrTiO3 substrates by radio frequency (RF) magnetron sputtering. The tetragonal axis of the films grown up to a 35 nm thickness is perpendicular to the film surface and the basal lattice parameters are imposed by the cubic structure of the substrate. Furthermore, the emergence of a spontaneous electric polarization below ~32 K points to the stabilization of an improper ferroelectric phase at low temperatures, which is not observed in bulk GdMnO3. This work shows how strain engineering can be used to tailor the structure and properties of strongly correlated oxides.

Tuning the Stoichiometry of Ag2S Thin Films for Resistive Switching Applications.

In this work silver-rich and sulfur-rich silver sulfide (Ag2S) thin films were fabricated using a diversified set of experimental methods, namely ion beam deposition and atmosphere- and solution-based sulfurizations. The composition of the Ag2S thin films was studied using X-ray diffraction, Raman spectroscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. We found that it strongly depends on the fabrication conditions, such as sulfurization time and temperature. These conditions, in turn, affect the electrical characteristics of the thin films, namely the resistivity and resistive switching. We were able to control the Ag2S stoichiometry and infer its dependence on the fabrication parameters for all the followed methods.