[en] The reflectance of ceramic YMnO₃ formed by randomly oriented large crystallites (average size about 10 µm) was measured at near normal incidence in the spectral range 100–1200 cm⁻¹. A calibration method was used in order to compensate for the effect of diffuse reflection on the surface of the sample. The spectrum was modelled by averaging the single-crystal reflectivity over all possible crystalline orientations. This anisotropy averaging method was found to adequately describe most of the spectrum, but failed in the regions where one of the principal refractive index became smaller than unity. We argue that these discrepancies are due to enhanced resonant scattering, and discuss the effect with the help of a simplified model of independent spheres based on the Mie theory of light scattering. (paper)

[en] In this work, we have studied in detail the temperature dependence of the electric polarization of Eu_0.8Y_0.2MnO₃ aimed at clarifying the controversial issues concerning the ferroelectric nature of the lower temperature magnetic phases and hence its multiferroic character. The existence of a spontaneous polarization in 30 K < T < 22 K, provides clear evidence for the ferroelectric character of the re-entrant non-collinear spiral-antiferromagnetic phase, stable in that temperature range. Contrary to results published in previous works, our experimental data clearly show that the weak-ferromagnetic, canted antiferromagnetic phase stable below 20 K is not intrinsically ferroelectric. The misinterpretation, regarding the polar character of the lower temperature magnetic phases, stems from the existence of an induced polarization occurring below 30 K. The mechanisms associated with polar and magnetic properties, and their correlation with both spin and lattice structures are also discussed.

[en] Although Cd and Ca ions have the same valence and cation size, their incorporation into vacancy-doped La manganites induce different properties. While the incorporation of Ca leads to high Tc up to 250 K and induces a metallic-like behaviour, the incorporation of Cd severely reduces Tc and promotes insulator-like behaviour. In this work, the Cd hyperfine fields have been measured with the Perturbed Angular Correlations (PAC) technique after implantation and annealing of ^111mCd in La-Cd-MnO₃ samples. The PAC results are compared with measurements of the resistivity and magnetization performed on the same samples. The mixed La and Mn site Cd occupancy is suggested as a possibility to explain the properties of the La-Cd-MnO₃ system

[en] In this work we investigate the magnetic field dependence of A_g octahedra rotation (tilt) and B_2g symmetric stretching modes frequencies at different temperatures. Our field-dependent Raman investigation at 10 K is interpreted by an ionic displacive nature of the magnetically-induced ferroelectric phase transition. The increasing frequency of the A_g tilt mode with magnetic field gives evidence for the increase of the MnO₆ tilt angle, which in turn leads to a weakening of the ferromagnetic exchange interaction, yielding the necessary conditions for the onset of ferroelectricity on the basis of the inverse Dzyaloshinskii-Moriya interaction. The frequency change of the B_2g symmetric stretching mode at the ferroelectric phase transition enables one to estimate the shift of oxygen ion positions and the corresponding spontaneous polarization of 480 μC m^-2 in magnitude, which is of the same order as earlier reported values in single crystals.

[en] In this work, resistive switching in pulsed laser deposited ferroelectric lead-free 0.5Ba(Zr_0.2Ti_0.8)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ (0.5BZT–0.5BCT) thin films was investigated. This study reveals that films grown at 5.5 J cm⁻² have shown optimal ferroelectric and resistive switching response, which are attributed to high tetragonality, large grain size and less defect concentration. Au/0.5BZT–0.5BCT/Pt capacitors show the electroforming free resistive switching that is explained based on the polarization modulation of the Schottky-like barrier at the 0.5BZT–0.5BCT/Au interface. The polarization induced resistive switching is evidenced by its disappearance as the temperature increases to the Curie temperature. The capacitor based on film grown at 5.5 J cm⁻² shows resistive switching characterized by high switching ratio of 10⁶ at a low set/reset voltage  ≈1 V, and by a stable memory window, which are highly required for memory applications. (paper)

[en] Eu_1-xY_xMnO₃, compared against other magnetoelectric systems, exhibits very distinctive features. Its magnetoelectric properties are driven by the magnetic spin of the Mn³⁺ ion, but they can be drastically changed by varying the content of Y³⁺, which does not carry any magnetic moment. Although the x = 0.40 composition has been studied extensively, some basic areas still remain to be thoroughly understood. Thus, this work is aimed at studying some of its polar properties and their magnetic field dependence as well. The experimental results reported here show that this material is very easily polarizable under external electric fields, and so, whenever the polarization is obtained from time integration of the displacement currents, an induced polarization is superposed on the spontaneous one, eventually masking the occurrence of ferroelectricity. We have found clear evidence for the influence of a magnetic field in the polar properties of Eu_0.6Y_0.4MnO₃. The study of electric polarization of Eu_0.6Y_0.4MnO₃ under an external magnetic field yields a value with the same order of magnitude of the remanent polarization as was determined from polarization reversal experiments. The comparison of the magnetically induced changes in the polarization obtained for polycrystalline samples and single crystals confirms the threshold magnetic field value for the polarization rotation from the a-direction to the c-direction, and provides evidence of the importance of the granular nature of the samples in the polar response to the magnetic field.

[en] The Perturbed Angular Correlation (PAC) technique was applied to study the yttrium local environment in YMnO₃ multiferroic manganite. The electric field gradients (EFG) at the Y site have been measured as function of temperature, covering both ferroelectric and magnetic transitions. The results were compared with point charge model (PCM) calculations. The experimental results show two different EFG distributions for all temperatures. Only one can be directly attributed to the yttrium crystalline site in the hexagonal structure.

[en] Deliberately oxygen deficient potassium tantalate thin films were grown by RF magnetron sputtering on Si/SiO₂/Ti/Pt substrates. Once they were structurally characterized, the effect of oxygen vacancies on their electric properties was addressed by measuring leakage currents, dielectric constant, electric polarization, and thermally stimulated depolarization currents. By using K₂O rich KTaO₃ targets and specific deposition conditions, KTaO_3−δ oxygen deficient thin films with a K/Ta = 1 ratio were obtained. Room temperature X-ray diffraction patterns show that KTaO_3−δ thin films are under a compressive strain of 2.3% relative to KTaO₃ crystals. Leakage current results reveal the presence of a conductive mechanism, following the Poole-Frenkel formalism. Furthermore, dielectric, polarization, and depolarization current measurements yield the existence of a polarized state below T_pol ∼ 367 °C. A Cole-Cole dipolar relaxation was also ascertained apparently due to oxygen vacancies induced dipoles. After thermal annealing the films in an oxygen atmosphere at a temperature above T_pol, the aforementioned polarized state is suppressed, associated with a drastic oxygen vacancies reduction emerging from annealing process

[en] The ferromagnetically coupled cobalt ion is observed to create a magnetocrystalline anisotropy in the PrNi_5-xCo_x structure above a critical composition of x = 2. The competition of the anisotropy energies between Co and Pr sublattices gives rise to a spin reorientation (SR) phenomenon in PrNi_5-xCo_x compounds at a low temperature (∼150 K) which is then followed by a magnetic transition at a higher temperature. Co-doping has a strong influence on the Curie temperature, changing it from ∼60 K (x = 1.95) to ∼537 K (x = 3). The magnetic entropy change is associated with SR as well as a magnetic transition, and correspondingly a large full width at half maximum (δT_FWHM) is obtained for this series of compounds. For example, the PrNi_2.85Co_2.15 compound presents δT_FWHM = 166 K at a 1 T field. This series therefore has an appreciable relative cooling power, which makes this material a suitable magnetic refrigerant over a large temperature span.

[en] We report perturbed angular correlation measurements with ^111mCd/¹¹¹Cd and ¹¹¹In/¹¹¹Cd probes, at the ISOLDE-CERN facility, in the manganite compounds BaMnO₃, with the 6H and 15R polymorphs, and SrMnO₃, with the 4H polymorph. The electric field gradient (EFG) is measured, and found approximately constant in a large temperature range for all the compounds. The EFG is also calculated from first principles with density functional theory, and compared with experimental results by considering diluted substitutional Cd impurities. Based on the results, we assign as sites for the probes the Ba (for BaMnO₃-6H, 15R) and Sr (for SrMnO₃-4H) sites, apart from fractions of undetermined origin in the case of BaMnO₃-6H. We predict the hyperfine parameters in the recently synthesized multiferroic manganite Sr_0.5Ba_0.5MnO₃, and its variation with the structure and electric polarization, which is found to be very small. (paper)