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We present results of a multimethod investigation of the polar antiferromagnets $\mathrm{N}{\mathrm{i}}_{2}\mathrm{CoTe}{\mathrm{O}}_{6}$ and $\mathrm{NiC}{\mathrm{o}}_{2}\mathrm{Te}{\mathrm{O}}_{6}$, inspired by the colossal magnetoelectric effect present in $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$. Both compounds crystalize in the same polar space group $R3$ as $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$, preserving the crystal symmetry at least from room temperature down to 2 K. $\mathrm{N}{\mathrm{i}}_{2}\mathrm{CoTe}{\mathrm{O}}_{6}$ and $\mathrm{NiC}{\mathrm{o}}_{2}\mathrm{Te}{\mathrm{O}}_{6}$ undergo antiferromagnetic phase transitions at ${T}_{\mathrm{N}}=55$ and 52 K, and spin-flop transitions at an external magnetic field of approximately 8 and 4 T, respectively. Both compounds present an incommensurate antiferromagnetic helical structure with spins lying in the $ab$ plane, in contrast to the collinear one along the $c$ axis in $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$. Moreover, dielectric anomalies are observed at their antiferromagnetic phase transitions, suggesting a magnetoelectric behavior. Spin and lattice dynamics studies by a combination of infrared, Raman, and terahertz spectroscopies were performed. Below ${T}_{\mathrm{N}}$, in both $\mathrm{N}{\mathrm{i}}_{2}\mathrm{CoTe}{\mathrm{O}}_{6}$ and $\mathrm{NiC}{\mathrm{o}}_{2}\mathrm{Te}{\mathrm{O}}_{6}$, low-frequency spin excitations extremely sensitive to external magnetic field were observed. At least one of these magnons was simultaneously seen in Raman and THz spectra of both compounds, therefore we propose to assign them to electromagnons.

Employing a combination of infrared, Raman and time-domain THz spectroscopies, we studied spin and lattice excitations in Ni 3 TeO 6 and its isostructural compounds obtained by substitution of Ni with Mn or Co. Low-temperature THz spectra revealed THz-range excitations sensitive to magnetic field; some of them can be assigned to electromagnons.

We present a structural and spectroscopic study of the compound $\mathrm{N}{\mathrm{i}}_{2}\mathrm{MnTe}{\mathrm{O}}_{6}$, closely related to the polar antiferromagnet $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$ known to show a colossal magnetoelectric effect and pronounced elementary magnetoelectric excitations. We prepared single crystals and polycrystalline samples of $\mathrm{N}{\mathrm{i}}_{2}\mathrm{MnTe}{\mathrm{O}}_{6}$ showing the same polar structure as $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$ from room temperature down to 4 K with the $R3$ space-group symmetry. Magnetic and dielectric measurements have indicated an antiferromagnetic phase transition at ${T}_{\mathrm{N}}\ensuremath{\approx}70\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, almost 20 K higher than that of $\mathrm{N}{\mathrm{i}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$. Extensive infrared, Raman, and terahertz spectroscopy experiments were employed for investigating lattice and spin excitations, revealing all phonons predicted by the factor group analysis. Terahertz spectra below ${T}_{\mathrm{N}}$ reveal one new excitation, which is strongly influenced by external magnetic field, thus assigned to a magnon.

Pb2MnTeO6, a new double perovskite, was synthesized. Its crystal structure was determined by synchrotron X-ray and powder neutron diffraction. Pb2MnTeO6 is monoclinic (I2/m) at room temperature with a regular arrangement of all the cations in their polyhedra. However, when the temperature is lowered to similar to 120 K it undergoes a phase transition from I2/m to C2/c structure. This transition is accompanied by a displacement of the Pb atoms from the center of their polyhedra due to the 6s2 lone-pair electrons, together with a surprising off-centering of Mn2+ (d5) magnetic cations. This strong first-order phase transition is also evidenced by specific heat, dielectric, Raman, and infrared spectroscopy measurements. The magnetic characterizations indicate an anti-ferromagnetic (AFM) order below TN approximate to 20 K; analysis of powder neutron diffraction data confirms the magnetic structure with propagation vector k = (0 1 0) and collinear AFM spins. The observed jump in dielectric permittivity near similar to 150 K implies possible anti-ferroelectric behavior; however, the absence of switching suggests that Pb2MnTeO6 can only be antipolar. First-principle calculations confirmed that the crystal and magnetic structures determined are locally stable and that anti-ferroelectric switching is unlikely to be observed in Pb2MnTeO6.

Very recently there was a report of the discovery using piezoelectric force microscopy (PFM) of a switchable ferroelectric polarization in 1.6 % tensile strained TiO2 thin film with anatase crystal structure (see N. Deepak et al. Adv. Funct. Mater. 2014, 24, 2844). The polarization disappeared only above 450 K, which was assigned as a Curie temperature, Tc. Here we have performed X-ray diffraction, second-harmonic generation (SHG) and infrared investigations of the same films. Phonon frequencies exhibit less than a 10 % shift down with the tensile strain and no anomaly near expected TC. SHG experiment did not reveal any signal characteristic for inversion symmetry breaking, as expected in ferroelectric phase, and the c-lattice parameter exhibits no anomaly on heating near expected TC. Based on these results, we can summarize that we were not able to confirm the previously discovered ferroelectricity in tensile-strained anatase-TiO2 thin films.

International audience; 2 count the nearest and next-nearest neighbor exchange interactions, two Dzyaloshinskii-Moriya interactions and an easy-axis anisotropy. 31,35 The same model successfully described the low-energy inelastic neutron scattering spectra. 30 In contrast, Komandin et al. 15 showed IR transmission spectra with an excitation at approximately 47 cm −1 which had not been previously reported by experimental or theoretical studies. The intensive discussion in the literature concerning the nature of the spin ex-citations raises the question: which excitations are pure magnons (i.e., contribute only to the magnetic perme-ability µ) and which are electromagnons (i.e., influence at least partially the permittivity ε *)? It is worth noting that according to a recent symmetry analysis, 36 BiFeO 3 allows directional dichroism and therefore spin waves can be simultaneously excited by the electric and magnetic components of electromagnetic radiation. For more details on BiFeO 3 spin dynamics, see the review of Park et al. 37 In the current work, we report spin and lattice exci-tations in BiFeO 3 ceramics, as measured by the combination of IR reflectivity and time-domain THz transmission spectroscopy, in a temperature range from 10 to 900 K. All 13 IR-active phonon modes are observed, exhibiting softening on heating. Five low-frequency spin modes are detected from 5 K up to RT, the highest two appearing at 53 and 56 cm −1. This corresponds to the frequency range where such excitations were theoretically predicted, 31,32,35 but not experimentally confirmed up to now. At 5 K, the low-energy spin dynamics in the THz range were also studied in a varying magnetic field of up to 7 T. Softening of the (electro)magnon frequencies upon increasing the magnetic field was observed. Additionally, a BiFeO 3 epitaxial thin film grown on an orthorhombic (110) TbScO 3 single crystal substrate was studied for the first time via IR reflectance spectroscopy. II. EXPERIMENTAL DETAILS BiFeO 3 ceramics were prepared by the solid-state route. A stoichiometric mixture of Fe 2 O 3 and Bi 2 O 3 powder oxides with a purity of 99.99% was ground and uniaxially cold-pressed under 20-30 MPa pressure into 8 mm diameter pellets. The pellets were then covered by sacrificial BFO powder to avoid bismuth oxide loss from the pellet, and sintered in a tube furnace at 825 • C for8 h in air. To avoid any secondary phase formation, the samples were quenched to RT. Polished disks with a diameter of 4 mm and thicknesses of ca. 600 and 338 µm were used for the IR reflectivity and THz transmission measurements, respectively. An epitaxial BiFeO 3 thin film with a thickness of 300 nm was grown by reactive molecular-beam epitaxy on a (110) single crystal substrate. The growth parameters were the same as for the samples reported in Ref. 38. Near-normal incidence IR reflectivity spectra of the BiFeO 3 ceramics and film were obtained using a Fourier-transform IR spectrometer Bruker IFS 113v in the frequency range 20-3000 cm −1 (0.6-90 THz) at RT; for the low and high temperature measurements the spectral range was reduced to 650 cm −1. Pyroelectric deuterated triglycine sulfate detectors were used for the room-and high-temperature measurements up to 900 K, whereas a He-cooled (operating temperature 1.6 K) Si bolometer was used for the low-temperature measurements down to 10 K. A commercial high-temperature cell (SPECAC P/N 5850) was used for the high-temperature experiments. The thermal radiation from the hot sample entering the interferometer was taken into account in our spectra evaluation. THz measurements from 3 cm −1 to 60 cm −1 (0.09-1.8 THz) were performed in the transmission mode with the use of a custom-made time-domain terahertz spectrometer. In this spectrometer, a femtosecond Ti:sapphire laser oscillator (Coherent, Mira) produces a train of femtosecond pulses which generates linearly polarized broadband THz pulses in a photoconduct-ing switch TeraSED (Giga-Optics). A gated detection scheme based on electrooptic sampling with a 1 mm thick [110] ZnTe crystal as a sensor allows us to measure the time profile of the electric field of the transmitted THz pulse. The same high-temperature cell as for the IR reflectivity was used for the THz range high-temperature measurements. Oxford Instruments Opti-stat optical cryostats with mylar and polyethylene windows were used for the low-temperature THz and IR measurements , respectively. THz experiments in an external magnetic field H ext ≤ 7 T were performed upon decreasing H with an Oxford Instruments Spectromag cryostat in the Voigt configuration, where the electric component of the THz radiation E THz was set perpendicular to H ext. The IR reflectivity and THz complex (relative) per-mittivity spectra ε * (ω) were carefully fit assuming the factorized form of the dielectric function based on a generalized damped-harmonic-oscillator model: 39 ε * (ω) = ε ∞ N j=1 ω 2 LOj − ω 2 + iωγ LOj ω 2 TOj − ω 2 + iωγ TOj , (1) where ω TOj and ω LOj are the frequencies of the j-th transverse optical (TO) and longitudinal optical (LO) phonons, γ TOj and γ LOj are the corresponding damping constants, and ε ∞ denotes the high-frequency (elec-tronic) contribution to the permittivity, determined from the RT frequency-independent reflectivity tail above the phonon frequencies. The reflectivity R(ω) is related to the complex dielectric function ε * (ω) by: R(ω) = ε * (ω) − 1 ε * (ω) + 1. (2) To evaluate the IR reflectance spectra of the BiFeO 3 /TbScO 3 thin film, a model corresponding to a two-layer optical system was used. 40 The IR reflectivity spectra of the bare TbScO 3 substrate were fit first at each

A thermodynamically stable series of superlattices, (ZnO)kIn2O3, form in the ZnO-In2O3 binary oxide system for InO1.5 concentrations from about 13 up to about 33 mole percent (m/o). These natural superlattices, which consist of a periodic stacking of single, two-dimensional sheets of InO6 octahedra, are found to give rise to systematic changes in the optical and vibrational properties of the superlattices. Low-frequency Raman scattering provides the evidence for the activation of acoustic phonons due to the folding of Brillouin zone. New vibrational modes at 520 and 620 cm-1, not present in either ZnO or In2O3, become Raman active. These new modes are attributed to collective plasmon oscillations localized at the two-dimensional InO1.5 sheets. Infrared reflectivity experiments, and simulations taking into account a negative dielectric susceptibility due to electron carriers in ZnO and interface modes of the dielectric layer of InO2, explain the occurrence of these new modes. We postulate that a localized electron gas forms at the ZnO/InO2 interface due to the electron band alignment and polarization effects. All our observations suggest that there are quantum contributions to the thermal and electrical conductivity in these natural superlattices. © 2016 Author(s).