Controlled generation of coherent spin waves with highest possible frequencies and shortest possible wavelengths is a cornerstone of spintronics and magnonics. Here, using Heisenberg antiferromagnet RbMnF3, we demonstrate that laser-induced THz spin dynamics corresponding to pairs of mutually coherent counter-propagating spin waves with the wavevectors up to the edge of the Brillouin zone cannot be understood in terms of magnetization and antiferromagnetic (Néel) vectors, conventionally used to describe spin waves. Instead, we propose to model such spin dynamics using the spin correlation function. We derive a quantum-mechanical equation of motion for the latter and emphasize that unlike the magnetization and antiferromagnetic vectors the spin correlations in antiferromagnets do not exhibit inertia.

Experimental studies demonstrate that the low-frequency dielectric permittivity of many magnetic materials is sensitive to the spin ordering, but the microscopic mechanisms of the observed phenomena still remain poorly understood. Here we report on the study of lattice dynamics and the spontaneous magnetodielectric effect using far-infrared and low-frequency dielectric spectroscopy in the model fluoroperovskites ${\mathrm{KCoF}}_{3}$ and ${\mathrm{RbCoF}}_{3}$ in the temperature range of 5--300 K, which includes the antiferromagnetic transition at ${T}_{N}=115$ K and 101 K, respectively. We show that the dielectric permittivity is mainly defined by the transverse TO and longitudinal LO infrared-active phonons and their specific contributions were determined. The anomalous growth of the low-frequency dielectric permittivity observed in ${\mathrm{KCoF}}_{3}$ with cooling is explained by the increase of the $\text{LO-TO}$ splitting of the lowest frequency phonon caused by about $7\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ softening. An important conclusion is that, microscopically, the spontaneous magnetodielectric effect is caused by the frequency shifts of only those TO and LO phonons which change the ${180}^{\ensuremath{\circ}}$ angle of the superexchange ${\mathrm{Co}}^{2+}\phantom{\rule{0.16em}{0ex}}\ensuremath{-}\phantom{\rule{0.16em}{0ex}}{\mathrm{F}}^{1\ensuremath{-}}\phantom{\rule{0.16em}{0ex}}\ensuremath{-}\phantom{\rule{0.16em}{0ex}}{\mathrm{Co}}^{2+}$ pathway, thus resulting in its modulation due to the spin-phonon coupling. The observed anomalous softening of the lowest frequency phonon and increase of the low-frequency dielectric permittivity were interpreted as a manifestation of the ferroelectric instability in cubic fluoroperovskites.

Recent theoretical studies predict potential multiferroicity in ABF(3) fluoroperovskites, challenging their experimental confirmations. Here, we show that magnetic manganese fluoroperovskites NaMnF3, RbMnF3, and CsMnF3 reveal clear evidence of intrinsic lattice instability via the temperature behavior of the dielectric permittivity epsilon(0) (T). Mechanisms of the instability closely correlate with the geometric tolerance factor t and differ from those in oxide perovskites. In particular, a dramatic 170% growth of the permittivity epsilon(0) (T) takes place at low temperatures in NaMnF3 with the smallest tolerance factor t = 0.78. Below T-N, this growth is accompanied by a huge magnetodielectric effect of about 25%. These findings allow us to call NaMnF3 an incipient multiferroic and open up opportunities for designing multiferroics on the basis of fluorides using chemical and strain engineering.

The dielectric response, conductivity, and domain structure of (Na 1/2 Bi 1/2 )TiO 3 single crystals are studied in the temperature range of 290–750 K for the [100], [110], and [111] crystallographic directions. It is shown that the region of optical isotropization is observed in polarized light in the temperature range of 570–620 K. In this case, the birefringence (Δ n ) decreases and disappears (together with the image of the domain structure) for the [100] directions. The region of optical isotropization in the [111] directions is characterized by the disappearance of the image of the domain structure and by the existence of individual regions with partial quenching. The domain structure in the [110] directions remains distinguished against the background of a significant decrease in Δ n in the indicated temperature range. The region of isotropization is also manifested in the temperature dependence of the imaginary part of the dielectric response and is determined by the isotropic character of the conductivity in the range of 570–620 K. The bulk conductivity has a thermally activated character with activation energies E a = 50−60 meV at T < 500 K and E a = 700−900 meV for T > 620 K. The low-frequency dispersion of the dielectric response is determined by the Maxwell–Wagner mechanism and is due to an increase in the ionic conductivity at temperatures above 620 K. The anisotropy of the susceptibility holds in the entire studied ranges of frequencies (25 Hz–1 MHz) and temperatures.

The f–f absorption and emission spectra of Er3+ impurities were observed and identified in KTaO3 and K1 – x Li x TaO3 single crystals. It was shown that Li off-centers related random fields markedly influence intensity, width and position of zero-phonon lines. The detail analysis of the absorption spectra obtained allowed one to determine energies of the Stark sublevels of excited states for Er3+ dominant centers of noncubic symmetry.

We used optical ellipsometry to study and elucidate the nature of the negative thermo-optic effect in KTaO3 single crystals, which consists of an increasing refraction index magnitude in the visible region upon cooling (dn/dT < 0). The optical constant functions were determined within the 1–6 eV region. The energies of the band edge indirect and direct optical transitions and their temperature evolutions in the region of 80–300 K were carefully investigated. The absorption edge in the region of the lowest indirect R → Γ optical transitions, 3.7–4.2 eV, was observed to shift to higher energies upon cooling. Simultaneously, an increase and ‘red shift’ of the optical absorption in the region of the direct X5'upper → X3 transitions (4.3–4.8 eV) were observed, which should be responsible for the nature of the observed negative thermo-optics in KTaO3.

Brillouin light scattering studies of the low-frequency region of vibrational spectra of a partially disordered ferroelectric crystal Na1/2Bi1/2TiO3 (NBT) from 300 to 850 K are presented. It is shown that the light scattering spectra contain a multicomponent quasi-elastic scattering (QELS) component observed in a broad frequency range, from 800 to 0.7 GHz. Reconstruction of the inelastic light scattering spectra in NBT in the frequency region from 0.7 to 800 GHz did not reveal relaxation processes (α and β relaxations) typical of disordered compounds (glasses, supercooled liquids, etc.). The fractal approach also proved to be inapplicable to the description of QELS in NBT. Detailed analysis of the Brillouin spectra with different free spectral ranges (“frequency windows”) showed that different contributions to the quasi-elastic light scattering manifest themselves in different frequency windows. These contributions are associated with the structural phase transitions and other processes responsible for the emergence of QELS (domain structure evolution, heterophase fluctuations, etc.). Thus, we suggest a new approach to the analysis of the temperature behavior of quasi-elastic scattering. It allows one to study the critical dynamics of the crystal lattice and the nonphonon contributions into vibrational spectra of partially disordered crystals.

Single crystals of Na1/2Bi1/2TiO3-KTaO3 (NBT-KT) solid solutions have been grown for the first time. The elemental composition of as-grown single crystals determined by X-ray diffraction corresponds to 0.81 NBT-0.19 KT. Measurements of the temperature dependences of dielectric characteristics showed that the obtained NBT-KT crystals possess clearly pronounced relaxor properties.

The temperature dependences of the dielectric constant and dielectric hysteresis loops in ceramic samples of (1 − x )SrTiO 3− x KNbO 3 and (1 − x )SrTiO 3− x KTaO 3 (0 ≤ x ≤ 0.3) solid solutions prepared using different heat treatments have been investigated. Phase diagrams of the studied solid solutions have been constructed in the T - x coordinates. It has been shown that, after quenching of samples (spontaneous cooling at room temperature after long-term heating at the sintering temperature of the ceramic samples), the temperature of the induced phase transition increases because of the weakening of random electric fields associated with nonisovalent impurities due to their “frozen” nonequilibrium redistribution. For small concentrations x , strong dielectric relaxation is observed in the temperature range of 150–250 K. A model of relaxing centers, which is based on the local charge compensation of heterovalent impurities, has been proposed.

We present a detailed investigation of structural properties of manganese doped strontium titanate ceramics and Verneuil grown single crystals, the last prepared with Sr deficiency compensated by proper Mn amount. EXAFS, TEM, ESR and Raman techniques, combining local and lattice probes, allows to inspect the local chemical environment and electronic structure of manganese taking into account possible chemical non-homogeneities and/or phase segregation. On the basis of experimental evidences, structural models for Mn insertion in SrTiO3 are discussed considering possibility for Mn ions to occupy both Ti4+ and Sr2+ sites as well as manganese segregation and Mn incorporation related non-homogeneities.

Strongly anisotropic photoinduced electron paramagnetic resonance spectrum was found in the KTa1-xNbxO3 crystals with x = 0.012 at T < 10 K, which is described well within the model of the center with S = 1, g(parallel to) = 0.82 +/- 0.04, g(perpendicular to) = 0.52 +/- 0.04, and D = 0.44 +/- 0.03 cm(-1). Analysis shows that this spectrum originates from the Nb4+-O- polaronic exciton in the triplet state.

Abstract The results of experimental studies of the crystal and domain structures of NaO, 5BiO, 5TiO3 as well as its lattice dynamics, dielectric, optical and other physical properties are considered. Nature of peculiar temperature points is discussed.

The formation of the photo-polaronic excitons in ABO_{3} perovskite type oxides has been detected experimentally by means of the photoinduced electron paramagnetic resonance studies of KTa_{0.998}Nb_{0.012}O_{3} crystals. The corresponding microwave X-band spectrum at T < 10 K consists of a narrow, nearly isotropic signal located at g ~ 2 and a strongly anisotropic component. The first signal, which has a rich structure due to hyperfine interactions with the lattice nuclei, is attributed to the single trapped charge carriers: the electrons and/or the holes. The anisotropic spectrum is caused by the axial centers oriented along the C_{4} pseudo-cubic principal crystalline axes. The spectrum angular dependence can be described well by an axial center with S = 1, g_{\parallel) = 0.82, g_{\perp} = 0.52 and D = 0.44 cm^{-1}. The anisotropic spectrum is attributed to the Nb^{4+}-O^{-} polaronic excitons. The temperature dependence of the anisotropic component is characterized by two activation energies: the internal dynamics activation E_{a1} = 3.7\pm0.5 meV, which makes the EPR spectrum unobservable above 10 K, and the destruction energy E_{a2} = 52\pm4 meV. By comparing the anisotropic photo-EPR spectrum and the photoinduced optical absorption temperature dependencies, we found that the Nb^{4+}-O^{-} polaronic excitons also manifested themselves via the ~0.7 eV wide absorption band arising under UV light excitation in the weakly concentrated KTaO_{3}:Nb crystals.

We report the first experimental observation of the photoinduced EPR spectra in KTa1-xNbxO3 crystals (x = 0.012). The signal has two components: nearly isotropic with g(eff) = 2 and strongly anisotropic one. Isotropic signal originates most probably from single localized photo-carriers electrons or holes. Anisotropic spectrum is observed below 10 K and reveals a set of characteristic properties. It originates from the axial centers with axes coinciding with the C-4 axes of the crystal. Effective g-factor values are g(parallel to) = 2.106(3) and g(perpendicular to) < 0.8. Angle dependencies of the intensity and the width of the anisotropic signal as well as the resonance field can qualitatively be explained within the simple J = 1 model. The observed anisotropic photoinduced EPR spectrum is tentatively assigned to excitons or bipolarons.

The results of the study of Mandelstam-Brillouin light scattering in single-crystal Na 1/2 Bi 1/2 TiO 3 within a broad temperature interval covering successively the cubic, tetragonal, and rhombohedral phases are reported. Measurements conducted within the range 820–300 K have revealed quasi-elastic scattering and distortion of the phonon spectra in shape, which suggests possible coupling between the low-frequency transverse optical and acoustic modes. These two observed phenomena are believed to be closely interrelated.

The dielectric permittivity (100 Hz-1 MHz) and IR reflectivity of SrTiO(3): Mn (0.1 at.%) crystals and SrTiO3: Mn(0.1-3 at.%) ceramics prepared maintaining the ratio (Sr + Mn)/Ti = 1 or (Ti + Mn)/ Sr = 1 (A-and B-type ceramics, respectively) were studied. Surprisingly, the magnitude and epsilon'(T) dependence for the crystals and B-type ceramics with 0.1 at.% of Mn appeared to be nearly the same; the magnitudes rise on cooling and reach similar to 3000 at T = 7 K. At the same time, IR spectra of SrTiO3: Mn(0.1 at.%) crystals reveal that TO (transverse optical) phonons yield a magnitude of low frequency permittivity of similar to 4000 at T = 5 K, evidencing crystal inhomogeneity. Both crystals, like B-type SrTiO(3): Mn (0.1 at.%) ceramics, reveal faint low temperature relaxations with the Arrhenius activation energies similar to 37-40 meV and similar to 120-130 meV (P- and T-relaxations). These relaxations emerge in losses only and were tentatively attributed to reorientation of defects associated with Mn(Ti)(2+)-O(-)-type polaronic centres and O(h)(1)-D(4h)(18) phase transition related effects, respectively. In reduced crystals P-relaxation strongly enhances, becoming distinctly seen in the real part of the permittivity. More heavily Mn doped A- and B-type ceramics reveal strongly pronounced Arrhenius-type dielectric relaxation with activation energy similar to 59-68 meV depending on the Mn concentration.