L. D. Iskhakova

Magnetic and magnetoelectric properties of M-type substitution hexaferrites TScxFe12 − xO19 (T = Ba, Sr)

Single crystals of M-type substitution hexaferrites BaScxFe12 − xO19 and SrScxFe12 − xO19 (x = 1.3–1.7) have been grown. Their anisotropic magnetic and magnetoelectric properties have been studied at temperatures of 2–360 K in magnetic fields up to 200 kOe. In all compounds under study, a spontaneous transition from the collinear ferrimagnetic uniaxial phase to a conical structure has been detected with a decrease in the temperature at a temperature increasing with the concentration of Sc up to room temperature. In magnetic fields both parallel and perpendicular to the c axis, the conical phase is suppressed and the transition to the collinear ferrimagnetic structure occurs. Phase diagrams have been plotted in H-T coordinates. In conical magnetic structures, a magnetic-field-induced electric polarization (higher than 40 μC/m2 at 4 K) has been detected in the geometry corresponding to the inverse Dzyaloshinskii-Moriya interaction mechanism (spin current). A higher stability of the polar state with respect to magnetic fields (higher than 50 kOe) and temperatures (up to 100 K) has been revealed as compared to previous studies of the Ba system. Various possibilities of controlling the chirality of the conical structure determining the sign of the polarization, which is important for applications, have been demonstrated.

Magnetic properties and phase transitions in hexagonal DyMnO3 single crystals

DyMnO3 single crystals of hexagonal modification were grown for the first time by zone melting, and their anisotropic magnetic properties were studied in detail. At T ∼ 7 K, a small uncompensated magnetic moment (∼1 μB per formula unit) is shown to occur along the hexagonal axis. At temperatures 7–45 K, a transition from the purely antiferromagnetic state to a state with uncompensated magnetic moment was revealed; the transition is induced by a magnetic field H parallel to the c axis. This transition seems to be due to a ferrimagnetic ordering of the moments of Dy cations located in nonequivalent crystallographic positions.

Effect of BiFeO3 ceramics morphology on electrodynamic properties in the terahertz frequency range

The nature of the contribution (additional to the phonon one) to the permittivity of bismuth ferrite ceramics has been studied using submillimeter and Fourier transform infrared spectroscopy. It has been shown that its numerical value depends on morphological features of samples, and the nature is determined by the defectness and crystallite interfaces. The effect of structure-phase features of bismuth ferrite ceramics on antiferromagnetic resonance line frequencies is indicated.

Magnetic and dielectric properties of orthorhombic and hexagonal multiferroics Tb1 − xYxMnO3

The transition from a stable orthorhombic structure to a hexagonal structure has been revealed in Tb1−xYxMnO3 multiferroics at x = 0.2–0.4. It has been shown that almost single-phase crystals with an orthorhombic or hexagonal structure can be obtained by choosing the growth conditions. It has been found that the magnetic and dielectric properties of orthorhombic single crystals with x = 0.2–0.3 are similar to the properties of pure TbMnO3 and are characterized by a strong anisotropy of the magnetic susceptibility at low temperatures and by the presence of a number of magnetic phase transitions, including those to the ferroelectric state. New spontaneous (T ≤ 15 K) and magnetic-field induced (H | C6) phase transitions accompanied by the appearance of an uncompensated rare-earth magnetic moment ∼1 μB/mole have been observed in hexagonal single crystals with x = 0.3–0.5.

Electrodynamic properties of lead Zirconate-Titanate thin films in the terahertz frequency range

The transmission/reflection spectra of bilayer structures consisting of thin amorphous and polycrystalline Pb(Zr0.52Ti0.48)O3 ferroelectric films deposited on dielectric substrates of magnesium oxide MgO and sapphire α-Al2O3 were measured in the frequency range of 5–4000 cm−1. Based on these spectra and using the dispersion analysis method, the spectra of complex dielectric permittivity ɛ*(ν) and dynamic conductivity σ′(ν) of the films were simulated, the electrodynamic parameters of the films were determined, and the dielectric dispersion responsible for the formation of static permittivity was found.

Magnetic and magnetoelectric properties of substituted M -type SrSc x Fe 12 – x O 19 hexaferrites

The anisotropic magnetic and magnetoelectric properties of single crystals of substituted M-type SrScxFe12 – xO19 (x = 1.4–1.7) hexaferrites are studied at temperatures of 2–800 K in magnetic fields up to 50 kOe. A spontaneous transition from a collinear ferrimagnetic single-axis phase to a conical structure is detected in all compositions, the transition temperature increases with the Sc concentration, and the Curie temperature decreases with increasing Sc concentration. The conical magnetic structures exhibit an electric polarization (more than 40 μC/m2 at T = 4 K) induced by a magnetic field. The conditions of appearance of the polarization indicate that it is caused by the inverse Dzyaloshinskii–Moriya interaction. The dependences of the polarization on the value and the orientation of a magnetic field are investigated, and the possibilities of controlling the chirality of a conical structure, which determines the sign of polarization, are demonstrated.

Electrodynamic response of Ca 1 – x Pb x TiO 3 two-phase solid solution in a wide frequency range

The dielectric response of Ca1–xPbxTiO3 two-phase ceramics is studied in a wide frequency range, from 120 Hz to 100 THz, using impedancemetric radio-frequency methods and infrared spectroscopy. The problems of applications of various dispersion models to analyze infrared spectra of disordered ceramics in which high permittivities are formed in the microwave–terahertz ranges are discussed. The effect of low-frequency relaxation processes on the infrared spectrum and the formation of the constant level of dielectric losses is indicated.

Single crystals growth of hexaferrits M-type MTixCoxFe12−2xO19 (M = Ba, Sr) by floating zone and investigation of their magnetic and magnetoelectric properties

Floating zone melting method with optical heating is elaborated to grow single crystals of the substituted hexaferrites BaTixCoxFe12−2xO19 and SrxTixCoxFe12−2xO19 (0.8 ≤ x ≤ 2). The dynamics of the growth process is studied and results of the analysis of impurity phases appearing in the initial stages of the crystal growth are presented. Compositions and unit-cell parameters of crystals are determined. Electrical, magnetic and magnetoelectric properties of grown crystals are investigated at temperatures 2–365 K and magnetic fields up to 50 kOe. It is shown that the resistivity of annealed in oxygen crystals at room temperature is ∼106 Ohm·cm while at helium temperatures the crystals become good insulators. Magnetic measurements reveal conical spin structures in the crystals at some concentrations and temperatures. Magnetic field induced electric polarization of the low value (∼0.3 μC/m2) is detected at liquid helium temperatures for compositions with Ti and Co concentrations x = 0.8–0.9.

Electric and magnetic properties of titanium-cobalt-oxide single crystals produced by floating zone melting with light heating

Single crystals of spinel Co2TiO4, CoTiO3 of ilmenite structure and pseudobrookite CoTi2O5 were grown by means of zone melting equipment URN-2-ZM. The growth processes were performed in an air atmosphere with a speed ∼10 mm/h and a final annealing of the crystal at temperature of 1250 °C. Co2TiO4 and CoTi2O5 crystals were obtained with a diameter of 12–15 mm and length up to 60 mm, being free of any other phase inclusions and cracks. However, in the CoTiO3 crystals some controversial features were found: x-ray Laue analysis indicated high-structure perfection while an electronic microscopy revealed small amount of second phase inclusions in contradiction to known phase diagram of this system. Electrical and magnetic properties of grown Co2TiO4 and CoTiO3 single crystals were studied. Semiconducting behavior of the Co2TiO4 was established with the energy gap of ∼1.3 eV. No magnetic anisotropy was found in the cubic Co2TiO4 single crystals showing a magnetic behavior similar to polycrystals. The rhombohedra...

Temperature evolution of dielectric response spectra of stillwellite-like glasses in the terahertz and infrared ranges

The reflectance and transmittance spectra of the PrBGeO5 glass were measured in the terahertz and infrared ranges. At low temperatures, an increase in the permittivity and the appearance of narrow resonance-type lines in the terahertz range were revealed. The complex permittivity spectra were calculated from the Fresnel and Kramers-Kronig formulas.