Anatolii Belous

Vacancy-induced enhancement of magnetic interactions in "Ca, Na…-doped lanthanum manganites

Structural, electric, and magnetic properties of the bulk polycrystalline samples with nominal composition La0.7Ca0.3−xNaxMnO3 (x=0−0.10) have been investigated in the work. As follows from the results of chemical and x-ray structural analyses, the increase in x brings about the rise in the number of structural vacancies in both lanthanum and oxygen sublattices. The influence of the variation of the vacancy number on peculiar features of the transition from ferromagnetic metallic state to paramagnetic state with activated conductivity has been studied in detail. The conclusion is made that the rise in the number of structural vacancies gives rise not only to the enhancement of magnetic inhomogeneity and broadening of the magnetic transition, but also to the increase in the temperature of magnetic transition. It is demonstrated that the changes in magnetic parameters also result in the increase of the temperatures of resistivity and magnetoresistance peaks as well as in the change of the character of the t...

Abnormal behavior of the dielectric parameters of Ba6−xLn8+2x/3Ti18O54 (Ln=La–Gd) solid solutions

The temperature dependence of the dielectric parameters e and tan δ for the entire x range of barium lanthanide titanate solid solutions (BLT ss) using the general formula Ba6−xLn8+2x/3Ti18O54 (Ln=La, Nd, Sm, Gd) was measured over wide frequency and temperature ranges. The results show the presence of diffuse regions of e(T) and tan δ(T) anomalies, which are located below (in the La- and Nd-containing solid solutions) or above (in the Sm- and Gd-containing solid solutions) room temperature. However, no evidence of a structural transition responsible for these anomalies could be determined by differential scanning calorimetry and x-ray diffraction analysis. To examine the nature of the abnormal temperature behavior the dielectric parameters in the systems studied were considered in terms of a simple oscillator model. Within the framework of this model correlations among the average size of the A-site ions, tilting of the oxygen octahedra, and the lattice strain are discussed, and the effect of crystallogra...

Negative permittivity and left-handed behavior of doped manganites in millimeter waveband

The “effective plasma frequency” of strontium-doped lanthanum manganite was determined basing on its double negative properties. The zone of high transmission (double negative zone) for spatially partitioned layered manganite specimen, immersed into dielectric matrix, was studied experimentally in millimeter waveband. It turns out that frequency dependence of manganite permittivity is well described by Drude formula, which includes only one parameter—the “effective plasma frequency.” Additionally, the negativity of the refraction index has been directly proved in experiments studying electromagnetic waves refraction in manganite prism.

Anomalies in the temperature dependence of the microwave dielectric properties of Ba6−xSm8+2x/3Ti18O54

Measurements of the microwave dielectric properties of Ba6−xSm8+2x/3Ti18O54 solid solutions revealed the existence of a broad maximum in the temperature dependence of the permittivity as well as the dielectric loss. Submillimeter-wave frequency measurements confirmed the intrinsic origin of the observed anomaly, which could possibly be ascribed to phase transition with unknown nature in the Ba6−xSm8+2x/3Ti18O54 solid solution. The temperature of the anomaly, which is in the range of 90–120 °C, depends on composition of Ba6−xSm8+2x/3Ti18O54 and decreases with an increase in x.

Effect of vacancies on the structural and relaxor properties of (Sr,Ba,Na)Nb2O6

It has been shown that the aliovalent substitution of sodium ions for strontium or barium ions in strontium barium niobate relaxor ferroelectric causes a decrease in vacancy content of the samples and a linear variation of unit cell volume with sodium concentration. The variation of c lattice parameter with sodium concentration is determined by deformations of the NbO6 oxygen octahedrons in the 001 direction. Dielectric studies carried out in a wide frequency range showed that the decrease in the vacancy content of the Sr0.6−xBa0.4Na2xNb2O6 and Sr0.6Ba0.4−yNa2yNb2O6 results in suppression of relaxor ferroelectric properties, which was manifested in both low-frequency and submillimeter region. Simultaneously, rise of the ferroelectric phase transition temperature was observed with increasing sodium concentration.

Temperature curve of magnetization and left-handed properties of La0.775Sr0.225MnO3

The left-handed features of Sr doped manganite lanthanum ceramics La0.775Sr0.225MnO3 in the vicinity of its Curie temperature are studied. The left-handed features are responsible for appearance of the additional transparency peak in the forbidden band of the photonic crystal bounded with the above manganite. The magnitude of the critical exponent β (β ∼ 0.50) determined in the work agrees with results reported in the literature and allows supposing that long-range interactions play an important role in the phase transition from conductive ferromagnetic to nonconductive dielectric paramagnetic state of the manganite in its Curie point.

Application of positron annihilation and Raman spectroscopies to the study of perovskite type materials

Defect properties of perovskite type materials, Ba3B′Nb2O9 (where B′=Mg, Zn, or Co), with near-stoichiometric compositions were studied by positron annihilation and Raman spectroscopies. Theoretical simulations of stoichiometric perovskites revealed a dependence of the positron bulk lifetime on the degree of ordering. In Ba3MgNb2O9 (BMN) the positron bulk lifetime for a completely disordered structure is 195 ps versus 237 ps for a completely ordered one. The predicted bulk lifetimes for Ba3ZnNb2O9 (BZN) and Ba3CoNb2O9 (BCN), with Pm3¯m symmetries are 193 ps and 194 ps, respectively. It was found that deviation from stoichiometry results in the appearance of secondary Ba- and Nb-rich phases, which according to theoretical simulations have bulk lifetimes much longer than that of the host material. Positron lifetime spectroscopy was used to monitor changes in the concentration of these second phases. The difference between predicted defect lifetimes and the bulk values for the studied perovskites was less th...

Synthesis of Iron Oxide Nanoparticles by Different Methods and Study of their Properties

In this work, iron oxide nanoparticles have been synthesized by precipitation in diethylene glycol, by cryochemical synthesis, in microemulsions using surfactants Triton X-100, Brij-35 and CTAB. Comparative spectroscopic, thermal, X-ray diffraction, 57Fe zero-field Mössbauer and magnetic studies of the synthesized nanoparticles have been carried out. Magnetic fluids prepared from synthesized nanopowders have been characterized by calorimetric measurements of specific loss power (SLP).

Magnetic Properties and AC Losses in AFe2O4 (A = Mn, Co, Ni, Zn) Nanoparticles Synthesized from Nonaqueous Solution

Nanosized particles of AFe2O4 (A = Mn, Co, Ni, or Zn) spinel ferrites were synthesized by coprecipitation from nonaqueous solutions using nitrate salts as starting reagents. The particles were characterized by X-ray diffraction, transmission electron microscopy, and magnetic measurements. Quasistatic magnetic measurements show superparamagnetic behavior with blocking temperature below room temperature for cobalt, nickel, and zinc spinel ferrite nanoparticles. Characteristic magnetic parameters of the particles including average magnetic moment of an individual nanoparticle and blocking temperature have been determined. The specific loss power which is released on the exposure of an ensemble of synthesized particles to a magnetic field is calculated and measured experimentally. It is shown that among all nanoferrites under study, the ZnFe2O4 nanoparticles demonstrate the highest heating efficiency in AC magnetic fields. The key parameters responsible for the heating efficiency in AC magnetic field have been determined. The directions to enhance the SLP value have been outlined.

Lithium Ion Conductors Based on System (Li,Na,La){Ti,Nb,Та}O with Perovskite Structure

Solid solutions with defect perovskite structure have been obtained in the systems Li0,5-уNayLa0,5TiO3, Li0.5-yNayLa0.5◊Nb2O6 and Li0.5-yNayLa0.5◊Та2O6 at 0 ≤ y ≤ 0.5. Their structure has been shown to undergo partial disordering with increasing sodium content in the system Li0.5-yNayLa0.5◊Nb2O6 as in the system Li0.5-yNayLa0.5◊Та2O6 structure has been ordering. Lithium diffusion in system Li0,5-уNayLa0,5TiO3 follows a percolation model for the lithium diffusion. The ionic conductivity as a function of sodium content in the system Li0.5-yNayLa0.5◊Nb2O6 has a maximum. The ionic conductivity of Li0.5-yNayLa0.5◊Ta2O6 samples decreases with sodium content.