Yu. P. Stepanov

Preparation and magnetic properties of LaMnO3 + δ (0 ≤ δ ≤ 0.154)

The structural, electrical, and magnetic properties of ceramic perovskite manganites LaMnO3 + δ (δ = 0–0.154) are investigated. It is found that, in a weak magnetic field (B = 2 G), the LaMnO3 + δ manganite with δ = 0.065 at temperatures below the Curie temperature TC of the paramagnet-ferromagnet phase transition has a mixed (spin glass + ferromagnet) phase. In LaMnO3 + δ manganites with the parameter δ = 0.100–0.154, this phase transforms into a frustrated ferromagnetic phase. A similar transformation was observed previously in La1−xCaxMnO3 compounds at calcium contents in the range 0 ≤ x ≤ 0.3. This similarity is explained by the fact that, in both materials, the Mn4+ concentration and, accordingly, the hole concentration c change equally in the concentration range from ∼0.13 to 0.34 with an increase in x or δ. However, the magnetic irreversibility, the concentration dependences of the Curie temperature TC(c) and the magnetic susceptibility X(c), and the critical behavior of the temperature dependence of the susceptibility X(T) in the vicinity of the Curie temperature TC differ substantially for these two materials. The observed differences are associated with the distortion of the cubic perovskite structure, the decrease in the degree of lattice disorder, and a more uniform distribution of holes in the LaMnO3 + δ manganites as compared to the La1 − xCaxMnO3 compounds.

DSC INVESTIGATION OF OXYGEN SUBSYSTEM IN Ba-Y-Cu-O SUPERCONDUCTORS

DSC investigations have been performed for a series of compounds Ba2YCu3Oy with the oxygen content varying in the rangey=6.0...6.9 by means of various heat treatments at 800–1200 K followed by quenching, or through the chemical extraction of oxygen by placing the sample in dihydrogen at 470–490 K. The sample preserving a constant oxygen content during heating in nitrogen exhibited exothermal effects between 450 and 850 K. It has been shown that the ΔH vs. y function reaches maximum aty ≈ 6.5. Kinetic measurements have shown that the diffusive mobility of oxygen atoms in the lattice is responsible for these effects, viz. the Arrhenius and cooperative processes of reorganization in the non-equilibrium oxygen subsystem of the bulk.ZusammenfassungFür eine Reihe von Verbindungen der allgemeinen Formel Ba2YCu3Oy und einem Sauerstoffgehalt vony=6.0....6.9 wurden mittels verschiedener Wärmebehandlungen bei 800–1200 K, gefolgt durch Abschrecken oder beim chemischen Sauerstoffentzug durch Einbringen der Probe in Diwasserstoff bei 470–490 K DSC-Untersuchungen durchgeführt. Wird die Probe in Stickstoff erhitzt, so behält sie ihren Sauerstoffgehalt bei und zeigt bei 450–850 K einen exothermen Effekt. Es wurde gezeigt, daß die Funktion ΔH(y) bei etwa y=6.5 ein Maximum erreicht. Kinetische Messungen zeigen, daß für diesen Effekt die diffusive Beweglichkeit der Sauerstoffatome im Gitter verantwortlich ist.

Effect of copper doping on charge ordering in La1/3Ca2/3Mn1 − yCuyO3 (0 ≤ y ≤ 0.07)

The effect of copper doping on charge-orbital ordering in La1/3Ca2/3Mn1 − yCuyO3 (0 ≤ y ≤ 0.07) is studied by measuring the temperature dependences of the magnetization, the electrical resistivity, and the heat capacity in combination with an electron microscopic investigation of the structure. It is demonstrated that copper doping leads to a lowering of the charge ordering temperature TCO and that this decrease is proportional to the decrease in the Mn3+ ion concentration. In the temperature range 5–300 K, the semiconducting pattern of the electrical resistivity persists for all values of 0 ≤ y ≤ 0.07. Electron microscope studies have shown that the presence of copper suppresses the formation of a regular superstructure, which is characteristic of the undoped starting compound, beginning already from low concentrations (y = 0.01). Differential scanning calorimetry revealed a substantial decrease in the transition entropy at the onset of charge ordering in copper-doped samples as compared to the starting compound. Doping with copper destroys long-range charge-orbital ordering and retains apparently only short-range order.

Effect of doping with iron on the charge ordering in La0.33Ca0.67Mn1−y Fe y O3 (y = 0, 0.05) manganites

The formation and specific features of the superstructure in La0.33Ca0.67Mn1−yFeyO3 (y = 0, 0.05) manganites doped with iron are investigated using transmission electron microscopy. The electron diffraction patterns of the manganites are studied in the temperature range 90–300 K, and the high-resolution electron microscope images recorded at temperatures of 91–92 K are analyzed. In both manganites, the structural transition that is accompanied by the formation of the superstructure and which is directly observed from the appearance of additional peaks in the electron diffraction patterns occurs at a temperature that is in close agreement with the charge ordering temperature TCO determined from the temperature dependences of the magnetization M(T). In the temperature range 90 < T < 200 K, the undoped compound has a commensurate superstructure characterized by the vector q = 1/3a* and triple the unit cell «3a × b × c» (where a ≈ b ≈ √2ac, c ≈ 2ac, and ac ∼ 3.9 Å is the lattice parameter of a simple perovskite). The doping with iron (5 at. %) brings about a decrease in the charge ordering temperature TCO by 50 K and the formation of an incommensurate structure for which the magnitude of the vector q is smaller by approximately 15%. The unit cell of the superstructure in the iron-doped compound is not triple the unit cell but involves defects of ordering, such as quadrupling of the unit cell, numerous translations by ac √2 along the a direction, and dislocation-type defects in the stripe structure of the charge ordering. These pseudoperiodic defects lead to a decrease in the magnitude of the vector q and are responsible for the incommensurability of the structure. A decrease in the charge ordering temperature TCO due to the doping with iron and the incommensurability of the superstructure correlate with the change in the concentration of Mn3+ Jahn-Teller ions as a result of their replacement by Fe3+ non-Jahn-Teller ions.

Effect of silver content on the mechanical and electrical properties of the YBaCuO/Ag ceramic

Doping the YBaCuO superconducting ceramic with 3–5 wt % silver is shown to substantially increase its microplasticity and strength. Temperature spectra of microplastic strain rates for YBaCuO/Ag ceramics with 0, 1.6, 2.9, 4.2, and 7.6 wt % silver were obtained. The structure of the spectra and the relation of one of the peaks to the superconducting transition are discussed. The specific features of the I–V characteristics of samples cooled in a magnetic field are considered. It is shown that the position of the rising branch of a I–V characteristic depends on its origin; this observation can be used to fix two or more stable states of an HTSC-based memory cell.

Effect of excess oxygen on the electrical and magnetic properties of La1 − xAgxMnO3 (x = 0.05, 0.1, 0.2) at 77 K < T < 300 K

A study has been made of the magnetic and electrical properties of oxygen-enriched La1 − xAgxMnO3.1 manganites at 77 K < T < 300 K. The samples have been produced by the sol-gel technique. The study has revealed an increase of the Curie temperature and a shift of the maximum in magnetoresistance toward high temperatures.

Luminescence and Raman spectra of sol-gel-derived ZnO microcrystals with a high iron content

The luminescence and Raman spectra of sol-gel-derived ZnO microcrystals with different iron and magnesium contents have been studied. It has been revealed that, in some cases, the doping material phases precipitated.

Electrical and magnetic properties of manganites La1 − xAgxMnO3 (x = 0.05, 0.1, and 0.2) at 77 K < T < 300 K

The electrical conductivity, magnetization, and magnetoresistance of manganites La1 − xAgxMnO3 have been investigated in the temperature range 78–300 K. The samples have been synthesized by the sol-gel method. At room temperature, the magnetic field of 0.6 T has no effect on the electrical conductivity. As the temperature decreases, an abrupt jump is observed in the magnetization curve due to the semiconductor-metal phase transition. This transition hardly affects the temperature dependence of the resistance.

Effect of oxygen deficiency on the electrical and magnetic properties of the manganites La1 − xCaxMnO3 − α (x = 0.5, 0.6, and 0.7)

The electrical conductivity of La1 − xCaxMnO3 − α (x = 0.5) alloys in a magnetic field of 0.6 T is investigated in the temperature range 78–300 K. According to the phase diagram, this compound at α = 0 corresponds to an intermediate region between the ferromagnetic conducting (x < 0.5) and antiferromagnetic semiconducting (x s> 0.5) phases and contains islands of the metallic phase. Under oxygen-deficient conditions, the fraction of the metallic phase increases and the semiconductor-metal transition takes place. The formation of bound islands of the metallic phase is observed under oxygen-deficient conditions in the La0.4Ca0.6MnO3 − α compounds.

Variable-range hopping conduction in LaMnO3 + δ

The temperature dependence of the electrical resistivity ρ(T) for ceramic samples of LaMnO3 + δ (δ = 0.100–0.154) are studied in the temperature range T = 15–350 K, in magnetic fields of 0–10 T, and under hydrostatic pressures P of up to 11 kbar. It is shown that, above the ferromagnet-paramagnet transition temperature of LaMnO3 + δ, the dependence ρ(T) of this compound obeys the Shklovskii-Efros variable-range hopping conduction: ρ(T) = ρ0(T)exp[(T0/T)1/2], where ρ0(T) = AT9/2 (A is a constant). The density of localized states g(ɛ) near the Fermi level is found to have a Coulomb gap Δ and a rigid gap γ(T). The Coulomb gap Δ assumes values of 0.43, 0.46, and 0.48 eV, and the rigid gap satisfies the relationship γ(T) ≈ γ(Tv)(T/Tv)1/2, where Tv is the temperature of the onset of variable-range hopping conduction and γ(Tv) = 0.13, 0.16, and 0.17 eV for δ = 0.100, 0.125, and 0.154, respectively. The carrier localization lengths a = 1.7, 1.4, and 1.2 Å are determined for the same values of δ. The effect of hydrostatic pressure on the variable-range hopping conduction in LaMnO3 + δ with δ = 0.154 is analyzed, and the dependences Δ(P) and γv(P) are obtained.