Manganite perovskite with A-site deficiency has been synthesized by a solid-state reaction technique. Our work does not support a general view that A-site deficiency gives a decrease on Curie temperature. The Curie temperature of (La 0.8 Ca 0.2 ) 0.975 MnO 3.01 is 188 K with maximum entropy change -ΔSM max = 1.6 J·kg -1 ·K -1 and refrigerant capacity (RC) of 27 J/kg under an applied field of 10 kOe. The nonstoichiometric (La 0.8 Ca 0.2 ) 0.975 MnO 3.01 may offer a wider temperature span of ~240% compared with those nanocrystalline La 0.8 Ca 0.2 MnO 3.01 that have similar RC, which has been reported by others.

Single crystalline Nd0.7Sr0.3MnO3 was grown by floating zone method. Maximum magnetic entropy change was -Delta S-M (max) = 2.44 J.kg(-1) . K-1 and refrigerant capacity obtained was 35.18 J/kg under external field of 10 kOe. Temperature span calculated from Gaussian fit of -Delta S-M (max) was 29.69 K. It is about 29 percent wider compared to other previous work reported on similar composition.

Manganite perovskite with A-site deficiency has been synthesized by a solid-state reaction technique. Our work does not support a general view that A-site deficiency gives a decrease on Curie temperature. The Curie temperature of (La0.8Ca0.2)0.975MnO3.01 is 188 K with maximum entropy change -ΔSM max = 1.6 J·kg-1·K-1 and refrigerant capacity (RC) of 27 J/kg under an applied field of 10 kOe. The nonstoichiometric (La0.8Ca0.2)0.975MnO3.01 may offer a wider temperature span of ~240% compared with those nanocrystalline La0.8Ca0.2MnO3.01 that have similar RC, which has been reported by others.

The electrical resistivity. and magnetization of La0.8Ca0.2MnO3.06, (La0.8Ca0.2)(0.975)MnO3.01, and La0.8Ca0.2Mn0.975O3.06 polycrystalline samples have been measured in magnetic fields 0 <= H <= 9 T at temperatures in the range 2 <= T <= 300 K. The results obtained for the first and second samples are typical of manganites with the colossal magnetoresistance effect: the electrical resistivity increases with a decrease in temperature to T approximate to T-C and then sharply decreases. The composition of the third sample differs from the composition of the first sample by the content of Mn vacancies, which is similar to 2.5% higher in the former case, and this difference leads to a very strong difference in the properties of the samples. First, the ferromagnetism in the third sample is almost completely suppressed and, at low temperatures (T similar to 50-70 K), there occurs a transition to the state close to the cluster glass state. Second, the electrical resistivity of the third sample at low temperatures and H = 0 is four orders of magnitude higher than that of the first sample, even though the third sample contains a considerably higher concentration of holes. It has been shown that, for all three samples over the entire temperature range under investigation, the local activation energy E depends on the magnetic field. Of particular interest is the fact that the local activation energy E significantly decreases with increasing field in the paramagnetic region, which is not typical of conventional semiconductors. These specific features of the properties of the samples have been explained by assuming the hypothesis according to which holes in all three samples can be in the form of Mn4+, as well as in the form of O-, and can transform into each other. It has been concluded that a decrease in the temperature T and an increase in the magnetic field H favor the formation of Mn4+ holes and result in a decrease in the values of. and E with the formation of the ferromagnetic order, whereas an increase in the content of vacancies in the Mn-sublattice favors the formation of O- holes and leads to an increase in the electrical resistivity rho and the suppression of ferromagnetism in the sample.

The electrical resistivity ρ and magnetization of La0.8Ca0.2MnO3.06, (La0.8Ca0.2)0.975MnO3.01, and La0.8Ca0.2Mn0.975O3.06 polycrystalline samples have been measured in magnetic fields 0 ≤ H ≤ 9 T at temperatures in the range 2 ≤ T ≤ 300 K. The results obtained for the first and second samples are typical of manganites with the colossal magnetoresistance effect: the electrical resistivity increases with a decrease in temperature to T ≈ T C and then sharply decreases. The composition of the third sample differs from the composition of the first sample by the content of Mn vacancies, which is ∼2.5% higher in the former case, and this difference leads to a very strong difference in the properties of the samples. First, the ferromagnetism in the third sample is almost completely suppressed and, at low temperatures (T ∼ 50–70 K), there occurs a transition to the state close to the cluster glass state. Second, the electrical resistivity of the third sample at low temperatures and H = 0 is four orders of magnitude higher than that of the first sample, even though the third sample contains a considerably higher concentration of holes. It has been shown that, for all three samples over the entire temperature range under investigation, the local activation energy E depends on the magnetic field. Of particular interest is the fact that the local activation energy E significantly decreases with increasing field in the paramagnetic region, which is not typical of conventional semiconductors. These specific features of the properties of the samples have been explained by assuming the hypothesis according to which holes in all three samples can be in the form of Mn4+, as well as in the form of O−, and can transform into each other. It has been concluded that a decrease in the temperature T and an increase in the magnetic field H favor the formation of Mn4+ holes and result in a decrease in the values of ρ and E with the formation of the ferromagnetic order, whereas an increase in the content of vacancies in the Mn-sublattice favors the formation of O- holes and leads to an increase in the electrical resistivity ρ and the suppression of ferromagnetism in the sample.

The effect of an oxygen excess δ on the magnetic and electrical properties of La 1− x Ca x MnO 3+δ ( x =0.10–0.15) has been studied over wide ranges of temperatures and magnetic fields. As δ increases, the magnetic ordering temperature T c decreases by 70–90 K, the magnetoresistance increases (the electrical resistivity decreases by a factor of up to 10 4 in a field of 9 T), and the effective moment μ eff of the paramagnetic susceptibility substantially exceeds the theoretical value at temperatures two to four times higher than T c and undergoes a jump, just as the activation energy of electrical resistivity, at T∼270 K. These results are attributed to the formation of cation vacancies, the localization of electrons in their vicinity with the subsequent formation of magnetic clusters, tunneling (or hopping) of carriers among them, changes in the sizes of clusters with variations in the temperature and magnetic field strength, the onset of frustrations initiated by the competition among different types of exchange interaction, and the dependence of the cluster parameters on the annealing conditions. Annealing of the oxygen-excess samples at high temperatures in vacuum (above 1100°C) restores the samples to a nearly initial state with the magnetic and magnetotransport properties characteristic of weakly doped manganites, as a result of the removal of cation vacancies.

A method for evaluation of the critical temperature T c and the width of the superconducting transition ΔT c in HTSC single crystals has been developed. By this method, the first derivative of the temperature dependence of the resistivity, \(\frac{\partial \rho (T)}{\partial T}\), is constructed. A technique for synthesis of YBa2Cu3O7−x single crystals with highly reproducible physical-mechanical properties has been described. A standard sample with T c=94 K and ΔT c=0.25 K has been synthesized and certified.

The magnetotransport and magnetic properties of La 1 − x Ca x MnO 3 polycrystalline samples ( x = 0–0.3) annealed under vacuum and in the oxygen environment are investigated in the temperature range from 77 to 400 K. The magnetic studies of lightly doped manganites reveal persistence of short-range magnetic order up to a temperature T * ≈ 300 K, which is about 2–3 times higher than their Curie temperature T C . The temperature dependence of the electrical resistivity measured from T * down to nearly T ≈ T C is fitted by the relation logρ ∼ T −1/2 , which is characteristic of granular metals with electrons tunneling among nanoclusters of magnetic metals embedded in a dielectric host. The magnetoresistance of polycrystalline samples annealed in the oxygen environment has been observed to increase. The electrical, magnetic, and magnetotransport properties of the manganites can be accounted for by the formation of magnetic nanoclusters below T *, tunneling (or hopping) of carriers among the nanoclusters, variation in the magnetic cluster size, and tunneling barrier thickness with variations in temperature and magnetic field strength, as well as by the effect of annealing in different media on the cluster properties.

The mechanochemical method is shown to be a relatively simple method for producing nanostructural manganites LaMnO3 + δ with crystallite size D ≥ 10 nm. An increase in the treatment duration in a planetary mill from 1 to 13 h decreases the size D and increases microstrains. The Curie temperature of the nanostructural manganites decreases insignificantly and the phase transition is smeared as D decreases. A decrease in the unit-cell volume and the temperature dependences of the inverse magnetic susceptibility 1/χ(T) indicate an increase in the Mn4+ ion concentration with the milling duration. The variation of the magnetic properties of LaMnO3 + δ nanostructural powders is explained by the competition of a number factors, such as variations of the composition, the cation-sublattice defect structure, the size effect, and the microstrain level.

The present work presents the original data concerning using a high pressure torsion method and a shock wave loading technique to produce compact oxide nanomaterials and investigations of the effect of severe plastic deformation on a microstructure, crystal lattice and stability of these compounds. This allowed us to compare two ways of deformation action that can be characterized as quasi-static and dynamic effect, correspondingly. Particular attention was paid to a stoichiometry and surface composition changes upon severe plastic deformations. A procedure for studying chemistry of the oxide nanomaterials by means of nuclear microanalysis and Rutherford back scattering has been worked through. For surface studies the X-ray photoelectron spectroscopy has been used. It was shown that both distortion methods permit to produce massive nano-scale oxide materials from the coarse-grained powder during a single technological cycle. Bulk nanocrystalline materials based on LaMnO3, TiOy and ZrOZ:YzO(3) were obtained by the quasi-static deformation technique. Nanoscaled ceramics of CuO, Mn3O4 and LaMnO3 were produced by the dynamic deformations. The density of the nanoceramics comes to 99%. Size effects and specific imperfection of the nanoceramics obtained lead to a set of particularities of physical properties.

The giant decrease of the electrical resistance of HgCr 2 Se 4 (more than by a factor of 200) caused by magnetic field-induced changes in the carrier mobility and concentration, the quadratic dependences of magnetoresistance and normal Hall constant on magnetic induction in the paramagnetic region, as well as the deviations from these dependences observed to occur as one approaches the Curie temperature, are discussed within a model involving carriers of several types (holes in the valence band, electrons localized at ferron-type impurity centers, and electrons hybridized in the impurity and conduction bands).

The optical absorption spectra of single-crystal CuO bombarded with 5-MeV electrons exhibit reduced absorption in the region of the fundamental absorption edge at 17 eV, which corresponds to the b 1 g → e u transition with charge transfer in CuO 4 6− . A simultaneous increase in absorption is observed in the middle infrared and in the region of high energies centered on 2.9 eV. The experimental results obtained are interpreted in terms of ideas on the phase-inhomogeneous nano-agglomorated structure in copper oxides that occurs as a result of the nucleation of polar centers (CuO 4 5− , CuO 4 7− ) under electron bombardment.

The temperature dependences ofresistivity rho(ab)(T), rho(c)(T) and Hall coefficient R(H)(T) ofYBa2Cu3Ox (x=6.7, 6.95) single crystals in the range from 50 to 300 K were investigated under uniaxial stress up to P= 5 kbar applied along the c-axis (P parellel-to c) and perpendicular to the c axis (P perpendicular-to c). The critical temperature T(c) was found to increase at the rate 0.3 K/kbar in YBa2Cu3O6.,and 0.05 K/kbar in YBa2CU306.95 for P perpendicular-to c, while rho(c), first decreases and then rises quickly with pressure enhancement up to P=2 kbar. No change is observed in the values of rho(ab), R(H) and T(c), while rho(c), monotonically decreases with the pressure increase up to P=5 kbar for P parallel-to c. Possible physical reasons for the observed effects are discussed.

Mixtures with an excess of BaO/CuO are mostly used for crystal growth of YBa2Cu3O7 - x where at higher temperatures considerable amounts of Cu(I) are formed. We investigated this system by thermogravimetry and differential thermal analysis, especially under reduced oxygen partial pressure, and estimated the Cu(II)/Cu(I) ratio of the melts. At reduced oxygen partial pressure the liquidus line is shifted to lower temperature and crystal growth at this temperature is possible. By varying the oxygen partial pressure the Cu(II)/Cu(I) ratio is changed, which enables isothermal crystal growth. Crystals with dimensions of 5 X 5 X 0.1 mm3 in a 5 ml melt volume grow at a constant temperature of T = 910°C and a variable oxygen partial pressure in the range Po2 = 5 X 102 - 2 X 104 Pa.