Yu.P. Stepanov

Preparation of superconducting YBa2Cu3O7-δ nanopowder by deoxydation in Ar before final oxygenation

Abstract Superconducting YBa2Cu3O7−δ nanopowder is prepared by the citrate-gel modification of the sol–gel method. Most important advantages of this method are the homogeneity at atomic level and possibility to reduce the calcination temperature to limit thermal agglomeration of the particles. A preparation temperature ⩽800 °C is used with a deoxydation step in Ar before the final oxidation. The X-ray diffraction data confirm the orthorhombic structure of the nanopowder. Superconducting properties are investigated with a SQUID magnetometer showing that the global Tc=92 K. Single grains of the powder have average diameter of 60–80 nm, depending on the calcination process used. The grains are weakly connected stacks of flat particles having thickness of a few nanometer.

Low-field magnetic properties of LaMnO3+δ with 0.065≤δ≤0.154

In a weak ðB ¼ 2G Þ magnetic field LaMnO3þd exhibits at d ¼ 0:065 below the paramagnetic-to-ferromagnetic (FM) Curie temperature, TC; a mixed (spin-glass and FM) phase followed by a frustrated FM phase at d between 0.100 and 0.154. The same behavior is observed in La12xCaxMnO3 with x between 0 and 0.3. This can be understood by the similar variation of the Mn 4þ concentration, c between < 0.13 and 0.34, in both materials when x or d is increased. On the other hand, considerable differences are found between these compounds in the values of the magnetic irreversibility, in the dependencies of TCðcÞ and the magnetic susceptibility, xðcÞ; as well as in the critical behavior of xðTÞ near TC: These differences can be explained by distortions of the cubic perovskite structure, by the reduced lattice disorder and by the more homogeneous hole distribution in LaMnO3þd than in La12xCaxMnO3.

Effect of Fe doping on structure, charge ordering, magnetic and transport properties of La0.33Ca0.67Mn1−yFeyO3 (0≤y≤0.06)

The relationships between incommensurability, charge ordering and magnetic and electric transport properties were studied in La0.33Ca0.67Mn1−yFeyO3 (0≤y≤0.06) polycrystalline samples. Incommensurability and charge ordering were directly observed by electron diffraction versus temperature and lattice imaging at low temperature in transmission electron microscopy. Both the charge ordering and the ferromagnetic Curie–Weiss temperature linearly decrease with increasing Fe concentration y. A mean field analysis yields an antiferromagnetic exchange J(Mn–Fe) = −22 K. The undoped samples showed predominantly a commensurate charge ordering superstructure with q-vector 1/3a* over the whole temperature range<200 K. For doped compositions, the q-vector (1/3−e)a* is reduced by 12–15% for 5% Fe doping, as compared to the undoped composition. The low-temperature superstructure of the Fe-doped manganite does not correspond exactly to the threefold cell with lattice parameters 3a, b, c, and defects inducing shifts along the a direction create a disturbance in long-range charge ordering. Jahn–Teller effects on Mn3+ centres are proposed to play a crucial part in the superstructure formation as well as in the suppression of charge ordering temperature by doping with non-Jahn–Teller impurity Fe3+ on Mn3+ sites.

Trapping and relaxation of inter- and intragrain vortices in YBa2Cu3O7−δ

Abstract Isothermoremanent magnetization and its relaxation with time have been investigated in ceramics and powders of YBa 2 Cu 3 O 7−δ at different temperatures after magnetizing the sample in a field of H ext eff ≈0.7 eV, independent of H ext . In the ceramic material U eff was found to depend strongly on the applied field reaching a value of 5 eV at 10 Oe and decreasing steeply when the magnetizing field was increased. The field dependence is discussed by using a model which takes into account the interaction between vortices.

Quantum dimensional effect in powders of the high-Tc superconductor YBa2Cu3O7-δ

Abstract Low-field magnetization has been investigated in powders of YBa 2 Cu 3 O 7- δ using samples with different maximal characteristic particle sizes d max of between 2.4 and 7.4 μm. Measurements of the thermoremanent magnetization reveal the existence of a threshold field H th corresponding to trapping of a single flux quantum Φ 0 in microcrystal: H th =(1.1±0.2) Φ 0 / d 2 max . Flux creep was found to depend on the grain size, giving evidence for the presence of a surface barrier of flux trapping. A reversible temperature dependence of the thermoremanent magnetization has been observed. This effect is attributed to a local field created by particles containing trapped flux.

Temperature dependence of the Meissner penetration length in submicron vortex-free and vortex-trapped YBaCuO particles: test for applicability of the modified London equation

To test the applicability of the modified London equation (MLE) the method based on comparison of the temperature dependencies of the penetration length λab in the Meissner and the vortex states is proposed. For experimental testing we investigate the magnetic properties of submicron superconducting YBaCuO particles prepared by a citrate gel modification of the sol-gel technique. It is found that the dependence λab2(0)λab2(T) obeys a power law with the exponent n = 2.0 ± 0.2 and n = 2.5 ± 0.05 in the Meissner and the vortex states, respectively. This difference indicates that MLE is not valid in the vortex state at least in the scale of the used particles i.e. of some hundred A. Possible reasons for this are discussed.

Temperature dependence of the magnetic field penetration depth versus oxygen ordering in submicron YBCO particles

Abstract The influence of oxygen ordering on the temperature dependence of the magnetic field penetration depth λ is investigated in submicron YBa 2 Cu 3 O 6+ x particles obtained by low-temperature ( T s = 800 − 850°C) synthesis from a sol-gel precursor. Additional OO bonds created at low temperature synthesis hinder the oxygen ordering in Cu(1)O plane despite of its high content ( x > 0.82). Oxygen exchange as well as additional annealing stimulate tetra-ortho transition and oxygen ordering. A wide linear range is observed in λ ab −2 ( T ) for samples with highly disordered oxygen. This is attributed to strong phase fluctuations caused by local strains. A gauge-glass model with random distribution of order parameter phases describes the behavior of λ ab −2 ( T ), revealed at initial stage of the hindered tetra-ortho transition. Transformation of the linear λ ab −2 ( T ) fuction to a quadratic one with oxygen ordering is related to decreasing of local deformations and to transformation from a glass state to the usual superconducting orthophase. Two essentially different ways of oxygen ordering, reoxidation and additional annealing, result in the same final state with a similar power law of λ ab −2 ( T ).

Magnetization of submicron superconducting YBCO particles: isolated single vortices versus critical state model

Abstract Strong suppression of the ratio of the thermoremanent (MTR) and the field-cooled (MFC) magnetizations MTR/MFC, exceeding two orders of magnitude, when decreasing the size of superconducting YBCO particles from micron to submicron scale has been observed. A high saturation field (Hs > 8 kOe) of MTR (H) in submicron particles excludes the applicability of critical state models for interpretation of the results. A model of trapping and relaxation of isolated single vortices in particles having size D ⪅ λ (λ is the magnetic field penetration depth) is introduced showing that interaction between a vortex and its mirror image results in a fast relaxation of MTR with size-dependent relaxation time τ ∝ D2. The reduction of the remanence and its temperature and field dependencies in submicron particles are explained by the fast relaxation resulting in vortex-free particles and threshold fields of vortex trapping.

Slow relaxation of AC-susceptibility of YBaCuO microcrystalline powders: magnetic field-induced dynamical alignment of crystallites

Abstract The response of microcrystalline YBaCuO powder AC susceptibility Δx to abrupt changes of weak DC magnetic field has been studied. The presence of slow relaxation of ΔX (during hours) exhibiting different signs of the effect for different mutual orientations of AC and DC fields, and which cannot be attributed to a flux creep, has been observed. An essential role of powder vibrations in this process has been revealed. One could distinguish between 3 stages of the relaxation: (1) “instantaneous” changes of X (due to an intragrain weak links contribution), (2) relatively steep ΔX(t) behavior at t⩽10 min, attributed to grain alignment in the course of their field-and vibration-induced rearrangement, (3) “residual” relaxation at t⩾10 min where irreversible effects are observed. This relaxation is attributed to “topological” grain rearrangement involving changes of grain short-range ordering due to grain motion. The model suggested qualitatively explains the main details of the observed behavior.

YBCO nanopowder: novel material for PLD preparation of thin films

Preparation rotes of YBCO nanopowders based on a citrate-gel method and calcination after reduction of oxygen from the precursor powder are discussed. High-quality thin films have been deposited by a XeCl laser using targets pressed from these powders.