E. V. Blinov

Preparation of one to three unit cell thick powders and investigation of their magnetic and microwave properties

Nanocrystalline powders have been prepared by a citrate gel modification of the sol - gel technique. Using a relatively low temperature, , and (i) a process of extracting oxygen and reoxidizing the precursor powder or (ii) a sequence of vacuum annealing and prolonged heating in air, it was possible to avoid sintering of the particles. According to x-ray powder diffraction analysis the particles have orthorhombic structure pertinent to the phase. Measurements made by atomic force microscopy show that they have flat shape with two maxima in the thickness distribution, one at 1.2 nm and the other at 3.8 nm , corresponding to one or three orthorhombic unit cell lengths of the crystal. The average width of the particles is 40 nm. A similar value could be obtained from XRD measurements and the width of microwave absorption line around zero applied field. Superconducting properties of the powders were investigated with magnetization measurements.

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.

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.

Reentrant reversible remanent magnetization in superconducting submicron YBaCuO particles

Abstract Reversible properties of the remanent magnetic moment, M rem , in superconducting submicron- and micron-size Y–Ba–Cu–O particles and thin films are investigated with a SQUID magnetometer. In micron-size particles and thin films a monotonic increase of M rem was observed when decreasing the temperature. In submicron particles, the same monotonic behaviour is observed at first but at low temperatures an unusual reentrant behaviour was found: M rem decreased when cooling the sample below 6 K. This phenomenon is independent of the direction of the thermal cycle. Possible mechanisms of this effect, such as midgap states in the quasiparticle excitation spectrum and defect-induced localization of Cooper pairs, are discussed.

Relaxation of trapped magnetic moment in thin YBaCuO films at incomplete flux penetration

Abstract Relaxation of the isothermo-remanent magnetic moment (MIR) of thin superconducting films is investigated in low magnetic fields when the magnetic flux penetrates into the sample incompletely. Dependence of the MIR relaxation rate on the magnetizing field H0 is determined, showing that the inverse relaxation rate decreases when MIR grows up to the saturation. This observation agrees well with the theoretical prediction.

Influence of the spatial distribution of trapped magnetic flux on the effective pinning energy of vortices in ceramic Y-Ba-Cu-O

Abstract Relaxation of the isothermoremanent magnetization has been investigated in ceramic Y-Ba-Cu-O samples with different grain sizes. It is found that the effective pinning energy U eff determined at different temperatures by using relaxation data of the remanent magnetic moment M IR and the relationship dln M IR / dln t =- kT / U eff has a maximum at a value of the magnetizing field H 0 corresponding to the most rapid increase of M IR with increasing H 0 . A phenomenological theory based on a critical- state model has been developed to describe these phenomena. The main conclusion of our analysis is that the dependence of U eff on H 0 can be attributed to the evolution of the distribution of the trapped flux (as H 0 is varied) without involving any assumptions about the dependence of the pinning barrier on the field.

Investigation of glass behaviour of vortex arrays in superconducting submicron YBCO powders by magnetic relaxation measurements

Magnetic relaxation measurements are made in the remanent state of pure and Zn-doped submicron YBCO powders. A characteristic feature of these samples is dipolar interaction between single vortices trapped in different particles. Similar to other systems with dipolar interactions, such as powders of fine ferromagnetic particles, our samples show glassy behaviour: different metastable states with various pinning properties can be prepared by varying the strength of the magnetizing field, i.e. history of the sample. It is found also that the relation between the pinning energy and the magnetic moment can be described by an expression similar to the vortex glass interpolation formula.

Line shape of the field dependence of the trapped moment in Y–Ba–Cu–O superconducting powders in low magnetic fields: manifestation of vortex polaron

Abstract The structure of trapped magnetic flux is investigated in small superconducting Y–Ba–Cu–O particles, attributing the trapped moment Mpol to an ensemble of vortex polarons which consists of vortices surrounded by diamagnetic matter. To distinguish between the contributions of the vortices and their diamagnetic surrounding to the magnetic moment, measurements of the field dependence of Mpol are made with a SQUID magnetometer around zero magnetic field H. It is found that the value of Mpol grows with increasing field and has a narrow dip at H=0. A similar line shape but with opposite sign is found in the field dependence of the zero field cooled susceptibility confirming the polaron nature of the trapped flux.

Remanence magnetization in powders and ceramics of YBaCuO

Abstract Thermoremanent magnetization (TRM), isothermoremanent magnetization (IRM) and time dependence of IRM have been investigated in powders and ceramics of YBa 2 Cu 3 O 7−δ in low magnetic fields. Measurements of TRM of powders with different particle sizes show trapping of single flux quanta in microcrystals. Time dependence of IRM for powders and ceramics shows the difference in dynamics of vortices when there were present in the sample only Abrikosov vortices or both Abrikosov and Josephson vortices, respectively. Also a reversible temperature dependence of the remanent magnetization, observed only in powders but not in ceramics, is reported. This reversibility is explained by the increase of the penetration depth λ when the temperature of the sample is lowered.