Magnetic field dependence of the relative critical current density in γ‐irradiated polycrystalline YBa2Cu3O7

The effect of different doses of γ‐rays on the behavior of the critical current density, J_c in an YBa₂Cu₃O₇ polycrystalline sample has been investigated at high temperatures. The samples were irradiated at room temperature by a ⁶⁰Co γ‐ray source at a dose rate of 0.5 MR/h. J_c was found to increase significantly with after irradiation dose of 10 MR. Further irradiation dose of 50 MR produced a slight and field dependent enhancement of J_c above its values at 10 MR. The most interesting result is that the relative change in the critical current density was found to have a non‐monotonic behavior with the applied magnetic field. These results are discussed in terms of the roles of several mechanisms created by γ‐rays in the regions of the grain boundaries combined with the effect of the magnetic field on these mechanisms. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Voltage‐current characteristics of Bi‐2223 superconducting tape near Tc under γ irradiation

Voltage‐current characteristics at four different applied magnetic fields (7, 20, 30, and 40 mT) of Bi₂Sr₂Ca₂Cu₃O_x superconducting tape were measured in the temperature range from 100 to 115 K. They were also measured at zero magnetic field before and after γ irradiations up to 10 MRad at different temperatures just below the critical temperature. The data were fitted to a power law expression V = I ^β(T) in which the exponential parameter β under 20 mT field and after irradiation is found to fluctuate around three and then drops to unity near the critical temperature which may be interpreted as a sign of Kosterlitz‐Thouless transition. The electrical properties of the tape were found to be very sensitive to γ irradiation where most of the changes take place in low γ doses. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magneto‐transport properties of polycrystalline YBa2(Cu1‐xMx)3O7‐δ (M = B and Mn)

The magnetic and transport properties of polycrystalline YBa₂ (Cu_1‐xM_x)₃ O_7‐δ (M = B and Mn) superconductor was investigated. Samples of YBa₂(Cu_1‐xB_x)₃O_7‐δ doped with several concentrations of boron B(x = 0.05 and 0.1) were investigated using magnetization measurements. A YBa₂(Cu_1‐xMn_x)₃O_7‐δ sample doped with Mn with concentration of x = 0.02 was investigated using current‐voltage (I‐V) measurements. Our results on the YBa₂(Cu_1‐xB_x)₃O_7‐δ samples reveal a considerable increase in the hysterisis width of the magnetization, M versus the applied magnetic field H with increasing boron concentration. The lower critical field was also found to be enhanced by boron doping. The critical current density, J_c was found to be significantly enhanced in the Mn‐doped sample. The enhancement of J_c was found to be more significant at the lower temperatures for all applied magnetic fields used (0 Oe, 300 Oe, and 500 Oe). Thus, chemical doping is suggested to enhance the vortex pinning forces in the YBCO samples. From the resistivity (R‐T) measurements, chemical doping of the samples was found to have no significant effect on the critical temperature, T_c. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Three‐dimensional unsteady thermocapillary flow under rotating magnetic field

Under a rotating magnetic filed (RMF), the instability of thermocapillary flow and its evolution with increasing Marangoni number (Ma) for semiconductor melt (Pr = 0.01) in a floating liquid bridge model (As = 1) are investigated numerically. Under 5 mT RMF, the thermocapillary flow is steady and axisymmetric with Ma < Ma_c, and the critical Marangoni number Ma_c for convection instability is 29.5, which is obtained by the direct numerical simulation. When the Ma is a little bit beyond the Ma_c, the thermocapillary flow loses stability to become a three‐dimensional rotating oscillatory convection, and a periodic oscillation is confirmed by the fast Fourier transform analysis, the oscillatory main frequency decays with increasing Ma. Under 1 mT–6 mT RMF, the Ma_c increases roughly with the magnetic strength except the Ma_c at 4 mT, where the corresponding change of flow mode after the instability is observed. The oscillatory convection occurs with a smaller Ma in the RMF than that without magnetic field. In addition, no instability toward a three‐dimensional steady convection, which is the state of thermocapillary flow without magnetic field after the first instability, is observed under the RMF.     

Point and extended defects in superconductors

The effect of point defects created by γ‐irradiation and extended defects created by Pb‐ion irradiation on the behavior of the critical current density, J_c in several superconductors have been investigated. We have found a limitation of the role of both types of defects on enhancing J_c. We report an explanation of the effect of γ‐irradiation and Pb‐ion irradiation doses on J_c in these materials. The explanation is based on combining several competing mechanisms of irradiation which we believe to take place mainly in the regions of the grain boundaries. The influences of these mechanisms were found to vary with the irradiation dose level. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhancing the pinning strengths in polycrystalline MgB2

The dependence of the critical current density, J_c, on the gamma irradiation dose in MgB₂ polycrystalline superconducting specimens has been studied. The MgB₂ samples were irradiated with 10 MR and 20 MR doses of γ‐rays. The critical current density, J_c was found to be enhanced almost at all temperatures after irradiation. The enhancement of J_c was found to be linear with the irradiation dose at all temperatures. We also report on the correlation between the superconducting transition temperature and the residual resistivity ratio. Most of our results agree with the Testardi and Mooij rules which suggest that the dominant pairing mechanism of MgB₂ is the phonon‐mediated interaction. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microstructural,thermal, and electrical properties of Bi1.7V0.3Sr2Ca2Cu3Ox glass‐ceramic superconductor

A glass‐ceramic Bi_1.7V_0.3Sr₂Ca₂Cu₃O_x superconductor was prepared by the melt‐quenching method. The compound was characterized by scanning electron microscopy, x‐ray diffraction, differential thermal analysis, current‐voltage characteristics, transport resistance measurements, and Hall effect measurements. Two main phases (BSCCO 2212 and 2223) were observed in the x‐ray data and the values of the lattice parameters quite agree with the known values for 2212 and 2223 phases. The glass transition temperature was found to be 426 °C while the activation energy for crystallization of glass has been found to be E_a = 370.5 kJ / mol. This result indicates that the substitution of vanadium increased the activation energy for the BSCCO system. An offset T_c of 80 K was measured and the onset T_c was 100 K. The Hall resistivity ρ_H was found to be almost field‐independent at the normal state. A negative Hall coefficient was observed and no sign reversal of ρ_H or R_H could be noticed. The mobility and carrier density at different temperatures in the range 140‐300 K under different applied magnetic fields up to 1.4 T were also measured and the results are discussed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth,structure, and superconducting properties of Bi2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2Ca2Cu3O10‐y crystals

Large and high‐quality single crystals of both Pb‐free and Pb‐doped high temperature superconducting compounds (Bi_1‐xPb_x)₂Sr₂Ca₂Cu₃O_10‐y (x = 0 and 0.3) were grown by means of a newly developed “Vapour‐Assisted Travelling Floating Zone” technique (VA‐TSFZ). This modified zone‐melting technique was realised in an image furnace and allowed for the first time to grow Pb‐doped crystals by compensating for the Pb losses occurring at high temperature. Crystals up to 3×2×0.1 mm³ were successfully grown. Post‐annealing under high pressure of O₂ (up to 10 MPa at T = 500°C) was undertaken to enhance T_c and improve the homogeneity of the crystals. Structural characterisation was performed by single‐crystal X‐ray diffraction (XRD) and the structure of the 3‐layer Bi‐based superconducting compound was refined for the first time. Structure refinement showed an incommensurate superlattice in the Pb‐free crystals. The space group is orthorhombic, A2aa, with cell parameters a = 27.105(4) Å, b = 5.4133(6) Å and c = 37.009(7) Å. Superconducting studies were carried out by A.C. and D.C. magnetic measurements. Very sharp superconducting transitions were obtained in both kinds of crystals (ΔT_c ≤ 1 K). In optimally doped Pb‐free crystals, critical temperatures up to 111 K were measured. Magnetic critical current densities of 2�10⁵ A/cm² were measured at T = 30 K and μ₀H = 0 T. A weak second peak in the magnetisation loops was observed in the temperature range 40‐50 K above which the vortex lattice becomes entangled. We have measured a portion of the irreversibility line (0.1‐5 Tesla) and fitted the expression for the melting of a vortex glass in a 2D fluctuation regime to the experimental data. Measurements of the lower critical field allowed to obtain the dependence of the penetration depth on temperature: the linear dependence of λ(T) for T < 30 K is consistent with d‐wave superconductivity in Bi‐2223. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetoresistance,voltage‐current characteristics,and Hall effect measurements of bulk MgB2 superconductors

Bulk MgB₂ samples were prepared from the commercially available powder (Alfa‐Aesar). One of the samples was used in measuring the transport properties by the DC four‐probe technique while the other was used in measuring Hall effect using the van‐der‐Pauw configuration. From the transport measurements, we noticed that the R‐T curves shift to lower temperatures under applied magnetic field without any observed enlargement of the transition width. T_c gradually decreases from 37 K at zero field to 32 K at B = 1.4 T. Our V‐I data were found to obey a power law expression V ∝ I^{β(T, B)}. The change of β with temperature and magnetic field was shown and discussed. R_H is positive under positive applied magnetic field. The 1/ R_H linear dependency on T, usually observed in high temperature superconductors, could not be observed in our case. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influence of Pb‐ion irradiation on melt‐textured YBa2Cu3Ox crystals

Melt‐textured YBa₂Cu₃O_x crystals have been irradiated along the c‐axis with ²⁰⁸Pb⁵⁶⁺ ions corresponding to dose matching fields, B_Φ = 0.5 T and B_Φ = 2.0 T. Magnetization measurements were conducted along the ab plane of the samples. The strength of pinning sites was investigated by measuring magnetization hysterisis and the saturation remanent magnetization M_R at several temperatures. We have found that the pinning strength was considerably enhanced after irradiation at both doses. Interestingly, the pinning strength at a Pb‐ion irradiation which corresponds to the dose matching field B_Φ = 0.5 T, was found to be significantly larger than that at the dose matching field B_Φ = 2.0 T at all temperatures. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Role of Silver on Superconducting Properties of BPSCCO (2223) System

The role of silver for the superconducting properties of the Bi Pb Sr Ca Cu O (2223) system have been investigated systematically. Samples with various concentrations of Ag were prepared by the matrix method. D.C. resistance results showed that T_c(0) varied between 100–109 K for various compositions of Ag. J_c measured at LNT from I–V data shows an increase with silver addition. J_c decreased from 90 Amp/cm² at zero field to 16 Amp/cm² at 100 Oe. The T_c(0) obtained form the susceptibility data agreed with those obtained from resistance measurements. X-ray diffraction results showed that the dominant phase in BPSCCO is the high-temperature (2223) phase and the same is improved in silver added BPSCCO samples. The scanning electron micrographs indicated that silver is precipated along the grain boundaries, separating the superconducting grains. These results have showed that silver addition does not destroy the superconductivity and at the same time enhances the critical current density.     

Magnetic Susceptibility in 2D Superconductor NaxHfNCl System

Abstract

Superconducting critical temperature, T_c and the shielding volume fraction, SVF, of layered nitride superconductor Na_xHfNCl have been studied as a function of x, i.e. Na concentration. Although T_c decreases gradually with increasing x from 20.0 K at x=0.11 to 16.5 K at x=0.85, SVF has a sharp peak around x=1/6, where strong coupling between local ordering of Na atoms and Fermi surface instability can be expected. Structural disorder in the samples above x=0.5, observed by powder neutron diffraction, does not affect the superconductivity appreciably. Electronic specific heat coefficient, γ, is estimated to be about 7.7 mJ/mol/K² by its difference of magnetic susceptibility between HfNCl and Na_0.5HfNCl. The γ value is relatively small compared with the high T_c value, revealing double honeycomb lattice system as new potential higher T_c superconductor series by intercalation.     

Sensitivity of YBa2Cu3O7 thick films to gamma rays

Superconducting YBa₂Cu₃O₇ (YBCO) thick films have been prepared using the screen printing method. Effects of gamma irradiation on their structure and transport properties have been investigated up to a gamma dose of 50 MR. A considerable decrease in the intensities of the major diffraction peaks of the XRD pattern was observed due to gamma irradiation. A gradual increase in the normal state resistance, with a gradual increase in the slope of the resistance‐temperature (R‐T) curves was observed with increasing gamma doses. However, slight decrease in the transition temperature has been observed. The Voltage‐current (V‐I) data were fitted to the power law V ∼ I^β in which the power exponent β was found to increase gradually with gamma dose. The sensitivity of the R‐T and V‐I curves to gamma rays where precisely estimated and from which the regions of linear response were deduced. The possibility of using YBCO thick film as a candidate device for gamma rays sensing was presented. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High‐quality LaCoO3 single crystal grown by optical floating zone method and its electromagnetic properties

A LaCoO₃ single crystal with 4 mm in diameter and 30 mm in length has been grown by optical floating zone method. The as‐grown crystal is highly crystalline with the rhombohedral perovskite structure (R3c) and grows parallel to the (121) direction. The room temperature resistivity of the as‐grown crystal is 0.12 Ω·cm and the insulator‐metal transition occurs around 500 K. The coercivity and the remanence of the as‐grown crystal are 5 Oe and 6.61×10^–5 μ_B/f.u. at 5 K, respectively. In 1000 Oe under zero‐field cooling, the magnetic susceptibility of the as‐grown crystal shows an upturn in a Curie tail fashion below 35 K, and appears a wave crest over the interval 55 K≤T≤90 K. In addition, a slope change of 1/χ(T) at about 12 K is observed in 50000 Oe under zero‐field cooling. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Heat‐treatment influence on the microstructure and magnetic properties of rare‐earth substituted SrFe12O19

Rare‐earth substituted strontium ferrite nanopowders SrFe_12‐xR_xO₁₉ (R = La, Gd and Er; x = 0.2, 0.5 and 1) were prepared by sol‐gel‐autocombustion method and subsequent heat treatments. Structural and magnetic properties of SrFe_12‐xR_xO₁₉ powders heat treated at 800, 900 and 1000⁰C, for various times, were characterized with an X‐ray diffractometer, a vibrating sample magnetometer and a scanning electron microscope. The results of X‐ray diffraction measurements showed the M‐type hexagonal structure formation by heat treatments. Magnetic properties, such as specific saturation magnetization σ_s, specific remanent magnetization σ_r and coercivity H_c, as well as microstructure depend on the heat treatment conditions (temperature and time). The coercivity H_c exhibits a great increase after a critical heat treatment time. When the heat treatment time increases, one obtains an increase in H_c after a shorter heat treatment time. This jump of H_c was explained by a transition from the superparamagnetic state to normal state of the single domain nanoparticles. The occurrence of an agglomerated structure composed of magnetically interacting ultrafine crystallites also contributes to the increase of H_c.The heat treatment determines a reduced grain growth due to the internal stress generated by R ions. With increasing R content the σ_s and σ_r decrease due to the dissolution of R ions into the hexaferrite lattice. We believe that by selecting the time and temperature of the heat treatment, microstructure and magnetic properties suitable for magnetic recording media application can be obtained. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the synthesis and characterization of La doped MgB2 superconductor

We report the synthesis of La doped MgB₂ superconductors with nominal compositions Mg_1‐xLa_xB₂ (with x = 0.01, 0.03, 0.05, 0.07) by solid state reaction at ambient pressure. A special encapsulation technique has been used by us to prepare high quality superconducting MgB₂ samples. The bulk polycrystalline samples possess superconducting transition temperature T_c(R=0) ranging between 36‐39 K. It has been found that transport critical current density J_c of the samples change significantly with the doping level of La. A high transport (J_c) value ∼1.9 x 10³ A/cm² at 15 K has been achieved for Mg_0.97La_0.03B₂ sample. The XRD and TEM investigations indicate that the samples prepared by encapsulation method are devoid of MgO, which is generally found when synthesis of MgB₂ is done through sintering of Mg and B powders. The detailed microstructural investigations of Mg_0.97La_0.03B₂ specimens by transmission electron microscopy (TEM) reveal the presence of partial dislocation network, moiré fringes and superlattice structure in the as synthesized samples. The higher transport critical current density observed in Mg_0.97La_0.03B₂ superconductor has been attributed to the partial dislocations which are capable of providing pinning centres. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Time resolved thermal lens measurements of the thermo-optical properties of Nd2O3-doped low silica calcium aluminosilicate glasses down to 4.3 K

In this work, the thermal lens spectrometry was applied to measure the thermo-optical properties of Nd₂O₃-doped low silica calcium aluminosilicate glasses as a function of temperature, between 4.3 and 300 K. The thermal relaxation calorimetry was used to determine the specific heat, c_p. The results showed a decrease of the thermal diffusivity of about one order of magnitude from 4.3 K up to 300 K, with a T^?1 dependence in the interval between 20 and 70 K and a T^?0.35 between 4.3 and 20 K. The fluorescence quantum efficiencies of the doped samples were calculated down to 50 K, showing a variation of the order of 12% and 25% for the samples with 0.6 and 1.04 mol% of Nd₂O₃, respectively. In addition, the temperature corresponding to the maximum in c_p/T³, the so-called boson peak, was observed at about 17 K for the undoped sample and at lower temperatures for the doped glasses. In conclusion, our results showed the ability of the time resolved thermal lens to determine the thermo-optical properties of glasses at temperatures lower than 300 K, bringing new possibilities for experiments in a wide range of optical materials.     

Some physical and magnetic properties of Mg‐Zn ferrite

Some physical properties (such as lattice parameter, density and porosity) and magnetic properties of the system Mg_1‐xZn_xFe₂O₄; where x=0,0.1,0.2,0.3,0.4,0.5and 0.6 have been studied. It was found that the lattice parameter increases with increasing the zinc concentration. The composition dependence of the physical properties is divided into two regions. The first one is for x ≤ 0.3 and the second one is for x > 0.3. From the magnetization measurements, the basic composition (MgFe₂O₄) shows the lowest magnetization, while the composition of x=0.4 shows the highest one. The behaviour of magnetization M versus composition shows also two regions for x <>0.3. The behaviour of M versus x was discussed in the bases of cation distribution. From the B‐H loops, the remanence induction B_r, saturation induction B_s and the coercive force H_c were determined and studied with x. The Curie temperature T_C was determined from the measurements of the initial permeability μ_i versus temperature. It was found T_C decreases with increasing Zn‐content. Also paramagnetic temperature T_P was determined from the behaviour of M_S vs. T. In general it was found T_P > T_C by about 7‐10 K.     

Resistance and inductance studies of Bi2−xPbxCa2Sr2Cu3Oy superconducting compounds

Bi_2−xPb_xCa₂Sr₂Cu₃O_y compounds with (x = 0, 0.1, 0.2, 0.25, 0.3 and 0.4) were prepared by solid state reaction. The resistance and inductance results showed that transition temperature increases with increase in lead concentration. The highest value of T_c(0) observed is 109 K for Bi_1.70Pb_0.30Ca₂Sr₂ · Cu₃O_y. The change of inductance per unit volume at liquid nitrogen temperature as a function of composition showed maximum change in x = 0.30 Pb-doped sample. The results showed that the inductance change is maximum in samples which also showed highest T_c.     

Elliptic NdCrO3 microplate by a simple one‐step hydrothermal synthesis route and its characterization and magnetic properties

Elliptic NdCrO₃ microplates were synthesized by a simple and facile one‐step hydrothermal method of processing temperature 280 °C for 3 days. The products prepared in this paper have been characterized by X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), X‐ray fluorescence (XRF), Fourier transform infrared spectroscopy (FTIR) and field‐emission scanning electron microscopy (FESEM). The magnetic properties of the final sample are also studied. The XRD pattern indicates the pure orthorhombic phase for NdCrO₃ particles, the XPS, XRF and FTIR results further demonstrate the composition and purity of the final product. A possible growth mechanism for elliptic NdCrO₃ microplates is proposed. Through the investigation of magnetic properties, it can be generally concluded that the orthorhombic elliptic NdCrO₃ microplates exhibit typical behaviors of magnetic transition, spin reorientation transition and magnetic exchange bias. The Néel temperature is 218 K and the spin reorientation transition temperature is 46 K. The hysteresis loop under 5 K shows that the value of exchange bias field (H_ex) is 12 Oe and the shift of remanent magnetization (ΔM) is 0.008 emu/g, respectively.