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Flux jumps have been observed in a large (138 mg) single crystal of La 1.86 Sr 0.14 CuO 4 . Both complete and incomplete flux jumps are observed with the energy dissipated by complete flux jumps consistent with temperature rises to nearly T c . The systematics of these flux jumps with respect to field history, temperature and the time duration of the hysteresis measurements are reported. The field at which flux jumps first occur and their subsequent spacing are not consistent with a simple adiabatic theory of flux jumps for our experimental conditions. Instead, the crystal is observed to be stabilized against flux jumps by flux creep which occurs during the time interval over which field steps are accomplished as well as during imposed time intervals between field steps.

Magnetic relaxation and hysteresis studies have been performed for a c-axis-oriented single crystal of ${\mathrm{La}}_{1.86}$${\mathrm{Sr}}_{0.14}$${\mathrm{CuO}}_{4}$. We demonstrate that the effective activation energy for flux creep, ${\mathit{U}}_{\mathrm{eff}}$, is a strongly nonlinear function of the current density J (as given by the irreversible magnetization). For fixed fields of 0.5, 1.0, 2.0, and 3.0 T and temperatures between 4 and 15 K (as consistent with full field penetration of the sample), ${\mathit{U}}_{\mathrm{eff}}$(J) is seen to vary approximately logarithmically with J. A scaling relationship, ${\mathit{U}}_{\mathrm{eff}}$(J)=[-${\mathit{U}}_{1}$g(T)/${\mathit{H}}^{\mathit{n}}$]ln(J/${\mathit{J}}_{\mathit{c}}$), is shown to be obeyed for this crystal with n\ensuremath{\simeq}0.9 (H in T), ${\mathit{U}}_{1}$\ensuremath{\simeq}180${\mathit{k}}_{\mathit{B}}$, and ${\mathit{J}}_{\mathit{c}}$(1 T)\ensuremath{\simeq}3.2\ifmmode\times\else\texttimes\fi{}${10}^{5}$ A/${\mathrm{cm}}^{2}$. Several functional forms of g(T) have been explored and shown to give reasonable fits if monotonically decreasing in temperature over the range where flux creep is observed. We also explore the ramifications of this functional dependence in explaining an observed exponential temperature dependence of the hysterically determined critical current density. Finally, we show that comparison of the temperature dependence of logarithmic relaxation rate A=dM/d ln(t) and the derivative of the magnetization with respect to ln(T) at fixed time, ${\mathit{dM}}_{0}$/d ln(T), can be used to set a scale of the attempt frequency for flux motion consistent with the other measurements.

Identical polycrystalline samples of YBa/sub 2/Cu/sub 3/O/sub 7-x/ have been irradiated with fast neutrons (E>0.1 MeV) in eight steps between 0 and 2.1*10/sup 18/ n/cm/sup 2/. Notable irradiation effects include a T/sub c/ depression of nearly 2 K at the highest fluence and large improvements in the critical current density for fields from 0-9 T and temperatures from 4-80 K. Critical currents approaching 2*10/sup 7/ A/cm/sup 2/ are observed for optimally irradiated materials at 5 K (in zero field), while at 77 K J/sub c/(0 kOe) approaches 5*10/sup 5/ A/cm/sup 2/. Irradiation is seen to take a nearly equilibrium magnetization curve at 77 K and broaden it to a significantly hysteretic curve. A substantial shift in the effective pinning potential as a function of current density is inferred from magnetic relaxation measurement at H=1 T. This is the first such measurement in which systematically increased activation energies for flux creep (as a function of current density) are noted.

An examination of weak-link coupling between grains has been made using a noncontact magnetic technique. Intergranular J/sub c/(T) measurements for unaligned and grain-aligned sintered YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBC) toroids are reported. Supercurrent is deduced from measurement of the net current encircling the core, and J/sub c/ is found from supercurrent saturation in contrast to onset pickup voltage criteria. Agreement with recently reported direct transport measurements on the aligned sample suggests that the conduction mechanism is the same as for direct transport. A universal normalized J/sub c/(T) behavior which follows the same junction critical current temperature dependence previously reported in bicrystal transport measurements is found. It is proposed that this technique may be extended for the measurement of critical currents in thin-film samples and that present bulk intergranular current densities appear to be high enough to produce a soft-material limited superconducting electromagnet at 77 K.

We have investigated magnetic relaxation on a grain-aligned sample of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} powder embedded in epoxy with the magnetic field along the {ital c} axis. Through a thermal-cycling procedure and through variations in the measuring temperature over the range 10--30 K, we manage to commence magnetic relaxation from a range of values of the magnetization, {ital M}, or current density {ital J}. Analysis of relaxation measurements at a magnetic field of 1.0 T over the temperature range 10--30 K reveals a nonlinear dependence of the effective activation energy {ital U}{sub {ital e}} on {ital M} or {ital J} that is consistent with the logarithmic dependence obtained by Zeldov {ital et} {ital al}. from transport measurements. This demonstration of a nonlinear dependence of {ital U}{sub {ital e}}({ital M}) is capable of resolving anomalies in the magnitudes and temperature dependence of flux-creep activation energies obtained from magnetic-relaxation studies.

This paper summarizes the on-going work in our laboratory on two aspects concerning the magnetic properties of high Tc superconducting oxides. In the first part, the paramagnetic susceptibility results of RBa2Cu3O7−x compounds (where R = Rare earth ion such as Nd, Er, and Dy) were semi-quantitatively analyzed. Crystal field theory together with the recent published results on the neutron inelastic scattering spectroscopy reveal the considerable degree of anisotropy in the susceptibility. Evaluation of the torque shows that the alignment of grains may be achieved at low external fields and that the alignment direction may be predicted by the nature of the rare earth ion. The second aspect reported in this paper deals with the optimization of processing techniques to obtain a high Tc Bismuth oxide-based superconductor. It has been found that heat treatment of Bi2Sr2Ca2Cu2.8O x (2223) for long periods of time provides a material with a reasonably high critical temperature. In a separate set of exper...

This paper summarizes the on-going work in our laboratory on two aspects concerning the magnetic properties of high Tc superconducting oxides. In the first part, the paramagnetic susceptibility results of RBa2Cu3O7−x compounds (where R = Rare earth ion such as Nd, Er, and Dy) were semi-quantitatively analyzed. Crystal field theory together with the recent published results on the neutron inelastic scattering spectroscopy reveal the considerable degree of anisotropy in the susceptibility. Evaluation of the torque shows that the alignment of grains may be achieved at low external fields and that the alignment direction may be predicted by the nature of the rare earth ion. The second aspect reported in this paper deals with the optimization of processing techniques to obtain a high Tc Bismuth oxide-based superconductor. It has been found that heat treatment of Bi2Sr2Ca2Cu2.8O x (2223) for long periods of time provides a material with a reasonably high critical temperature. In a separate set of exper...

Magnetic susceptibility measurements of RBa2Cu3O7 − x (where R ≡ rare earth ion) indicate that these compounds are paramagnetic above Tc. When R  Dy, Er or Nd, there appears to be a considerable degree of anisotropy in the magnetic susceptibility between the c axis and the a-b plane. This property may help align these compounds in external magnetic fields. The nature and extent of this anisotropy is of major theoretical interest. In this paper, we report on calculations of the magnetic susceptibility in the paramagnetic regime of the RBa2Cu3O7 − x (where R  Dy, Er and Nd and 0 < x < 1). The point charge model was employed utilizing the Stevens operator equivalents. The calculations show that the crystalline field acting on the 4f electrons of the rare earth ions plays a reasonable role in modifying the susceptibility to temperatures up to 300 K. The susceptibility was also found to be anisotropic. The crystal field coefficients evaluated from the point charge model appear to be consistent with experimental results for R  Dy, Er and Nd and for oxygen stoichiometry between 6 and 7.

We have investigated the magnetization behavior of Bi{sub 1.6}Pb{sub 0.4}Sr{sub 1.6}Ca{sub 2}Cu{sub 2.8}O{sub {ital x}} (Bi 2:2:2:3) and YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} (Y-Ba-Cu-O) as a function of field, temperature, and time. We find a logarithmic time decay in both zero-field-cooled magnetization and remanent magnetization, consistent with the Anderson-Kim flux-creep model with activation energies of {ital U}{sub 0}/k=700 K (0.06 eV) for Bi 2:2:2:3 and {ital U}{sub 0}/k=1700 K (0.15 eV) for Y-Ba-Cu-O in an applied field of 1 kOe. Measured activation energies remain essentially temperature independent for {ital T}/{ital T}{sub {ital c}} less than 0.6 while {ital J}{sub {ital c}}(T) decays rapidly as a result of the relatively small {ital U}{sub 0}.