Magnetic flux motion and flux pinning in superconductors

Abstract

<p>Magnetization measurements on the two high-temperature superconductors Nd[subscript]1.85Ce[subscript]0.15CuO[subscript]4-y and Ba[subscript]0.6K[subscript]0.4BiO[subscript]3 have been undertaken in order to study the reversible and irreversible magnetic behavior and compare these two different materials with other high-temperature superconductors (HTSC). While the other HTSC are known to have hole carriers and layers of CuO[subscript]2 planes in anisotropic crystal structures, Nd[subscript]1.85Ce[subscript]0.15CuO[subscript]4-y (NCCO) has electron carriers as well as hole carriers, and Ba[subscript]0.6K[subscript]0.4BiO[subscript]3 (BKBO) has an isotropic crystal structure with no CuO[subscript]2 layers;For NCCO, the field-dependence of the specific heat derived from the reversible magnetization, C[subscript]0-C[subscript] H, exhibited peak broadening with increasing field and a field-independent onset temperature, both of which are observed in YBa[subscript]2Cu[subscript]3O[subscript]7-[delta]. The value at the highest available field, C[subscript]0-C [subscript]2T = 1.2 mJ/cm[superscript]3K, establishes a lower limit of the calorimetric specific heat jump [delta]C[subscript] p. Flux creep and hysteresis data were parameterized in terms of a single effective pinning potential U[subscript] eff in order to study the crossover from reversible to flux-pinning behavior in NCCO. There is a band in the H-T plane below H[subscript] irr(T) where U[subscript] eff/kT gradually rises from 2 to 20. The position of the U[subscript] eff/kT lines for NCCO are in approximately the same location as those for the hole carrier Tl(2223), when a reduced temperature scale is used;For BKBO, magnetization-vs-temperature data were found to be similar to that expected from the standard Ginzburg-Landau treatment in that there were sections of the M-vs-T curves in which the slopes dM/dT were nearly constant for increasing fields. Rounding of the M-vs-T curves near T[subscript] c was still observed, however, as might be expected if fluctuations are important. In this reversible region the specific heat jump values C[subscript] 1T-C[subscript] H derived from the free energy curves are also qualitatively similar to the high-T[subscript] c cuprates, the onset temperature is constant in field, and the peak broadens as field is increased. A maximum value of C[subscript] 1T-C[subscript] 5T = 1.8 mJ/cm[superscript]3K sets a lower limit on the value of [delta]C[subscript] p. U[subscript] eff/kT rises relatively quickly from 20 to 100 for BKBO below the irreversibility crossover, compared to the more gradual increase in U[subscript] eff/kT for NCCO. The isotropic BKBO has a smaller reversible region and stronger effective flux pinning potentials than layered NCCO. Fast-neutron irradiation of BKBO with a fluence of 1 x 10[superscript]22 n m[superscript]-2 did not increase critical current density in this sample. There was a small (<2 K) shift in temperature in the (C[subscript] 1T-C[subscript] H)-vs-T data, but the qualitative features of this thermodynamic data remained intact following the irradiation.</p>