Vortex pinning in heavily Pb-doped Bi2212 crystals

Abstract

Magnetic vortex pinning processes were studied for heavily Pb-doped Bi 2− x Pb x Sr 2CaCu 2O y crystals. The atomic ratio of Bi to Pb atoms was 1.66:0.34, as obtained by EDS. The hysteresis loops of the crystals exhibited a very pronounced secondary peak effect, in the temperature range 20 K< T< T c. J c0 at T=0.3 T c was 75 kA/cm 2, several times higher than for a pure Bi2212. The current-dependence of the activation energy was obtained from the magnetic relaxation measurements at different temperatures. Temperature scaling U∼(1−( T/ T c) 2) was employed. Fitting of the activation energy by the interpolation formula U( J)= U c[( J 0/ J) μ −1] gave μ=0.33 and 0.1 for the fields at the secondary peak and away from the peak, respectively. This shows that variable range hopping (VRH) processes define the magnetic relaxation at the secondary peak, whereas single vortex pinning was obtained at higher fields. It was discussed that increased c-axis coupling with Pb-doping contributed to the increased pinning in both field ranges. The nanometer-size laminae, observed in heavily Pb-doped Bi2212, were suggested to be responsible for the variable range hopping processes at the secondary peak. The nanometer-size amorphous regions were most probably responsible for the increased pinning at higher fields.