Flux Pinning Mechanisms and Competition Between $\delta l$ and $\delta {T_c}$ Pinning in Polycrystalline Nano-Alumina-Doped YBa2Cu3O7- y

Abstract

Flux-pinning mechanisms of Al2O3-doped YBa2Cu3O7- y (Y-123) have been investigated by electrical and magnetic measurements. Using the Ambegaokar and Halperin model applied to magnetoresistivity ρ ( T, H ), we calculated the critical current density in the grain boundaries at zero temperature. The field dependence of the critical density ${J_c}(B)$ was analyzed within the collective pinning model. A crossover field B sb from the single vortex to the vortex bundle-pinning was observed. It has been found that there is competition between $\delta l$ and $\delta {T_c}$ pinning in the nanosize Al2O3-doped Y-123 (Y-123:Al) samples. The $\delta {T_c}$ and $\delta l$ pinning mechanisms play an important role, each of them in its own temperature region; $\delta l$ and $\delta {T_c}$ are dominant mechanisms at low and high temperatures, respectively. It is noted that $\delta l$ and $\delta {T_c}$ mechanisms coexist at 50 K for the Y-123:Al sample. We have also plotted the new B–T phase diagram of Y-123:Al samples.