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

Flux-pinning mechanisms of Al
₂
O
₃
-doped YBa
₂
Cu
₃
O
_7-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 δl and δT
_c
pinning in the nanosize Al
₂
O
₃
-doped Y-123 (Y-123:Al) samples. The δT
_c
and δl pinning mechanisms play an important role, each of them in its own temperature region; δl and δT
_c
are dominant mechanisms at low and high temperatures, respectively. It is noted that δl and δT
_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.