Magnetic relaxation and critical currents of NdBCO coated conductors*

A magnetic study of the critical current density $J_c$ and magnetic relaxation (`creep') effects in thin NdBa$_2$Cu$_3$O$_{7-\delta}$ (NdBCO) superconducting films of thicknesses 0.7 and 2.1 $\mu$m was conducted. These films, doped with BaZrO$_3$, were deposited by a PLD process on `IBAD' substrates. The $J_c$ values display broad peaks near the c-axis. This is associated with densely spaced columnar defects distributed about this axis. We analyzed the magnetic relaxation data using the Maley expression for the activation energy $U$ as a function of current density $J$. The data are described fairly well by the collective creep interpolation formula $U=U_0\left[\left(J_{c0}/J\right)^\mu-1\right]$ with $U_0$, $J_{c0}$, and $\mu$ treated as fitting parameters.