Enhancement of the critical current density of MgB2 thick films by Ag and Cu impurity layers

We report on Ag and Cu doping in MgB2 thick films using amorphous Ag- and Cu-impurity layers with various thicknesses, 4, 8, 16, and 32 nm. Firstly, Ag- and Cu-impurity layers were deposited on Al2O3(0 0 0 1) substrates at room temperature by using pulsed laser deposition system. Subsequently MgB2 films were grown on the top of Ag or Cu/Al2O3 substrates at 560 and 600 °C by using hybrid physical–chemical vapor deposition. The effect of Ag and Cu doping as well as growth temperature on the microstructure and critical current density (Jc) of MgB2 films were investigated. It was found that both of Ag/Cu doping and low growth temperature can significantly enhance Jc without suppression of Tc. The increase in Jc results from improved grain connectivity and strong flux pinning by high density of grain boundaries and other types of defects introduced by Ag- or Cu-impurity layers.