Fabrication and Characterization of MgB2 Films With Two-Dimensional Artificial Pinning Centers Using ZnO Nanowires

We fabricated MgB₂ thin films on off-cut c-plane sapphire substrates with ZnO nanowires arranged along the edges of nanosteps to investigate the effect of two-dimensional artificial pinning centers (2D-APCs) on superconducting thin films. First, the ZnO nanowires were fabricated on the c-plane sapphire substrates with the MOCVD technique. Then, the MgB₂ thin films (200 nm) were deposited on the substrates with a carrousel-type magnetron sputtering system. The AFM images and XRD (2 thetas/thetas) patterns of samples with and without ZnO nanowires show that fabricating ZnO nanowires on the substrates has a negligible effect on the crystallinity of the MgB₂ thin Alms. The surface resistance (R_s) of these thin films was measured at 38 GHz with a dielectric resonator method both with and without an external dc magnetic field. A dc magnetic field of up to 2.1 T has been applied using a superconducting magnet. The R_s of the MgB₂ thin films with nanowires was found to be larger than that of thin films without nanowires in the absence of external magnetic field, whereas, the R_s of the MgB₂ thin films with nanowires was found to be smaller than that of those without nanowires in the external dc magnetic field from 0.7 T to 2.1 T. These results suggest that the ZnO nanowires acted as 2D-APCs.