Transport properties of superconducting MgB ₂ fibers under high magnetic fields

Superconducting MgB ₂ fibers have been grown by using a diffusion method, in which B filaments are exposed to Mg vapor inside a folded Ta foil over a wide range of temperatures and growth times. The fibers have a diameter of about 110 μm. The radial distribution of Mg ions into B is observed over the cross-sectional area, and the transport properties of the MgB ₂ fibers are examined by using a physical property measurement system. The MgB ₂ fibers grown at 900°C for 2 hours show a superconducting transition at 39.8 K with ΔT _c < 2.4 K. The resistance of the MgB ₂ sample at room temperature is 4 Ω, and the residual resistance ratio (RRR) is estimated as 4.72. The electrical resistance of the fibers is measured in a magnet field from 0 T to 8 T. From this measurement, the upper critical field H _c2 at 4 K is estimated to be more than 16 T. A small amount of magneto-resistance is detected at high magnetic fields. The current vs. voltage characteristics are measured to get critical currents.