Mixed-state magnetotransport properties of MgB₂ thin film prepared by pulsed laser deposition on an Al₂O₃ substrate

Abstract

A MgB₂ film was deposited on an Al₂O₃ substrate (1102) using pulsed laser deposition (PLD) and in situ annealing at 700 °C for 30 min. The thickness of the film was 600 nm. The film was investigated using a PPMS system from Quantum Design. The film presented a critical temperature of 36 K, and XRD analysis showed that the film is preferentially c-oriented. Transport properties that were measured or estimated included the following: upper critical field, H_C2, irreversibility field, H_irr, activation energy, U_o, magnetoresistance, MR and I–V characteristics. We completed investigations under different magnetic fields (0–7 T) applied perpendicular to the film at different temperatures. The results showed the remarkable dependence of both T_C and ΔT_C on the field. ΔT_C increased continually from 1.6 K at no field up to 3.8 K for 7 T. The strong dependence of U_o on the magnetic field was observed, but U_o decreased faster for high fields from 451 meV at 0 T down to 210 meV at 7 T. The sharp decrease of U_o indicated that the weakening of the effective pinning forces is more rapid at higher fields. H_C2 and H_irr were estimated using the Arrhenius law, and the H_C2(0) value was 21 T. The current–voltage (I–V) characteristics measurements at different conditions showed hysteresis in the critical current, I_C, which was temperature- and magnetic field-dependent. Although the critical current degraded linearly with increasing magnetic field, the hysteresis width, ΔI, tended to decrease in value more quickly at higher temperatures. Hysteresis has been attributed to weaker intergrain coupling and larger effective fields at the grain boundaries in the film.