Mixed-state flux dynamics in bulk MgB2

Electric field vs. current density (E-J) isotherms in the mixed-state of a bulk sample of the high-temperature superconductor MgB2 (Tc=38.5 K), synthesized under 200 MPa pressure by hot isostatic pressing (HIPing), have been measured and analyzed in terms of the critical scaling model. Magnetization data reveal distinctly different critical current density (Jc) behaviors in high and low magnetic field critical scaling regions. E-J isotherm sets at fields ranging from 2 to 90 kOe conform to the vortex-glass (VG) scaling anzatz. Scaling analysis, resistivity data and Jc data suggest that a Bragg-glass state may exist for H<8 kOe. The critical scaling exponents depend on field and range from ν∼0.6, z∼3.1 in low fields (H<8 kOe) to ν∼0.9, z∼3.4 in high fields. A more complete magnetic field-temperature (H-T) phase diagram, containing the upper critical field Hc2(T), the VG transition line Hg(T), and the magnetic irreversibility line Hirr(T) has been established for bulk MgB2