Dependence of transport critical current of magnetic field processed Bi₂Sr₂CaCu₂O₈/AgMg tapes on the background magnetic field and magnetic field direction

The background magnetic field dependence of the critical current (I _c) is studied on magnetic field processed (MFP) Bi₂Sr ₂CaCu₂O_8+δ (Bi-2212) multifilamentary tapes in increasing and decreasing magnetic fields. The critical currents are measured in magnetic fields up to 5 T at 4.2 K, with several orientations of the applied magnetic field relative to the tape plane. Two field dependences of I_c are found: a stronger one when the magnetic field is applied perpendicular to the tape plane (B ∥ c) and a weaker one when it is applied parallel to the tape plane (B ⊥ c). The angular dependence of I _c as a function of B sin θ, which is the component of the magnetic field perpendicular to the tape plane, exhibits that I_c(B sin θ) coincides with I_c(B ∥ c) in the high-field region, indicating that the tape behaves as a two-dimensional (2D) superconductor. Because of the existence of misalignment in Bi-2212 grains, an average misalignment angle φ_avg is introduced and is found within the range of 4.5°-6° for the MFP Bi-2212 tape. φ_avg is independent of the magnetic field, indicating that it is an intrinsic property of the tape. To determine the critical current behaviour for all orientations of the magnetic field, a scaling function f(θ) is introduced. The field-angle dependence of the critical current can be scaled to one curve where the field applied perpendicular to the tape plane at 4.2 K within the field range between 0 and 5 T, both in increasing and decreasing fields, which is of primary importance.