Dependence of critical current densities in composite conductors with ultrafine filaments on the magnetic field application

Critical current densities of Nb₃Sn composite conductors with ultrafine filament (d_f=36.7 nm~145 nm) have been observed to depend on the magnetic history of the composite. Magnetic fields were applied as follows. (A) (1) The bias field is increased to higher values and then reduced to B₀ , (2) the bias field is increased directly from zero to B ₀; and (3) the composite is cooled down within B₀. (B) After establishing B₀, a second field B_m is temporarily superimposed parallel or antiparallel to B₀ and then removed. Then the critical current density is measured by increasing the current. A new cycle of B_m of the same sign may follow, with the process being repeated