Effect of Weak DC Magnetic Fields on the AC Magnetization of a Melt-Processed YBa₂Cu₃O_y

We studied, both theoretically and experimentally, the ac magnetic response of type-II superconductors superimposed on a weak H_dc field. Theoretical simulations were made within the construct of the Kim-Anderson critical-state model, where the critical current density J_c is assumed to be given by J_c= k / (B₀+|B_i|). k and B₀ are temperature-dependent material parameters, and B_i is the local flux density in the sample. We also measured complex susceptibilities X_n=X′_n-iX″_n (n=1,2,3,5,7) of a melt-processed YBa₂Cu₃O_y specimen and analyzed the observed X_n by applying the magnetization equations derived from the Kim-Anderson model. We found that the experimental results were qualitatively well reproduced. Present results indicate that, for a specimen of which J_c sensitively varies with B₁, the effect of a weak H_dc should be explicitly taken into consideration for studies of the ac magnetization.