Irreversibility lines in halogen- and hydrogen-doped YBa₂Cu ₃O_7-δ compounds

We measure the real and imaginary parts of the ac magnetic permeability of YBa₂Cu₃ O_7-δA_χ (A = Cl, F, H) ceramics and powders as functions of temperature and ac magnetic field amplitude H₀, applying uncommonly low and widely ranged excitation fields (1 mOe ≤ H_o ≤ 200 Oe). We determine the temperature dependence of two loss peaks, H_p(T) ∼ (T_c - T) ^2.7 at low fields in ceramics and H_m(T) ∼ T _s0 - T at higher fields in both powders and ceramics. The extrapolated field H_p(0) and characteristic temperature T_c depend on χ and δ, whereas H_m(0) and T_s0 depend mainly on δ. The real part of the permeability indicates a two-step flux-penetration process with two threshold fields for flux penetration. The latter govern the temperature dependence of maxima in the imaginary part as well as the peaks' widths. We propose a field-dependent percolation description of this process, derived from a previous temperature-dependent percolation model. It involves two penetration depths, the classic Ginzburg-Landau one and a low-field abnormally large penetration depth due to chemical disorder. Correspondingly, two types of vortices describe high- and low-field dissipation peaks.