It is first demonstrated experimentally that there are two different irreversibility temperatures, Tirrcer and Tirrgr (Tirrcer(Tirrgr), in high-T ceramic superconductors of the compositions Bi1.6Pb0.3Sb0.1Sr2Ca2Cu2.8Oy and YBa2Cu4O8. At T(Tirrcer, both intragranular and intergranular irreversible currents are induced by pinning. When T)Tirrgr, there are no irreversible currents. It is found that Tirrgr(Tc for bismuth ceramics and Tirrgr approximately Tc for YBa2Cu4O8.

The paper is devoted for the theoretical and experimental study of the picture of the low magnetic field penetration into HTSC ceramics, the field being much smaller than the critical one of grains. Within the critical state theory for the low-field electrodynamics of the random Josephson medium the expressions are obtained for the harmonics of the induction B(t) as a function of the external parameters in two asymptotic cases of a low field and thin sample. In particular, for a thin sample the harmonics are shown to be an oscillation function of the ac field amplitude. The behavior of the linear and nonlinear susceptibility in the ac field, hysteresis loops and dc magnetization curve are studied experimentally in a wide temperature and field range. A satisfactory agreement in the experimental and theoretical results is outlined. A number of the magnitudes characterizing the low-field electrodynamics in HTSC and their temperature dependences are obtained. The explicit expression for the function J(c)(H, T) = = j0H0(2)(T)/(H-2 + H0(2)(T)) is determined in the experiment.

We have investigated, theoretically and experimentally, the penetration of magnetic fields into ceramic HTSC for fields below the lower critical field of the grains. In the framework of a critical state theory in the low-field electrodynamics of a random Josephson medium, expressions for the harmonics of the induction B̄(t) as a function of external parameters were derived for two asymptotic cases — the case of the weak field and the case of the thin sample. The behavior of the magnetic induction B̄ was investigated in AC and DC fields in a wide range of temperature and field amplitudes. Rather good agreement between experimental and theoretical results has been found. From the experimental results the values of number of quantities characterizing the low-field electrodynamics of HTSC and their temperature dependences as well as an explicit expression for the critical current densityjc(H, T) = j0(T)H20(T)H2+H20(T)) were derived.

We have investigated experimentally the penetration of low magnetic fields into Bi-based ceramics. The behavior of the harmonics of the induction was investigated in ac and dc fields in a wide range of temperatures and field amplitudes. From the experimental results obtained in the asymptotic case of a weak field the values of a number of quantities characterizing the low-field electrodynamics of HTSC and their temperature dependences as well as an explicit expression for the critical current density jc(H, T)=jo(T)Ho(T)[|H| + Ho(T)] were derived.