Offset susceptibility of superconductors

The novel concept of offset susceptibility χ₀ is presented for superconductors in applied fields H_dc+H_ac sinωt. χ₀ represents the dc component of the time-dependent magnetization M(t). For sintered YBa₂Cu₃O₇, χ₀ was measured as a function of H_ac, H_dc, and temperature using a Hall probe magnetometer and Fourier analysis. For intragranular diamagnetism, χ₀ behaves as the real part of complex fundamental susceptibility χ₁^′, confirming a London-like field penetration. However, χ₀ arising from intergranular coupling is positive at temperatures just below the onset of the coupling transition, in good agreement with theoretical predictions of a simplified Kim critical-state model of magnetization.