Fundamental and harmonic susceptibilities of YBa₂Cu₃O_7-δ

We have examined the complex harmonic magnetic susceptibilities χ_n=χ_n^′-iχ_n^″ (n=1,2,3,...,10) of the sintered high-critical-temperature (high-T_c) superconductor YBa₂Cu₃O_7-δ. The experimental variables for the measurements of χ_n were the sample temperature T (10⩽T⩽110 K), the ac magnetic field amplitude H_ac (1.4 μT⩽μ₀H_ac⩽8.5 mT), frequency f (7.3⩽f⩽1460 Hz), and the magnitude of a superimposed dc field H_dc (|μ₀H_dc|⩽8.5 mT). As functions of temperature, χ₁^′ and χ₁^″ depend on both H_ac and H_dc. In particular, the χ₁^′ transition curve can shift to higher temperatures with increasing H_dc. Odd-harmonic susceptibilities were measured as functions of temperature below T_c for zero H_dc; both even and odd harmonics were observed for nonzero H_dc. At fixed temperature, the odd-harmonic susceptibilities are even functions of H_dc, while the even-harmonic susceptibilities are odd functions of H_dc. We compared the experimental intergrain coupling characteristics of χ_n^′ and χ_n^″ with theoretical susceptibility curves based on magnetization equations derived by Ji et al. from a simplified Kim model for critical current density. The theoretical curves are in good agreement with the temperature- and field-dependent features of χ_n^′ and χ_n^″, and, therefore, the intergrain coupling component of a sintered high-T_c superconductor behaves as a type-II superconductor.