FUNDAMENTAL AND HARMONIC SUSCEPTIBILITIES OF YBA2CU3O7-DELTA

We have examined the complex harmonic magnetic susceptibilities n=n2-in (n=1,2,3, ..., 10) of the sintered high-critical-temperature (high-Tc) superconductor YBa2Cu3O7. The experimental variables for the measurements of n were the sample temperature T (10 T 110 K), the ac magnetic field amplitude Hac (1.4 T 0Hac 8.5 mT), frequency f (7.3 f 1460 Hz), and the magnitude of a superimposed dc field Hdc (|0Hdc| 8.5 mT). As functions of temperature, 12 and 1 depend on both Hac and Hdc. In particular, the 12 transition curve can shift to higher temperatures with increasing Hdc. Odd-harmonic susceptibilities were measured as functions of temperature below Tc for zero Hdc; both even and odd harmonics were observed for nonzero Hdc. At fixed temperature, the odd-harmonic susceptibilities are even functions of Hdc, while the even-harmonic susceptibilities are odd functions of Hdc. We compared the experimental intergrain coupling characteristics of n2 and 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 n2 and n, and, therefore, the intergrain coupling component of a sintered high-Tc superconductor behaves as a type-II superconductor. © 1990 The American Physical Society.