Equilibrium behavior and critical current density in polycrstalline MgB2

A simple "effective media" approach is used to calculate the constitutive equilibrium field Be(H), and hence also the equilibrium magnetization Me(I-I), relations for a polycrystalline, anisotropic Type 11 superconductor with random grain orientation., Mutually-consistent scaling of experimental M versus H isotherms to the calculated Me(H) relation, for an as-prepared polycrystalline MgB2 specimen, allows for the determination of a self-consistent set of values for the anisotropic G-L parameters and for the critical fields H-c1(T), and H-c2(T) for the,material. The calculated B-e(H) relation, together with explicit critical current density, J(c)(B), trial functions, allows for the determination of flux density profiles [B(r)](H) and. also the nonequilibrium magnetization M(H) behavior, which is compared with the experimental M versus H isotherms. Optimum fits are obtained with, a Kramer-like relation of the form: J(c)(B, T) proportional to H-c1(n) (T)(1-B/B-0) B-2(-1/2), where B-0 (T) approximate to mu(0) H-irr (T) is the irreversibility field, and n = 0.75. and 2.25 for T below and above 28 K, respectively. The general form of this relation suggests that J(c) in polycrystalline MgB2 is determined by vortex pinning at grain boundaries.