Flux pinning mechanism in unorientated grains of YBa2Cu3O7-δ

Abstract

Magnetization measurements of the intragranular pinning force in polycrystalline YBa2Cu3O7-δ and YBa2(Cu1-xMx)3O7-δ, where M = Fe,Ni, address two major questions of flux pinning: 1, what are the dominant pinning sites; and 2, what is the relevant pinning mechanism in these materials. It is demonstrated that the microstructure, i.e. the twin spacing, is unaltered by Ni, but drastically decreased by addition of Fe. Thus these systems provide an ideal tool with which to examine the role of twin boundaries in flux pinning. The volume pinning force is interpreted in terms of flux line shear, i.e. FP is related to the onset of dissipative flux flow within weakly pinning channels in the (inhomogeneous) superconductor. The theoretically expected field and temperature scaling agrees with the experimental data. A quantitative analysis of the data yields a channel width comparable to the flux line spacing, ao. This indicates that single rows of flux lines move once the Lorentz force exceeds the flow stress of the flux line lattice. The applicability of this pinning mechanism to YBa2Cu3O7-α more homogeneous than the metallurgically prepared powder used here is discussed. © 1990.