Flux pinning by grain boundaries in niobium bicrystals

Abstract

Bicrystals of niobium with symmetric tilt boundaries grown directly from the melt with bicrystal seeds have been used as a model system to measure the resistive critical current I_c as a function of the applied transverse magnetic field H and angle φ, between H and the grain boundary plane. The specimens were investigated in the as-received, chemically-polished and surface-oxidized states. Of these, the last state brought out the true grain boundary contribution to I_c by suppressing the surface I_C. Extremely narrow peaks in I_c(φ) with half-intensity width ≤ 1° and heights up to eight times the off-peak values were found for φ = 0. The present results provide direct evidence of the efficacy of grain boundary pinning. The simplicity of the pinning system allows the elementary pinning force ƒ_pL to be evaluated without the complications of statistical summation. Typically, ƒ_pL ∼ 7 × 10⁻⁶ Nm⁻¹ at b=0.67. From a consideration of the orientations of the grains, the possibility of grain boundary pinning occurring through the anisotropy of H _c2 is ruled out in our bicrystals. A calculation of the parelastic interaction between the grain boundary dislocations and the flux line lattice, shows no pinning. Our results suggest the presence of an inhomogeneous layer at the grain boundary as the cause of the grain boundary flux pinning.