Fluxoid Pinning by Small Nitride Precipitates in Niobium

Abstract

The magnetic hysteresis of superconducting niobium single crystals containing ∼80-Å-diam nitride precipitates was investigated. Introducing precipitates into the samples produced no significant change in the critical-current density $J_c$ except near the critical fields $H_{c1\mathrm{}}$ and $H_{c2\mathrm{}}$. The samples trapped flux below $H_{c1\mathrm{}}$ and exhibited dramatic peaks in $J_c$ within 3% of $H_{c2\mathrm{}}$. These observations are interpreted as clear evidence of a threshold for fluxoid pinning, and reasonable agreement with theory is obtained.