Application of NbN films to the development of very high field superconducting magnets

Abstract

We report the preliminary results of a program recently begun at Argonne National Laboratory to demonstrate the feasibility of using niobium nitride (NbN) films as practical high field superconductors. Films of varying thicknesses (2-9 μm) have been deposited on sapphire and Hastelloy substrates, using d.c. magnetron sputtering. The superconducting transition temperatures T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> of these films range from 12-15 K. Using the WHH extrapolation, upper critical fields H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</inf> (O) up to 36 T in the parallel direction and 43T in the perpendicular direction were obtained. Critical current densities J <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> (H,4.2 K) were <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\approx 1\times10^{4}</tex> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 20 T in both the parallel and perpendicular directions. The preliminary results from novel sample preparation technique for TEM studies of film cross-sections is briefly described. This technique shows the variation in film structure as a function of distance from the substrate.