Preparation of Nb₃Ge films by chemical transport reaction and their critical properties

Niobium-germanium films have been deposited on sapphire substrates at 900 ‡C by a chemical transport reaction method. The highest superconducting transition onset temperature T _C,on of 22.4 K is observed for a nearly stoichiometric Nb₃Ge film with the A15-type structure (thickness ~ 93.5 Μm). Lattice constants for the Nb₃Ge phase formed in the Nb-Ge films with both T _C,on above 22 K and T _C,midpoint above 21 K are found to extend from 5.143 to 5.153 ». Deposition rates for the obtained films are in the range of 2–10 Μm/min. Critical current densities for the Nb₃Ge film with the highest T _C,on value are observed to be relatively low (~ 10³ A/cm² at 19 K at self-field). This is due to the coarse grain structure of the film or the low density of effectual pinning centers in the film. Field variations of the pinning forces operating in this film in magnetic fields both parallel to the film surface and perpendicular to the film surface are found to follow closely b ^1/2(1\s-b)², to which the pinning force for flux pinning at the surface of normal regions, such as grain boundaries, film surfaces, etc., is proportional, and where b is the reduced magnetic induction (B/B _C2).A small increase in J _C at low fields is caused by the presence of a small amount of the Nb₅Ge₃ phase in a Nb₃Ge film, and seems attributable to additional flux pinning on Nb₅Ge₃-phase particles in the film.