High Field Properties of Pure Niobium Nitride Thin Films

Thin films of the rocksalt structure (B1) compounds of the transition metals can be prepared by the method of reactive sputtering. By a refinement of this technique we have produced, for the first time, thin films of niobium nitride (NbN) with transition temperatures similar to those of the bulk materials (≳ 15°K). Films have been prepared on both hastelloy and fused silica substrates. Measurements of the normal state resistivity ρ_n yield values in excess of 200 μΩ.cm. Preliminary studies of the resistive transitions in these materials near T_c indicate that H_r(J), the field at which ρ/ρ_n=1/2, is independent of current density below 200 A/cm² for films 1000-4000 Å thick. Identifying H_r(J) (J<50 A/cm²) with the upper critical field H_c2(t), the measurements indicate that H_c2(0) is significantly in excess of 200 kOe, and thus is substantially higher than the values obtained for the bulk material. These results are discussed in terms of the current theories of high field superconductivity with particular reference to the importance of Pauli spin paramagnetism and spin-orbit scattering in these materials.