Surface Ferromagnetism and Superconducting Properties of Nanocrystalline Niobium Nitride

We report magnetic, transport (electrical) and thermal (heat capacity) properties of nanocrystalline $\delta $-NbNx prepared by urea-nitridation method and heated at three different temperatures, 700, 800 and 900 $^{\circ}$C respectively. Particle size and their agglomeration increases with increasing synthesis temperature. The sample prepared at 900 $^{\circ}$C, showed the highest transition temperature, Tc(onset) $=$ 16 K with a transition width ($\Delta $Tc) of 1.8 K, as obtained from resistivity measurement on the cold-pressed bar. Above Tc, magnetization measurements revealed the presence surface ferromagnetism that coexists with superconductivity below Tc. Heat capacity measurements confirm the bulk nature of superconductivity with strong electron -phonon coupling. These results are compared with those of the samples prepared at 800 $^{\circ}$C with a lower Tc (10K) and 700 $^{\circ}$C, which is non-superconducting down to the lowest temperature measured.