Pinning investigation in NbN thin films

NbN thin films with Tc=16 K and a thickness of 100 nm were fabricated on MgO(100) substrates by UHV pulsed laser deposition by using a Nb (99.95%) target in nitrogen gas atmosphere. Low temperature magnetic force microscopic (LT-MFM) images were performed after field cooling in a magnetic field of -2 mT to study the temperature dependence of vortex distribution. At low temperatures vortices are organized in a slightly disordered Abrikosov lattice with an average inter-vortex distance of 930 nm. Increasing the temperature to above T=75%Tc leads to a movement of vortices during the MFM scan, which is interpreted as a thermally activated depinning process initiated by the stray field of the tip. The local pinning force has been estimated from MFM-images and compared with global VSM magnetization measurements. In addition, the temperature dependence of the penetration depth, λ, was extracted from the magnetic field profiles of single vortices.