Atomic resolution and vortex lattice studies of magnetic superconductors: A first approach in the nickel borocarbide TmNi2B2C

We present new scanning tunneling microscopy measurements in the superconductor TmNi2B2C. The topography shows in some areas flat surfaces, where atomic size modulations can be identified. We find a hexagonal vortex lattice between 0.15 T and 1.4 T, when the magnetic field is applied along the basal plane of the tetragonal crystal structure (B⊥c)(B⊥c), and a hexagonal to square transition around 0.15 T when the field is applied along the c  -axis (B‖c)(B‖c). Measured intervortex distance are smaller than expected at high field, due to the internal field being larger than the applied field.