INFLUENCE OF A FERROMAGNETIC TIP ON THE ABRIKOSOV VORTEX LATTICE IN NBSE2 STUDIED BY LOW-TEMPERATURE SCANNING-TUNNELING-MICROSCOPY

We use a high-vacuum low-temperature scanning tunneling microscope to study the Abrikosov vortex lattice in NbSe2 at 2.7 K. Vortex lattices imaged with a ferromagnetic Ni tip in an external field show a higher vortex density than lattices imaged with a nonmagnetic tip in the same field. The additional field induced by the Ni tip can be extracted fr om the vortex density and is found to be about 10 mT, independent of t he external field. In zero external field, the Ni tip generates vortic es with a density corresponding to 10 mT. The scanning motion of the N i tip neither affects single vortices, nor the complete vortex lattice . We attribute this to the homogeneity of the tip stray field.