  We present measurements of the superconducting upper critical field Hc2(T)
and the magnetic phase diagram of the superconductor ErNi2B2C made with a
scanning tunneling microscope (STM). The magnetic field was applied in the
basal plane of the tetragonal crystal structure. We have found large gapless
regions in the superconducting phase diagram of ErNi2B2C, extending between
different magnetic transitions. A close correlation between magnetic
transitions and Hc2(T) is found, showing that superconductivity is strongly
linked to magnetism.
