We present measurements of the superconducting upper critical field H c2(T) and the magnetic phase diagram of the superconductor ErNi 2B 2C 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 ErNi 2B 2C, extending between different magnetic transitions. A close correlation between magnetic transitions and H c2(T) is found, showing that superconductivity is strongly linked to magnetism.
