Anisotropic Superconducting and Magnetic Properties of a Single Crystal of ErRh4B4

The magnetic response of a single crystal of the tetragonal compound Er${\mathrm{Rh}}_4$ ${\mathrm{B}}_4$ in the $\overline{c}$ (hard) and $\overline{a}$ (easy) directions is presented. Extreme anisotropy is found in the magnetic and superconducting behavior, allowing a natural separation of the bare superconducting properties from those affected by magnetism. Below $T_{c2\mathrm{}}$ the data reconcile the discrepancy between the ferromagnetic moment per erbium atom measured by neutron scattering and by Mössbauer effect.