  Using a Ginsburg-Landau free energy functional, we study the $H-T$ phase
diagram of a weak magnetic superconductor, where the magnetization from the
magnetic component is marginal in supporting a spontaneous vortex phase in
absence of external magnetic field. In particular, the competition between the
spiral state and spontaneous vortex phase is analysed. Our theory is applied to
understand the magnetic properties of $ErNi_2B_2C$.
