Using a Ginsburg-Landau free-energy functional, we study the H-T phase diag
ram of a weak magnetic superconductor, where the magnetization from the mag
netic component is marginal in supporting a spontaneous vortex phase. In pa
rticular, the competition between the spiral state and spontaneous vortex p
hase is analysed. Our theory is applied to understand the magnetic properti
es of ErNi2B2C.