  We report a photoinduced change of the coercive field, i.e., a
photocoercivity effect (PCE), under very low intensity illumination of a
low-doped (Ga,Mn)As ferromagnetic semiconductor. We find a strong correlation
between the PCE and the sample resistivity. Spatially resolved dynamics of the
magnetization reversal rule out any role of thermal heating in the origin of
this PCE, and we propose a mechanism based on the light-induced lowering of the
domain wall pinning energy. The PCE is local and reversible, allowing writing
and erasing of magnetic images using light.
