  We demonstrate that light resonant with the bandgap forces the
antiferromagnetic semiconducor EuSe to enter ferromagnetic alignment in the
picosecond time scale. A photon generates an electron-hole pair, whose electron
forms a supergiant spin polaron of magnetic moment of nearly 6,000 Bohr
magnetons. By increasing the light intensity, the whole of the sample can be
fully magnetized. The key to the novel large photoinduced magnetization
mechanism is the huge enhancement of the magnetic susceptibility when both
antiferromagnetic and ferromagnetic interactions are present in the material,
and are of nearly equal magnitude, as is the case in EuSe.
