Light Switching of the Ferromagnetic Phase in the Magnetic Semiconductor EuSe

We demonstrate that light resonant with the EuSe bandgap forces the crystal lattice spins into complete ferromagnetic alignment, due to the generation of magnetic polarons. The magnetic polaron has a super gigantic magnetic moment approaching 6000 Bohr magnetons, which is an absolute record value, and more than two orders of magnitude larger than for polarons seen in diluted magnetic semiconductors. Hence a magnetic field of only 8mT is sufficient to fully align all polarons. The polaron is described by an effective exchange magnetic field of about 1 Tesla. This field is sufficient to drive the paramagnetic lattice into the ferromagnetic state. A self-consistent quantum-mechanical calculation of the magnetic polaron in EuSe supports the experimental findings. It also provides a clear-cut picture of the magnetic polaron inner spin structure, such as the radial distribution of spin orientation and exchange magnetic field.

We demonstrate that the electronic state generating the magnetic polaron is intrinsic, therefore light can be used to completely magnetize the layer simply by increasing the pulsed light excitation. The discovered phenomenon has the potential applicability to many other magnetic semiconductors in their disordered phase.