300 K Ferromagnetic Magnetic Circular Dichroism in Co2+-doped ZnO   Activated by Shallow Donors
  Cobalt-doped ZnO (Co2+:ZnO) films were studied by magnetic circular dichroism (MCD) spectroscopy. A broad 300 K ferromagnetic MCD signal was observed between 1.4 and 4.0 eV after introducing shallow donor states by exposure of paramagnetic Co2+:ZnO films to zinc vapor. The new sub-bandgap ferromagnetic MCD intensity is attributed to low-energy photoionization transitions originating from a spin-split donor impurity band in ferromagnetic Co2+:ZnO. 
Kevin R. Kittilstved, Jialong Zhao, William K. Liu, J. Daniel Bryan,   and Daniel R. Gamelin