  The discovery of reversible 300 K ferromagnetic ordering in a diluted
magnetic semiconductor is reported. Switching of room-temperature
ferromagnetism between "on" and "off" states is achieved in Co2+:ZnO by lattice
incorporation and removal of the native n-type defect, interstitial Zn.
Spectroscopic and magnetic data implicate a double-exchange mechanism for
ferromagnetism. These results demonstrate for the first time reversible
room-temperature ferromagnetic ordering in a diluted magnetic semiconductor,
and present new opportunities for integrating magnetism and conductivity in
semiconductor sensor or spin-based electronics devices.
