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The ferromagnetic coupling between Co and CrO2 , through an insulator (Cr2O3) was characterized by in situ magneto-optic Kerr effect. By evaporating 20–60 Å Co thin films on top of epitaxial CrO2 films, a Co/Cr2O3 /CrO2 trilayer system can be readily fabricated; this is possible because the native surface layer of CrO2 is Cr2O3 . In situ x-ray photoemission studies show that the Co is oxidized at the interface between Co and Cr2O3 , so that the system more resembles Co/CoO/Cr2O3 /CrO2 . The Co thickness and temperature dependence of the magnetic hysteresis loops indicate that magnetic coupling strength increases with increasing Co thickness and decreases with increasing temperature. The magnetic coupling through the insulator barrier may be related to defect states in the insulating barrier layer. © 2003 American Institute of Physics.