The electron spin resonance of Fe4+-OI was studied by using the optically detected magnetic circular dichroism of absorption technique. The charge state, Fe3+-OI, was present at the same time. It was concluded that the Fe4+-OI was created metastably at low temperatures by the light which was used to detect the magnetic circular dichroism.

D.Faust, H.J.Reyher, O.F.Schirmer: Solid State Communications, 1996, 98[5], 445-7