Clustering and electron spin resonance linewidths in Fe/MgO

Abstract

Electron spin resonance linewidths and integrated intensities for the 1/2 → −1/2 transition of Fe3+ in cubic sites in single-crystal MgO have been examined at 9 GHz between 293 and 90 K for iron concentrations between 100 and 12 900 ppm. Very little of the iron is present in cubic sites as isolated Fe3+ ions. It is suggested that this is partly caused by clustering, even at extremely low concentrations. The measured linewidths, which agree well with those reported previously, have been analysed in terms of Kittel and Abrahams' dipolar broadening theory. The highest measured Fe3+ concentrations give linewidths comparable with those predicted, but for most samples the measured linewidths are considerably broader than those calculated. This broadening may be caused by interactions with the undetected iron, which accounts for the major fraction. There is no evidence for exchange narrowing of the Fe3+ lines for the range of concentrations examined.