Exchange-Induced Autoionization in Eu-Rich EuO

Below the Curie temperature, with increasing magnetic order, Eu-rich EuO shows both a change of 0.15 mm/sec in the Eu¹⁵¹ isomer shift and an insulator-metal transition with a resistivity change of more than 13 orders of magnitude. These effects are interpreted on the basis of an exchange-induced configurational change (autoionization) of two electrons trapped at an oxygen vacancy.