Bound Magnetic Polarons and the Insulator-Metal Transition in EuO

A new model is presented for the insulator-metal transition in Eu-rich EuO: At low temperatures the electrons are only very weakly bound to oxygen vacancies and conduction is metallic; near and above $T_c$ the electrons localize and form bound magnetic polarons by ordering the ${\mathrm{Eu}}^{2+}$ spins neighboring the vacancy, thereby gaining exchange energy. Magnetic-susceptibility data are presented which support this model.