  We employ spin-polarized (SP) STM to study the spin-dependent tunneling
between a magnetite (111) sample and an antiferromagnetic tip through a vacuum
barrier at room temperature. Atomic scale STM images show significant magnetic
contrast corresponding to variations in the local surface states induced by
oxygen vacancies. The estimated variations in tunneling magnetoresistance (TMR)
of 250% suggest that the spin-transport properties are significantly altered
locally by the presence of surface defects.
