Vacuum-tunneling magnetoresistance: The role of spin-polarized surface states

We have studied magnetoresistance effects in Fe-vacuum-Gd tunnel junctions as a function of the applied bias voltage by using a scanning tunneling mic roscope operated under ultra-high-vacuum conditions. We found that the vacu um-tunneling magnetoresistance (VTMR) can be maximized by tunneling into hi ghly spin-polarized surface states. By tuning the applied bias to the energ etic positions of the spin-polarized surface states, a VTMR response as muc h as 31% at 70 K was obtained. This result is explained in terms of an enha ncement caused by the spin-polarized surface state and a suppression of spi n-flip tunneling processes compared to tunnel junctions with oxide barriers . (C) 1999 American Institute of Physics. [S0003-6951(99)03227-1].