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 microscope operated under ultra-high-vacuum conditions. We found that the vacuum-tunneling magnetoresistance (VTMR) can be maximized by tunneling into highly spin-polarized surface states. By tuning the applied bias to the energetic positions of the spin-polarized surface states, a VTMR response as much as 31% at 70 K was obtained. This result is explained in terms of an enhancement caused by the spin-polarized surface state and a suppression of spin-flip tunneling processes compared to tunnel junctions with oxide barriers. (C) 1999 American Institute of Physics. [S0003-6951(99)03227-1].