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.

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