Scanning tunneling microscopy experiments reveal that the degree of spin polarization of electrons tunneling from Ni into a semiconductor increases with decreasing potential barrier thickness. The results show that the highly polarized $3d$ bands as well as the low-polarized $4sp$ bands contribute to the tunneling current and that the ratio of their tunneling probabilities depends on the potential barrier thickness and height. Furthermore, the tunneling potential barrier for the $3d$-like levels is estimated to be $\sim$1 eV higher than for the $4sp$ contribution.

• Received 8 March 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.513