The Gd(0001) surface is ideally suited in order to demonstrate spin-polarized vacuum tunneling since it exhibits a dz2-like surface state that is exchange split into two parts with opposite spin polarization. Both, the majority as well as the minority spin parts appear as peaks in the tunneling spectra. The use of ferromagnetic probe tips leads to magnetic field dependent asymmetries in the differential tunneling conductivity at bias voltages which correspond to the energies of the spin components. By mapping the asymmetry parameter we can image the magnetic domain structure of the sample. The spin polarization of the differential tunneling conductivity is found to be in excellent agreement with (inverse) photoemission data. Prolonged exposure to the residual gas being present in the vacuum system leads to a significant decrease of spin polarization.

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