Spin-Polarized Positronium Time-of-Flight Spectroscopy for Probing Spin-Polarized Surface Electronic States

The energy spectrum of positronium atoms generated at_solid surface reflects the electron density of states (DOS) associated solely with the first surface layer.Using spin-polarized positrons, the spindependent surface DOS can be studied.For this purpose, we have developed_spin-polarized positronium time-of-flight spectroscopy apparatus based on_22 Na positron source and an electrostatic beam transportation system, which enables the sampling of topmost surface electrons around the Γ point and near the Fermi level.We applied this technique to nonmagnetic Pt(111) and W(001), ferromagnetic Ni (111), Co(0001) and graphene on them, Co2FeGa0 .5Ge0 .5 (CFGG) and Co2MnSi (CMS).The results showed that the electrons of Ni(111) and Co(0001) surfaces have characteristic negative spin polarizations, while these spin polarizations vanished upon graphene deposition, suggesting that the spin polarizations of graphene on Ni(111) and Co(0001) were mainly induced at the Dirac points that were out of range in the present measurement.The CFGG and CMS surfaces also exhibited only weak spin polarizations suggesting that the half-metallicity expected for these bulk states was not maintained at the surfaces