Anisotropy of spin-orbit interaction in W(110) by spin-polarized two-electron spectroscopy

The energy distributions I(E-1,E-2) of correlated electron pairs, excited from a single crystal of W(110) by 29 eV spin-polarized electrons, have been measured for two azimuthal orientations of the crystal: (a) the [100] direction is perpendicular to the scattering plane containing two-electron detectors and normal to the sample surface; (b) the sample was rotated by 90 degrees around the surface normal such that the [110] direction was perpendicular to the scattering plane. The polarization of the incident beam was chosen to be perpendicular to the scattering plane. The energy distributions were measured for spin-up I+(E-1,E-2) and spin-down I-(E-1,E-2) polarizations of the primary electrons and analyzed in terms of the spin asymmetry A=(I+-I-)/(I++I-). The asymmetry was visualized by extracting the energy sharing distribution for particular total energy of pairs from the two-dimensional distribution of the electron pairs. The asymmetry is nonzero due to a spin-orbit coupling and is different for the two azimuthal orientations of the sample. These results are discussed in terms of anisotropy of the spin-orbit coupling and anisotropy of the orbital moment in W(110). The relevance of these results to the magnetic anisotropy of thin Fe films on W(110) is discussed.