  Spin dephasing by the Dyakonov-Perel mechanism in metallic films deposited on
insulating substrates is revealed, and quantitatively examined by means of
density functional calculations combined with a kinetic equation. The
surface-to-substrate asymmetry, probed by the metal wave functions in thin
films, is found to produce strong spin-orbit fields and a fast Larmor
precession, giving a dominant contribution to spin decay over the Elliott-Yafet
spin relaxation up to a thickness of 70 nm. The spin dephasing is oscillatory
in time with a rapid (sub-picosecond) initial decay. However, parts of the
Fermi surface act as spin traps, causing a persistent tail signal lasting 1000
times longer than the initial decay time. It is also found that the decay
depends on the direction of the initial spin polarization, resulting in a
spin-dephasing anisotropy of 200% in the examined cases.
