  The spin- and energy-dependent interface reflectivity of a ferromagnetic (FM)
film in contact with a nonmagnetic (NM) film is calculated using a
first-principles transport method and incorporated into the superdiffusive spin
transport model to study the femtosecond laser-induced ultrafast
demagnetization of Fe|NM and Ni|NM (NM= Au, Al & Pt) bilayers. By comparing the
calculated demagnetization with transparent and real interfaces, we demonstrate
that the spin-dependent reflection of hot electrons has a noticeable influence
on the ultrafast demagnetization and the associated terahertz electromagnetic
radiation. In particular, a spin filtering effect is found at the Fe|NM
interface that increases the spin current injected into the NM metal, which
enhances both the resulting demagnetization and the resulting THz emission.
This suggests that the THz radiation can be optimized by tailoring the
interface, indicating a very large tunability.
