  We examine a modified drift-diffusion formalism to describe spin transport
near an ultrathin magnet whose thickness is similar to or less than the spin
dephasing length. Most of the previous theories on spin torque assume the
transverse component of a injected spin current dephases perfectly thus are
fully absorbed into the ferromagnet. However, in the state-of-art multilayer
systems under consideration of recent studies, the thicknesses of ferromagnets
are on the order of or less than a nanometer, thus one cannot safely assume the
spin dephasing to be perfect. To describe the effects of a finite dephasing
rate, we adopt the concept of transmitted mixing conductance, whose application
to the drift-diffusion formalism has been limited. For a concise description of
physical consequences, we introduce an effective spin transparency.
Interestingly, for an ultrathin magnet with a finite dephasing rate, the spin
transparency can be even enhanced and there arises a non-negligible field-like
spin-orbit torque even in the absence of the imaginary part of the spin mixing
conductance. The effective spin transparency provides a simple extension of the
drift-diffusion formalism, which is accessible to experimentalists analyzing
their results.
