  We develop a macroscopic description of the current-induced torque due to
spin transfer in a layered system consisting of two ferromagnetic layers
separated by a nonmagnetic layer. The description is based on i) the classical
spin diffusion equations for the distribution functions used in the theory of
CPP-GMR, ii) the relevant boundary conditions for the longitudinal and
transverse components of the spin current in the situation of quasi-interfacial
absorption of the transverse components in a magnetic layer. The torque is
expressed as a function of the usual parameters derived from CPP-GMR
experiments and two additional parameters involved in the transverse boundary
conditions. Our model is used to describe qualitatively normal and inverse
switching phenomena studied in recent experiments. We also present a structure
for which we predict only states of steady precession above a certain critical
current. We finally discuss the limits of a small angle between magnetic
moments of the ferromagnetic layers and of vanishing imaginary part of the
mixing conductance.
