  A Datta-Das spin field-effect transistor is built of a heterostructure with a
Rashba spin-orbit interaction (SOI) at the interface (or quantum well)
separating two possibly magnetized reservoirs. The particle and spin currents
between the two reservoirs are driven by chemical potentials that are
(possibly) different for each spin direction. These currents are also tuned by
varying the strength of the SOI, which changes the amount of the rotation of
the spins of electrons crossing the heterostructure. Here we investigate the
dependence of these currents on additional Zeeman fields on the heterostructure
and on variations of the reservoir magnetizations. In contrast to the particle
current, the spin currents are not necessarily conserved; an additional spin
polarization is injected into the reservoirs. If a reservoir has a finite
(equilibrium) magnetization, then we surprisingly find that the spin current
into that reservoir can only have spins which are parallel to the reservoir
magnetization, independent of all the other fields. This spin current can be
enhanced by increasing the magnetization of the other reservoir, and can also
be tuned by the SOI and the various magnetic fields. When only one reservoir is
magnetized then the spin current into the other reservoir has arbitrary tunable
size and direction. In particular, this spin current changes as the
magnetization of the other reservoir is rotated. The optimal conditions for
accumulating spin polarization on an unpolarized reservoir are to either apply
a Zeeman field in addition to the SOI, or to polarize the other reservoir.
