  We study theoretically the effective spin Hall properties of a composite
consisting of two materials with and without spin-orbit (SO) coupling. In
particular, we assume that SO material represents a system of grains in a
matrix with no SO. We calculate the effective spin Hall angle and the effective
spin diffusion length of the mixture. Our main qualitative finding is that,
when the bare spin diffusion length is much smaller than the radius of the
grain, the effective spin diffusion length is strongly enhanced, well beyond
the "geometrical" factor. The physical origin of this additional enhancement is
that, with small diffusion length, the spin current mostly flows around the
grain without suffering much loss. We also demonstrate that the voltage,
created by a spin current, is sensitive to a very weak magnetic field directed
along the spin current, and even reverses sign in a certain domain of fields.
The origin of this sensitivity is that the spin precession, caused by magnetic
field, takes place outside the grains where SO is absent.
