  In metallic ferromagnets, an electric current is accompanied by a flux of
angula r momentum, also called spin current. In multilayers, spatial variations
of the spin current correspond to drive torques exerted on a magnetic layer.
These torq ues result in spin precession above a certain current threshold. The
usual kind of spin current is associated with translation of the spin-up and
spin-down Ferm i surfaces in momentum space. We discuss a different kind of
spin current, assoc iated with expansion and contraction of the Fermi surfaces.
It is more nonlocal in nature, and may exist even in locations where the
electrical current density is zero. It is larger than the usual spin current,
in a ratio of 10 or 100, and is dominant in most cases. The new spin current is
proportional to the differenc e Delta-mu = 0.001 eV between spin-up and
spin-down Fermi levels, averaged over the entire Fermi surface. Conduction
processes, spin relaxation, and spin-wave emission in the multilayer can be
described by an equivalent electrical circuit resembling an unbalanced dc
Wheatstone bridge. And Delta-mu corresponds to the output voltage of the
bridge.
