  Spin pumping is the emission of a spin current by a magnetization dynamics
while spin transfer stands for the excitation of magnetization by spin
currents. Using Onsager's reciprocity relations we prove that spin pumping and
spin-transfer torques are two fundamentally equivalent dynamic processes in
magnetic structures with itinerant electrons. We review the theory of the
coupled motion of the magnetization order parameter and electron for textured
bulk ferromagnets (e.g. containing domain walls) and heterostructures (such as
spin valves). We present first-principles calculations for the
material-dependent damping parameters of magnetic alloys. Theoretical and
experimental results agree in general well.
