  We demonstrate the general principle which states that a dissipationless spin
current flows between two coupled ferromagnets if their magnetic orders are
misaligned. This principle applies regardless the two ferromagnets are metallic
or insulating, and also generally applies to bulk magnetic insulators. On a
phenomenological level, this principle is analogous to Josephson effect, and
yields a dissipationless spin current that is independent from scattering. The
microscopic mechanisms for the dissipationless spin current depend on the
systems, which are elaborated in details. A uniform, static magnetic field is
further proposed to be an efficient handle to create the misaligned
configuration and stabilize the dissipationless spin current.
