  In ferromagnetic metals, an effective electromagnetic field which couples to
conduction electron spin is induced by the sd exchange interaction. We
investigate how this effective field, spin electromagnetic field, interacts
with the ordinary electromagnetic field by deriving an effective Hamiltonian
based on the path-integral formalism. It turns out that the dominant coupling
term is the product of the electric field and the spin gauge field. This term
descrives the spin-transfer effect, as was pointed out before. The electric
field couples also to the spin electric field, but this contribution is smaller
than the spin-transfer contribution in the low frequency regime. The magnetic
field couples to the spin magnetic field, and this interaction suggests an
intriguing intrinsic mechanism of frustration in very weak metallic
ferromagnets under a uniform magnetic field. We also propose a voltage
generation mechanism due to a non-linear effect of non-monochromatic spin wave
excitation.
