  We study dynamics of tunnel Josephson junctions with a thin ferromagnetic
layer F [superconductor-insulator-ferromagnet-superconductor (SIFS) junctions].
On the basis of derived equations relating the superconducting phase and
magnetic moment to each other we analyze collective excitations in the system
and find a new mode which is a hybrid of plasmalike and spin waves. The latter
are coupled together in a broad range of parameters characterizing the system.
Using the solution describing the collective modes we demonstrate that besides
the Fiske steps new peaks appear on the I-V characteristics due to oscillations
of the magnetic moment M in the ferromagnetic layer. Thus, by measuring the I-V
curve of the SIFS junctions, one can extract information about the spectrum of
spin excitations in the ferromagnet F.
