  Coupling of Josephson-phase and spin-waves is theoretically studied in a
superconductor/ferromagnetic insulator/superconductor (S/FI/S) junction.
Electromagnetic (EM) field inside the junction and the Josephson current
coupled with spin-waves in FI are calculated by combining Maxwell and
Landau-Lifshitz-Gilbert equations. In the S/FI/S junction, it is found that the
current-voltage (I-V) characteristic shows two resonant peaks. Voltages at the
resonant peaks are obtained as a function of the normal modes of EM field,
which indicates a composite excitation of the EM field and spin-waves in the
S/FI/S junction. We also examine another type of junction, in which a
nonmagnetic insulator (I) is located at one of interfaces between S and FI. In
such a S/I/FI/S junction, three resonant peaks appear in the I-V curve, since
the Josephson-phase couples to the EM field in the I layer.
