  Magnetization dynamics in an artificial square spin-ice lattice made of
Ni80Fe20 with magnetic field applied in the lattice plane is investigated by
broadband ferromagnetic resonance spectroscopy. The experimentally observed
dispersion shows a rich spectrum of modes corresponding to different
magnetization states. These magnetization states are determined by exchange and
dipolar interaction between individual islands, as is confirmed by a
semianalytical model. In the low field regime below 400 Oe a hysteretic
behavior in the mode spectrum is found. Micromagnetic simulations reveal that
the origin of the observed spectra is due to the initialization of different
magnetization states of individual nanomagnets. Our results indicate that it
might be possible to determine the spin-ice state by resonance experiments and
are a first step towards the understanding of artificial geometrically
frustrated magnetic systems in the high-frequency regime.
