Magnetization waves in split-ring-resonator arrays: Evidence for retardation effects

We excite low-symmetry planar arrays of nanoscale magnetic split-ring resonators oscillating at around 200 THz frequency under oblique incidence of light. Due to the in-plane coupling of split-ring resonators, classical magnetic-dipole waves result in the plane. We measure the dispersion relation of antiferromagnetic and ferromagnetic modes, revealing backward waves and a wave-vector-dependent damping. The latter provides evidence for retardation effects, which play no role in the quantum-mechanical counterpart of classical magnetization waves, i.e., magnons. Our experiments are in good agreement with both simple heuristic modeling and microscopic theory.