  In magnetoplasmonics, it is possible to tailor the magneto-optical properties
of nanostructures by exciting surface plasmon polaritons (SPPs). Thus far,
magnetoplasmonic effects have been considered static. Here, we describe
ultrafast manifestations of magnetoplasmonics by observing the non-trivial
evolution of the transverse magneto-optic Kerr effect within 45-fs pulses
reflected from an iron-based magnetoplasmonic crystal. The effect occurs for
resonant SPP excitations, displays opposite time derivative signs for different
slopes of the resonance, and is explained with the magnetization-dependent
dispersion relation of SPPs.
