Response of a ferrofluid to traveling-stripe forcing.

We observe the dynamics of waves propagating on the surface of a ferrofluid under the influence of a spatially and temporally modulated field. In particular, we excite plane waves by applying a traveling lamellar modulation of the magnetization. By means of this external driving, both the wavelength and the propagation velocity of the waves can be controlled. The amplitude of the excited waves exhibits a resonance phenomenon similar to that of a forced harmonic oscillator. Its analysis reveals the dispersion relation of the free surface waves, from which the critical magnetic field for the onset of the Rosensweig instability can be extrapolated.