Non-Monotonic Dispersion of Surface Waves in Magnetic Fluids
  The dispersion relation of surface waves of a magnetic fluid in a magnetic field is studied experimentally. We verify the theoretically predicted existence of a non-monotonic dispersion relation. In particular, we demonstrate the existence of two different wave numbers occuring at the same frequency in an annular geometry. 
Thomas Mahr, Ingo Rehberg (Uni Magdeburg, Germany), Alexander Groisman   (Weizmann Institut, Israel)