  Ferromagnetic filaments in a rotating magnetic field are studied both
numerically and experimentally. The filaments are made from micron-sized
ferromagnetic particles linked with DNA strands. It is found that at low
frequencies of the rotating field a filament rotates synchronously with the
field and beyond a critical frequency it undergoes a transition to a three
dimensional regime. In this regime the tips of the filament rotate
synchronously with the field on circular trajectories in the plane parallel to
the plane of the rotating field. The characteristics of this motion found
numerically match the experimental data and allow us to obtain the physical
properties of such filaments. We also discuss the differences in behaviour
between magnetic rods and filaments and the applicability of filaments in
mixing.
