  Dynamics of flexible ferromagnetic filaments in an external magnetic field is
considered. We report the existence of a buckling instability of the
ferromagnetic filament at the magnetic field reversion, which leads to the
formation of a metastable loop. Its relaxation through three dimensional
transformation of the configurations is observed experimentally and confirmed
by numerical simulations. Bending modulus of the flexible ferromagnetic
filaments synthesized by linking micron size core-shell ferromagnetic particles
with DNA fragments is estimated by comparison of the parameters of the loops
observed in the experiment with theoretical calculations. Formation of the loop
and its relaxation are characterized by the numerically calculated writhe
number. The relaxation time of the loop allows us to estimate the hydrodynamic
drag of the filament.
