Viscous dynamics of vortices in a ferromagnetic film

We derive viscous forces for vortices in a thin-film ferromagnet. The viscous force acting on vortex i is a linear superposition F-i = -Sigma(j) (D) over cap V-ij(j), where V-j is the velocity of vortex j. Thanks to the long-range nature of vortices, the mutual drag tensor (D) over cap (ij) is comparable in magnitude to the coefficient of self-drag D-ii.