THEORY OF A DYNAMICAL COERCIVE FORCE IN MAGNETS WITH MICROSCOPIC DEFECTS

The peculiar dynamical drag force F(c) can arise at the domain wall mo tion in a magnet with microscopic defects, such that cannot pin domain wall in the usual way. The value of F(c) is finite as the domain wall velocity upsilon goes to zero. The reason for such behavior is the em ission of bending waves of the domain wall. The calculations for diffe rent types of impurities as well as for both bulk ferromagnet and ultr athin magnetic layers are presented. In 3D ferromagnet the F(c) value is finite at upsilon --> 0 and this is interpreted as the dynamical co ercive force. In 2D case (e.g. in monolayer films) the drag force dive rges at small velocities as F(c) is-proportional-to 1/upsilon1/2 . In the film with a finite thickness and very small upsilon the force has a viscous character: F(c) is-proportional-to upsilon.