Question

Why there is domain wall thickness (typical 40 nm) between magnetic domains for a typical ferromagnetic material? ( for ferroelectric materials, there is no thickness for the domain boundary)

Answer 1

domain wall split two domains wherein magnetic dipoles points in different direction. The exchange interaction which creates the magnetization tends to align nearby dipoles so that they point in same direction.forcing adjacent dipoles to point in different directions require energy. Therefore a domain wall requires extra energy which is proportional to area of the wall.

so there are two energies are in picture.one is exchange energy which tries to increase width as much as possible.other is anisotropic energy which makes the wall as thin as possible.competition between exchange energy and anisotropy energy makes the width of domain wall finite.