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Domain walls
Time:05/25/2011

Domain walls
The boundaries between adjacent domains in bulk  ferromagnetic materials are called domain walls, or Bloch walls. They are about four-millionths of an inch in thickness, and across this distance the direction of magnetization changes usually by either 180 or 90.
The width of domain walls is again determined by a balance between competing energy contributions. The exchange energy is optimized if adjacent magnetic moments are parallel, or as close to parallel as possible, to each other. This favors wide walls, so that the change in angle of the moments between adjacent planes of atoms can be as small as possible. However the magnetocrystalline anisotropy is optimized if the moments are aligned as closely as possible to the easy axes This favors narrow walls with a sharp transition between the domains, so that few moments have unfavorable crystalline alignment in the transition region. In practice a compromise is reached which minimizes the total energy across the boundary.
The most energetically favorable types of domain wall are those which do not produce magnetic poles within the material, and therefore don’t introduce demagnetizing fields. One such wall is the twist boundary, illustrated for a 180 boundary. Here the magnetization perpendicular to the boundary does not change across the wall; therefore no magnetic poles or demagnetizing fieldsarise. Also stable are 90 tilt boundaries.The magnetic moments rotate through the wall in such a way that they make a constant angle of 45 with both the wall normal and the surface.Another kind of domain wall, called a N´eel wall, occurs in thin films of magnetic materials. In N´eel walls the spins rotate around an axis normal to the surface of the film, rather than around an axis normal to the domain wall. The spin rotation in a N´eel wall is shown in plan . Neel walls are energetically favorable in thin films because free poles are formed on the wall surface, rather than the film surface, causing a reduction in magnetostatic energy.

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