Berry-phase interpretation of thin-film micromagnetism

Magnetic flux densities (B-fields) and field intensities (H-fields) in thin films are investigated from the viewpoints of Berry phase and topological Hall effect. The well-known origin of the topological Hall effect is an emergent B-field originating from the Berry phase of conduction electrons, but Maxwell’s equations predict the relevant perpendicular component Bz to be zero. This paradox is solved by treating the electrons as point-like objects in Lorentz cavities. These cavities can also be used to interpret magnetization measurements in the present and other contexts, but structural and magnetic inhomogeneities lead to major modifications of the Lorentz-hole picture.