What ARE Magnetic Bubble Domains?

Magnetic bubble domains exist in magnetic garnets and other materials that can be made into thin films whose magnetization is perpendicular to the surface. As shown in the figure below, in its natural "demagnetized" state, the film breaks up into long serpentine domains of alternating magnetic orientation, either into or out of the film plane, so that the net magnetization is zero. When a magnetic field is applied perpendicular to the film, one set of serpentine domains grows and the other shrinks into small round domains that are easily moved. The Faraday effect allows us to see these microscopic domains with polarized light, under which the round domains look like "bubbles", hence the name.