Spiral Walls in Thin Magnetic Films

Dense concentrations of walls were previously observed to appear in very thin magnetic films with the application of an ac demagnetizing field. Other observations showed roughly circular concentric walls around barely discernible imperfections. Particularly clear powder patterns of such walls that were obtained on the thin end of a Permalloy film of tapered thickness are presented. The patterns allow the deduction of the detailed magnetization distribution which takes the form of a pair of antiparallel domains that spiral in toward the central imperfection. A mechanism for the formation of spiral walls is presented, together with confirming experimental evidence. The experiments establish the nature of the central imperfection as a region of abnormally high uniaxial anisotropy where the magnetization is constrained to lie in one direction along the easy axis while the magnetization of the surrounding regions of the film can be rotated with an applied field.