Micromagnetic modeling of soft underlayer magnetization processes and fields in perpendicular magnetic recording

Micromagnetic processes in the soft underlayer (SUL) of a perpendicular magnetic recording medium are studied by dynamic simulation of the Landau-Lifshitz-Langevin equations in order to obtain estimates of the write field as a function of thickness, magnetization, and anisotropy of the SUL. In this way, the SUL thickness requirements are examined from the point of view of supporting a given input magnetic flux. It is shown that insufficient SUL thickness is characterized by large surface charges at the bottom of the SUL, leading to significant external flux leakage. For thicker SULs, the magnetic flux is conducted interior to the SUL and can involve spin-wave excitations with three-dimensional vortices. The wavelength of the spin waves is related to the vortex size and, therefore, to the exchange length of the SUL. The calculations also demonstrate the importance of proper scaling of the solid angle subtended by the write pole as seen by the data layer and SUL when the track density is increased.