Effective permeability and imaging of multilayered soft films for perpendicular recording media

Abstract

Summary form only given, as follows. A finite thickness soft magnetic film has an effective permeability for an equivalent semi-infinite layer that is lower than the bulk (infinite film) material permeability. Thinner soft magnetic film layers show significant variation of effective permeability with wave number or magnetic recording density. When the magnetic material is divided between several equally thick soft layers, the effective permeability of the composite films is reduced, particularly at lower wave numbers This effect increases with more layers in the composite soft magnetic film. A thin exchange-decoupling layer (3 nm thick) separates the magnetic layers. Multilayered soft magnetic films are one approach to reduce the domain noise in the soft magnetic imaging layer used in perpendicular magnetic recording media. However, the disk designer must be careful with the number and thickness of the layers in the composite film to avoid significant reduction in the imaging capability of these films. This imaging is important in writing perpendicular media since the total write field is that produced by the magnetic recording head plus the image of the head field from the soft magnetic underlayer. This imaging factor determines how strong the imaging field will be in enhancing the applied field from the head to write on the perpendicular magnetic media. At lower recording densities, (e.g. in servo writing) the imaging factor can be significantly reduced in soft magnetic multilayered films making writing high coercivity media more difficult.