High-moment FeCo-IrMn exchange-coupled soft underlayers for perpendicular media

Ferromagnetic/antiferromagnetic coupled multilayers provide a unique solution to several problems found with soft underlayers for perpendicular media. Properly designed, they demonstrate outstanding characteristics: a unique single-domain remanent direction, reversible hysteretic behavior over a wide angular range, optimum permeability μ∼100, and high saturation flux density 4πM_s. Initially, FeTaN-IrMn multilayers were prepared with these characteristics and a target ferromagnetic thickness of 200 nm, but 19 total layers were required and they were thermally unstable. These problems were first overcome by replacing FeTaN with high-anisotropy Fe₁₀Co₉₀, although 4πM_s was reduced from ∼19 to ∼16 kG. Further work, however, revealed that soft anisotropic Fe₆₅Co₃₅ films with an induced anisotropy field H_k of 30 Oe and 4πM_s∼23 kG could be successfully produced on Cu, NiFe, Ru, Ta-Cu, Ta-NiFe, or Cu-IrMn underlayers. An optimized structure was found to be glass/Cu(20 nm)/IrMn(10 nm)/[Fe₆₅Co₃₅(50 nm)/IrMn(10 nm)]₄/Fe₆₅Co₃₅(25 nm). Furthermore, postannealing will allow the number of layers to be reduced from 11 to 7.