Improvement of soft magnetic properties of Fe–Co–B underlayer with large saturation magnetization by Ni–Fe–O seedlayers

Abstract

Fe–Co–B layers were investigated for a soft magnetic underlayer with large saturation magnetization 4πMs in perpendicular magnetic recording media. The addition of B was examined to see the decrease of in-plane coercivity Hc∥ in the Fe–Co layer. The surface morphology of the films observed by atomic force microscopy revealed that each of the grains in the Fe–Co–B layer seemed to be separate. These separate grains seemed to cause the suppression of exchange coupling between grains in polycrystalline ferromagnetic films. A Ni–Fe–O seed layer was effective in reducing the grain size of the Fe–Co–B layer and inducing high uniaxial in-plane magnetic anisotropy. The grain size of the Fe–Co–B layer deposited on the Ni–Fe–O seedlayer was about 73% that of a Fe–Co–B single layer. The magnetic anisotropy field Hk was as high as 255 Oe in the Fe–Co–B/Ni–Fe–O layer. Furthermore, the relative permeability μr of Fe–Co–B/Ni–Fe–O was about 200 and the resonance frequency fr was above 3 GHz. Insertion of a Ni–Fe–O interlayer was also effective in decreasing Hc∥ of the Fe–Co–B layer. A [Fe–Co–B/Ni–Fe–O]8 multilayer exhibited Hc∥ as low as 11 Oe in the hard axis of magnetization.