Microwave properties and surface topography of soft magnetic films with high resistivity

A single layer of CoFeB and a multilayer of CoFeB--MgO films are prepared by means of DC/RF magnetron sputter deposition. The excellent microwave properties and high electrical resistivity are simultaneously achieved in the discontinuous multilayer structure of Co_44Fe_44B_12(0.7 nm)/MgO(0.4 nm)_40 film. This film has a high permeability (\mu \prime ) (larger than 100 below 2.1 GHz), a high magnetic loss (\mu'') (larger than 100 in a range from 1.5 to 3.3 GHz), a resistivity of 3.3\times 10^3\mu \Omega \cdot cm, a saturation magnetization of 1.2 T, and an in-plane uniaxial anisotropy field of 5.5 kA/m. The microstructure and the surface topography of the film are also analysed. The relatively large surface roughness for the discontinuous film is responsible for the wide frequency band of magnetic loss. The magnetic loss and the high resistivity indicate that the discontinuous CoFeB--MgO multilayer film has potential applications in microwave absorbers and electromagnetic interference (EMI) shielding materials in a GHz frequency range.