Study on the microstructure, soft magnetic and high frequency properties of obliquely sputtered [Fe65Co35/ZnO]50 multilayer thin films

•Good soft magnetic and high frequency characteristics are obtained in the [Fe65Co35/ZnO]50 multilayer thin films.•ZnO layers take on increasing crystallization intensity as ZnO layer thickness t increases.•The resonance frequency fr is almost invariable with increasing t and around 4 GHz. And the initial permeabilty μi can be adjusted from 48 to 136 by decreasing t from 3.6 nm to 1 nm.•[Fe65Co35/ZnO]50 multilayers have both high fr and high electric resistivity ρ (up to 12.6 mΩ cm).•The multilayers exhibits a high real permeability μ‘ (> 90) and a low imaginary component μ“ (< 1.55) when f=2 GHz, and the quality factor Q value is more than 150 at f=2 GHz, which implys that [Fe65Co35/ZnO]50 multilayer thin films are good candidate materials for high frequency applications on electromagnetic devices.

The effect of ZnO layer thickness t on the structure, static magnetic properties and high frequency properties is systemically studied for [Fe65Co35/ZnO(t)]50 multilayer films. The gradually increasing crystallinity of ZnO layer with increasing t is observed in the XRD patterns. The VSM measurements show good soft magnetic properties are obtained in these multilayer films, even though the ZnO layer is thick (up to 3.6 nm). For the typical sample with t=3.6 nm, an obvious in-plane anisotropy field and comparatively small coercivity are achieved, and high electric resistivity up to 12.6 mΩ cm is obtained. The measurement of permeability spectra indicates that the multilayers have an invariable ferromagnetic resonance frequency fr about 4.0 GHz; however, the initial permeability μi can be adjusted from 48 to 136 by decreasing t from 3.6 nm to 1.0 nm. The phenomenon that fr is almost invariable is the result of the decreasing saturation magnetization 4πMs and the increasing dynamic anisotropy field Hk-dyn with increasing t. For our samples, the real part (μ‘2G) of complex permeability at f=2.0 GHz remains a high value beyond 90, while the imaginary part (μ“2G) keeps a low value below 1.55, indicating that these multilayer films are promising for application in high frequency range.