Soft magnetic Co-Fe-Al-O thin films for ultrahigh frequency applications

The effect of microstructures on the electrical and magnetic properties of Co-Fe-Al-O thin films was studied using X-ray diffraction (XRD) analysis and conventional transmission electron microscopy (TEM). The results showed that excellent soft magnetic properties were associated with granular nanoscale grains of α-CoFe and α-Co(Fe) phases. The introduction of aluminium and oxygen to CoFe films led to a refinement of the granular microstructure. When the partial pressure of oxygen was less than 4%, oxide particles CoFe ₂O₄, CoAl₂O₄ or FeAl ₂O₄ were embedded in α-CoFe, α-Co(Fe) grains. A higher partial pressure of oxygen changed the crystalline oxide to amorphous oxide, resulting in the formation of smaller grains of α-CoFe and α-Co(Fe) phases. The partial pressure of oxygen caused drastic changes in the coercivity (H_c), anisotropy field (H_k), and electrical resistivity (ρ), the reason for which was discussed based on the microstructural observations and X-ray diffraction results. Co-Fe-Al-O thin films exhibit excellent high frequency characteristics: an effective permeability larger than 1000 and stability above 1 GHz.