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Fe 100- x  Co  x   ( x =30-40) alloys have the highest saturation magnetization, 4 πMs  ≥ 24 kGs. Therefore, FeCo thin flms have been widely used in microwave magnetic devices. However, the as-deposited FeCo films have a large coercivity, which is attributed to the large saturation magnetostriction and high magneto-crystalline anisotropy. On the basis of maintaining high saturation magnetization, adding an appropriate underlayer is a simple and effective method to reduce the coercivity of the films and facilitate the magnetic field-induced in-plane uniaxial magnetic anisotropy. Since these kinds of films are used in a high-frequency environment, the eddy current loss in GHz band must be considered. For a certain film material, the thinner the film is, the lower the eddy current loss is. However, for current research works, the thickness of ferromagnetic layer is generally tens of nanometers or even hundreds of nanometers, which will not help to suppress the eddy current loss at high frequency. In the present study, to obtain FeCo films with good soft magnetic properties and excellent high-frequency characteristics, Fe 65 Co 35  alloy films with a thickness of 13 nm and different underlayers (Cu, Co and Ni 80 Fe 20 ) were prepared by magnetron sputtering. The effects of different underlayer materials and that of different NiFe underlayer thicknesses on the structures and magnetic properties of FeCo films were studied. The results showed that introduction of underlayers could increase the in-plane uniaxial magnetic anisotropy of films, and the soft magnetic properties of films were significantly improved. The reason for obtaining good soft magnetic properties was attributed to the grains refinement, the dipolar interaction between layers, and the reduction of surface roughness. For different underlayer materials with the same thickness, NiFe underlayer could obviously improve the soft magnetic properties of FeCo films:the covercivity of easy axis was 23 Oe. By changing the thickness of NiFe underlayer, the dynamic magnetic properties of films could be adjusted. The resonance frequency changed from 3.13 GHz for NiFe(1 nm)/FeCo(13 nm) film to 2.78 GHz for NiFe(9.3 nm)/FeCo(13 nm) film. For all NiFe/FeCo bilayer films, real part of the permeability  μ ' at low frequency had a large value of 350-450, and the damping coefficient  α  showed a small value of 0.01-0.02. In addition, the smaller film thickness could reduce eddy current loss, which promotes its application in high-frequency microwave magnetic devices.

Effects of underlayer on structures and magnetic properties of Fe<sub>65</sub>Co<sub>35</sub> alloy films