Soft Magnetic And High Frequency Properties Of FeCoSiO/SiO₂ Nanogranular Multilayer Films

With the integration and miniaturization of electro-magnetic devices, the soft magnetic films with high saturation magnetization 4Ï€Ms, high electrical resistivity p, low coercive force Hc (therefore, high permeability μ), attracted more and more attention. Among them, multilayer nanogranular film, composited of magnetic metal and nonmagnetic insulator matrix, is one of the most interesting films because this kind of films takes the advantages of metal-insulator nanogranular films and nanomultilayer films, satisfying the above requirements for soft magnetic films.In this work, Series of FeCoSiO / SiO₂ multilayer nanogranular film was fabricated by magnetron sputtering. The structures and the compositional ratio of the FeCo of the samples were characterized by X-ray Diffraction(XRD) and Energy Dispersive X-ray(EDX) while the electrical and magnetic properties were systematically studied through vibrating sample magnetometer(VSM) and conventional four-probe method. Permeability was measured using a permeameter with one port strip transmission line at frequency range from 500MHz to 5GHz. The main contents are briefly introduced as follows:1. A series of FeCoSiO/SiO₂ multilayer nanogranular film were successfully fabricated by magnetron sputtering. The results of XRD show that SiO₂ is amorphous while FeCo particles exhibit body-centered cubic (bcc) crystal structures with the crystallite size under 10 nanometer. The crystallite size increases with the thickness of FeCoSiO nanogranular layer for the samples with SiO₂ layer thickness of 1nm. SXRD results show that the multilayer structure becomes better with the increasing of the SiO₂ layer thickness or the FeCoSiO layer thickness as the interface getting more smooth.2. The soft magnetic properties become better with the decreasing of the SiO₂ layer thickness. The samples with t(SiO₂) < 3nm exhibit in-plane uniaxial anisotropy due to the exchange coupling between the FeCoSiO layers. On the other hand, with the increasing of the insulator layer thickness, electrical resistivity increaseswhile saturation magnetization decreases. Meanwhile, coercive force increases due to the weakening of interlayer exchange coupleing. When t(SiC>2) > 3nm, the soft magnetic properties deteriorate.3. The complex permeabilities of multilayer nanogranular films with the different SiCh layer thickness of 0.5nm and lnm but the same FeCoSiO layer thickness (5nm) were measured from 500MHz to 5GHz. The FMR frequencies occur at 2.2GHz and 2.6GHz respectively, the real part and imaginary part of complex permeabilities are 120 and 60 for the sample with t(SiO2) = 0.5nm while 50 and 25 for the sample with t(SiC>2) = lnm, implying that exchange coupling between granular layers increases the complex permeabilities, and the films are promising for high frequency applications.4. The optimized multilayer nanogranular film (appropriate FeCoSiO and SiO2 layer thickness, i. e., t(FeCoSiO) = 5 nm, t(SiC>2) = 0.5 nm) shows good uniaxial magnetic anisotropy, saturation magnetization 47iMs = 9.95 kGs, coercive force He = 2.3 Oe, anisotropy field Hk = 50 Oe, FMR/= 2.2GHz5. When fixing the thickness of SiC>2 layer at lnm and increasing the thickness of FeCoSiO layers, the electrical resistivity p of the films decreases while the saturation magnetization 4rcMs increase, and both coercive force He and anisotropy field Hk keep constant.