Soft Magnetic And High Frequency Properties Of FeCoSi/native-oxide Multilayer Films And FeCoB-O Nanogranular Films

Recent develops in electronic devices have led to an urgent need for further miniaturization and higher frequency operation of magnetic devices. One of the important properties of magnetic materials required for such applications is high permeability at high frequency, which is limited by eddy current loss and ferromagnetic resonance (FMR) frequency. To produce a soft magnetic thin film with excellentμ-f response, the film must have not only high electrical resistivity (p), but also large saturation magnetization (4πMs) and appropriate in-plane uniaxial anisotropy field (Hk) so as to effectively suppress eddy current loss and to possess high ferromagnetic resonance (FMR) frequency fr. Metal-insulator granular films have demonstrated good soft magnetic properties and high resistivities due to the exchange coupling between granules, and were extensively studied. However, it is only at metal volume fractions approaching percolation threshold that soft magnetic properties are obtained, which leads to the decrease of effective saturation magnetization in proportion to the increase of oxide fraction. Recently, a newly introduced discontinuous metal/native-oxide multilayer structure, which has been exposed in situ to oxygen to form oxide interface layers, can significantly improve the soft magnetic properties and high frequency characteristics, because the oxide layers exhibit room temperature ferromagnetism. The excellent soft magnetic properties imply that the FeCoSi/native-oxide multilayer films are promising for application in high frequency range.In this dissertation, we deposited FeCoSi/native-oxide multilayer films by DC magnetron sputtering and exposure to oxygen in a sequence on glass and Kapton substrates, respectively. Soft magnetic granular films FeCoB-O system was fabricated by reactive radio-frequency sputtering using an Ar+O2 atmosphere on glass substrates. The microstructure, magnetic and electric properties, high frequency characteristics were investigated systematically by X-ray diffraction (XRD), high resolution transmission electronic microscope (HRTEM), vibrating sample magnetometer (VSM), conventional four-probe method, shorted microstrip transmission-line perturbation method.For FeCoSi/native-oxide multilayer films, the main contents are as follows:(1) FeCoSi/native-oxide multilayer films were fabricated by DC magnetron sputtering and exposure to oxygen in a sequence on glass and Kapton substrates, respectively(2) It can be seen from a bright-field HRTEM image shows that the FeCoSi metal layer is discontinuous as the thickness of FeCoSi metal layer d≤20 A. The results of XRD show that the films deposited on glass substrates have evident crystalline structure, while the films deposited on Kapton substrates is amorphous and nanocrystalline mixture structure.(3) According to the theoretical estimation, we found that FeCoSi/native-oxide multilayer films possess room-temperature ferromagnetism. A weak peak located around 35°, which can be ascribed to Fe3O4 oxide, was observed in each XRD pattern of the multilayer film deposited on Kapton substrates. The results imply that the native-oxide fabricate in this matter is Fe3O4 oxide.(4) VSM was employed to investigate the static magnetic properties of FeCoSi/native-oxide multilayer films. Research results show that both in situ oxidation time t and FeCoSi layer thickness d have a great influence on the magnetic properties of the films, and the larger in-plane anisotropy field Hk than that of metal-insulator granular films has been induced by the applied magnetic field. Since the native-oxide formed in this manner is magnetic, the exchange coupling between the metal particles through the oxide in this discontinuous multilayer film is expected to be stronger than that of granular films in which the oxide is nonmagnetic. In this case, the magneto-crystalline anisotropy of individual particle has been better averaged by the exchange coupling in comparison with the case of granular films, therefore the larger in-plane anisotropy field Hk than that of granular films has been induced by the applied magnetic field. (5) It is found that the films deposited on flexible substrates have relatively higher Hc and p than that on rigid substrates, which can be ascribed to the surface roughness, the defects and the stress of the substrates.(6) The excellent soft magnetic properties that obtained in FeCoSi/native-oxide multilayer films can be explained by Herzer's random anisotropy model.(7) FeCoSi/native-oxide multilayer film possesses excellent high-frequency characteristics. For films were deposited on glass substrates, a typical sample with the FeCoSi layer thickness d= 20 A, the saturation magnetization 4nMs=13.9 kG, a good in-plane uniaxial anisotropy field Hk= 130 Oe, andμ'is more than 100 and flattens almost to 2.5 GHz, then gradually decreases with frequency, while the imaginary partμ" is less than 50 for f< 2.5 GHz then increases rapidly to a maximum at f= 3.9 GHz, which can be ascribed to the FMR. For the film deposited on Kapton substrates, as d ranging from 1 to 4 nm, the ferromagnetic resonance frequency increase continuously from 2.9 to 7.9 GHz. Therefore, the high frequency performance of the [FeCoSi (d)/native-oxide]5o discontinuous films can be easily optimized by adjusting d to satisfy the demands of different applications.(8) Using Landau-Lifshitz equation, the experimentalμ-f curve was fitted quite well. This implies that the magnetization process in FeCoSi/native-oxide multilayer film should be rotational.For the FeCoB-O granular films, the main contents are as follows:(1) FeCoB-O granular films were successfully prepared by RF magnetron sputtering on glass substrates.(2) XRD measurement results of FeCoB thin films indicate that the grain size decreases as increasing the fraction of B element in FeCo alloy. The excellent soft magnetic properties are obtained as the content of B is equal to 19.9 at%. (3) The change of sputtering power has little effect on the magnetic properties of FeCoB-O granular films as P< 80 W, while the soft magnetic properties become worse as P> 80 W. This may be due to increasing the residual stress in thin film as the sputtering power increases, which result in the sputtering rate increases, so B element can not be completely oxidized.(4) The good soft magnetic properties can be obtained only for a certain appropriate region of film thickness. For our samples, the optimum film thickness is around 150 nm. This can be explained as follows:for ultrathin films, it is likely that in the interfacial phase there exists a large strain field, which is strong enough to impede the magnetization process and thus brings out a large coercivity; for thick films, both the larger grain size and demagnetization field effect can cause the deterioration of soft magnetic properties.(5) We investigated the influence of oxygen partial pressure (Ro2) on the soft magnetic properties in FeCoB-O granular films. Research results show that the Ro2 range of obtaining excellent soft magnetic properties under 80 W is wider than that under 40 W, which have important reference and actural using value.(6) For a typical sample:Using P= 40 W, the FeCoB-O granular film with Ro2=1.3%, the resistivity p= 3638μΩ·cm, andμ'is more than 146 and flattens almost to 2 GHz, the resonance frequency fr=3.0 GHz; Using P = 80 W, the FeCoB-O granular film with Ro2= 0.8%. An ultrahigh ferromagnetic resonance frequency fr up to 4.74 GHz.