Structure And Magnetic Properties OfFe-N Films Prepared By RF Reactive Magnetron Sputtering For High-frequency Applications

In recent years, with the rapid development of electronicand information industryhigher requirementsfor soft magnetic materials as the core of electronic devices and the need of miniaturization, high operation frequency and integration ofelectronic devices have been raised. Currently, research of soft magnetic materials havinga high magnetic permeability(μ),low coercivity(Hc), large saturationmagnetization(4πMs),high electrical resistivity(p)and appropriate in-plane uniaxial anisotropy field (Hk) have become an inevitabledirection ofsoft magnetic materialsdevelopment.Fe-Nferromagneticthin films, due to its excellentmechanicalproperties and goodresistance to oxidation, corrosion, abrasion andgood thermal stabilityproperties, have becomea promisingcandidate forthe head and themagnetic recordingmedium and received much attention. In addition, Fe-N films having ahigh saturationmagnetization, lowcoercivity, high initialpermeability and relativelyhigh electrical resistivity to suppress eddy current lossshow bright prospects inthe study ofthe high frequencymicrowave devices,thusimproving thesoft magneticproperties and high frequency responsecharacteristic ofFe-N thin filmsbecomes very importantand meaningful.In this dissertation, we mainly deposited polycrystalline y’-Fe4N films and amorphous or nanocrystalline Fe-N films by radio frequency reactive magnetron sputtering. The microstructure and magnetic properties of the samples were studied systematically by X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), vector network analyzer and electron paramagnetic resonance (ESR), and a series of valuable research results are obtained as follows:1. A systematic study about the effects of preparation conditions on the microstructure and magnetic properties of Fe-N films has been done. Throughthe variation of the N2flow ratio, sputtering pressure, sputtering time (film thickness) and seed layers, the optimal conditions for polycrystalline y’-Fe4N films are foundto be as follows. The substrate temperature, sputtering pressure and N2flow ratio (N2/Ar+N2) are250℃,0.5Pa and5%, respectively, and the substrate is Si (100).2. The presence of y’-Fe4N phase in the Fe-N film is very unfavorable for the in-plane uniaxial anisotropy. For the films prepared under optimal conditions for γ’-Fe4N and with an external magnetic field in the film plane simultaneously, the182nmthick sample shows a y’-Fe4N phase and is in-plane magnetic isotropic, while the samples≤119nm show the structure of FeN0.056nanocrystalline particles embedded in the amorphous matrix and have an evident in-plane uniaxial anisotropy. And with the increasingfilm thickness, the natural resonant frequency is also increased.When oblique sputtering method is used to induce in-plane magnetic anisotropy, it is found that obliquely sputtered films with different thicknesses all show the y’-Fe4N phase, and an obvious in-plane magnetic anisotropy is failed to obtained.3. With the N2flow rate ratio in the range of3%to6%, the nanocrystalline or amorphous Fe-N films obliquely sputtered at room temperature are easy to obtain in-plane uniaxial magnetic anisotropy.For the series of Fe-N films prepared at room temperature with different N2flow ratio (N2/Ar+N2), it is found that with increasing flow ratio of N2, the sample structure evolves from a-Fe to amorphous or nanocrystalline Fe-N, and finally s-Fe2occurs when N2flow is relatively large, and the saturation magnetization is monotonously decreased.With the N2flow rate ratio between3%and6%all samples exhibit good soft magnetic and in-plane uniaxial magnetic anisotropy, and with N2flow ratio increasing from3%to6%, the anisotropy field and the resonant frequency are reduced, and the resonance linewidth and corresponding damping factor are increased.By fitting magnetic permeability spectra and ferromagnetic resonance spectra, we have a clear understanding of the change of damping factor in Fe-N films.4. The in-plane uniaxial magnetic anisotropy is successfully tailored by oblique sputtering. It is foundthat when the N2flow rate ratiois5%, all Fe-N films have excellent soft magnetic properties, and the anisotropy fieldandeasy-axiscoercivityboth areincreased withthe sputteringincidence angleswhile the hard-axiscoercivity is maintained ata low value. At the sameobliqueangle, with the increase ofthe sputteringtime, theanisotropy fieldalso exhibitsa tendency to increase. Forthesample deposited for30min, with incidence anglebetween3°to32°,the resonant frequencyis adjustable from2.87GHzto6.05GHz.5. We also investigated the influence of obliquely sputtered Cu seed layer on magnetic properties of Permalloy thin films. For the samples of Cu (t nm)/Permalloy (50nm), a well-defined in-plane uniaxial anisotropy is obtained, and with increasing thickness t of the Cu seed layer, the damping factor of the sample is also increased gradually. For the samples of Cu (10nm)/Permalloy (d nm), there exists a critical thickness tc. When the Permalloy layer thickness d is less than tc,the sample displays an obvious uniaxial anisotropy. And with the increase of d, the uniaxial anisotropy field decreases;, when Permalloy layer thickness dis larger than tc, the stripe domain structure occurs in the sample, and with the increase of d, the stripe width is widened. We found that the obliquely sputtered Cu seed layer can effectively reduce the critical thickness of Permalloy films for stripe domain formation.