A.R. Chezan

Controlling the induced anisotropy in soft magnetic films for high frequency applications

Summary form only given. Soft magnetic materials with controllable uniaxial anisotropy are required for future ultrahigh frequency applications. Nowadays, the attention is focused on FeXN (X = Ta, Zr, Al). systems due to their high saturation magnetisation, excellent magnetic softness and good corrosion resistance. The films can be obtained in an as-deposited nanocrystalline state by sputtering Fe rich alloys in a reactive atmosphere (Ar + N/sub 2/). Such materials can be magnetically soft when the grain size becomes smaller than the ferromagnetic exchange length (as explained by the random anisotropy model). The anisotropy is created by a bias magnetic field applied during deposition or during a subsequent annealing. One limiting factor restricting the frequency range of applications is the natural ferromagnetic resonance. In order to push this limit in the GHz range, films with a uniaxial anisotropy field in excess of 20 Oe are desired. We have obtained films with excellent soft magnetic properties and controllable in plane uniaxial magnetic anisotropy by depositing Fe N films at different substrate temperatures. The induced in-plane anisotropy increases with increasing nitrogen content. Increasing the Zr/Fe ratio from 1/99 to 3/97 has little influence on the induced anisotropy.

Microstructure of nanocrystalline FeZr(N)-films and their soft magnetic properties

Abstract The microstructure of nanocrystalline FeZr(N) films, deposited by DC sputtering in a Ar+N 2 atmosphere was studied in correlation with their soft magnetic properties. The micromagnetic properties of the films were investigated using the Fresnel mode of Lorentz microscopy.

The influence of the surface topography on the magnetization dynamics in soft magnetic thin films

In this work we study the influence of surface roughness on the magnetization dynamics of soft magnetic nanocrystalline Fe–Zr–N thin films deposited (under identical conditions) onto a Si oxide, a thin polymer layer, and a thin Cu layer. The substrate temperature during deposition was approximately −25°C ensuring a nanocrystalline state. The demagnetizing factors due to sample roughness were calculated based on atomic force microscopy (AFM) analysis of the surface topography. A clear correlation between sample roughness and the width of the high-frequency response is observed. The local random demagnetizing field created by the nanocrystalline structure and the surface topography is responsible for the positive shift of the ferromagnetic resonance frequency. In addition, a pronounced effect of line broadening is induced by the surface topography at large wavelengths. Finally, we show a good agreement between the values of the average demagnetizing field 4πNMS as calculated from the AFM scans, and the valu...

Thermal stability of ultrasoft Fe-Zr-N films

The thermal stability of nanocrystalline ultrasoft magnetic (Fe98Zr2)1−xNx films with x = 0.10–0.25 was studied using thermal desorption spectrometry, positron beam analysis and high resolution transmission electron microscopy. The results demonstrate that grain growth during the heat treatment is accompanied by an increase of the free volume and nitrogen relocation and desorption. All these phenomena can drastically degrade the ultrasoft magnetic properties. The nitrogen desorption has already started at temperatures around 400 K. Nevertheless, most of the nitrogen leaves the sample at a temperature above 800 K. We found that nitrogen out-diffusion is significantly retarded compared with the prediction of the diffusion in bulk α-Fe. A qualitative model is proposed in which the nitrogen out-diffusion in nanocrystalline material is retarded by trapping at immobile defects, namely Zr atoms, and also by voids at grain boundaries. From a certain temperature, nitrogen migrates from the interior of the nanograins to the nanovoids at the grain boundaries and the out-diffusion to the outer surface is controlled by transport between the voids.

The role of microstructural aspects on the performance of coarse-grained superplastic Al alloys

Deformed under optimum conditions of temperature and strain rate, coarse-grained aluminum alloys show elongation to failure in excess of 300%. The strain rate sensitivity index and the activation energy point to solute drag creep as the principal mechanism, a mechanism that has virtually no grain size dependence. The present study summarizes microstructural effects that are grain size dependent and which can influence the values of the maximum tensile elongation that can be obtained in coarse-grained aluminum alloys. Such effects like inhomogeneous refinement of the microstructure accompanied by the increase of the ratio of low/high angle grain boundaries and that of the texture contributes to flow stress instabilities leading to necking and premature failure.

Relation between observed micromagnetic ripple and FMR width in ultrasoft magnetic films

Summary form only given. It was demonstrated that nanocrystalline FeXN films (X is an alloying element), obtained by sputtering or electrodeposition, have excellent ultra-soft magnetic properties with a saturation magnetization up to M/spl ap/2.0 T, a high magnetic susceptibility and a frequency range above 1 GHz. Here we report on a correlation between the microstructure, the micromagnetic ripples, and high frequency magnetism in the sputter-deposited FeZrN-films. The range of operating frequencies for the films is limited by the frequency of the ferromagnetic resonance (FMR) and by the width of FMR. Besides contributions to the FMR width due to dissipation sources, which are characteristic for crystalline and polycrystalline ferromagnetics, in nanocrystalline films there exists an additional contribution due to the local variation of the magnetic uniaxial anisotropy. Notwithstanding its importance, so far only a very few studies have been reported in the literature.

Evolution of nanaostructure of FeNi(Ti/Cr)N alloy during phase cycling

A possibility to manipulate the microstructure of cold rolled (down to less than 1% of the initial thickness) Fe94Ni4Ti2 and Fe93 Ni4 Cr3 foils via inter-phase cycling by nitriding and de-nitriding is investigated. The grains in the as-rolled material had a dominating (001)[110] texture and contained a complicated internal nanostructure due to a dislocation network. The interphase cycling α↔γ’-Fe4N and α↔ε-FexN (x~3) was investigated as a tool to change or destroy the texture. We observed that the α↔γ’-Fe4N phase transformations weaken but do not erase the texture. A stronger reduction of the texture was obtained in the α↔ε-cycling, where grains with a new orientation appear in XRD scans. During the α→γ’ phase transformation a lamella structure is formed which is coarsening with the number of cycles. It was found that the rates of the α↔γ’ and α↔ε interphase transitions were slowing down with the number of cycles. The observation is explained by an increase of kinetic barriers while removing/transforming the rolling-induced defects during the cycling.

On the GHz frequency response in nanocrystalline FeXN ultra-soft magnetic films

The periodicity and angular spread of the in-plane magnetization for ultrasoft nanocrystalline FeZrN films were estimated from an analysis of the ripple structure, observed in Lorentz transmission electron microscopy (LTEM) images. The influence of the micromagnetic ripple on the ferromagnetic resonance (FMR) width is analyzed using an approach based on the Landau-Lifshitz equation. A strong dependence of the resonance width on the magnetic moment dispersion is predicted. To a large extent this particular aspect explains the high frequency response in some of our films.

Evolution of free volume in ultrasoft magnetic FeZrN films during thermal annealing

The thermal stability of nanocrystalline ultra-soft magnetic (Fe98Zr2)(1-x)N-x films with x=0.10-0.25 was studied using high-resolution transmission electron microscopy (HRTEM), positron beam analysis (PBA) and thermal desorption spectrometry (TDS). The results demonstrate that grain growth during the heat treatment is accompanied by an increase of the free volume, by nitrogen reallocation and desorption. All this can drastically deteriorate the ultrasoft magnetic properties. The desorption starts already at slightly elevated temperatures, below 100 degreesC. However, most of the nitrogen leaves the sample at a temperature above 500 degreesC.