We examined the magnetic anisotropy of Fe-Al-O films with Bs∼1.8T and ρ∼1/μΩm. The films were sputtered using the compacted target of Fe and Al2O3 powders. The magnetic anisotropy of the as-deposited Fe-Al-O films strongly depends on Ar pressure during sputtering. That is, the phase of easy axis shifts to 90 degree as the pressure increase. The Fe-Al-O films have an anisotropic film morphology such as grain shape and crystal orientation. It seems that the origin of uniaxial magnetic anisotropy in the Fe-Al-O films closely relates to the following factors, i) Magnetic anisotropy which directly originates in the anisotropic microstructure. ii) Magnetoelastic effect causing by anisotropic residual stress which is induced to the anisotropic microstructure.
The annealing temperature dependence of the tunnel magnetore-sistance (TMR) ratio for ferromagnetic tunnel junctions prepared by various oxidation methods was measured. A junction prepared by plasma oxidation showed a TMR ratio of 31.4% before heat treatment, and this ratio increased to 49% after annealing at 300°C. On the other hand, the TMR ratio for a junction prepared by radical oxidation showed a maximum at 350°C. The enhancement of the thermal stability resulted from the different oxidation progress depending on the oxidation method.
In the past we fabricated and put to practical use novel noise suppressor sheets made of micro-forged Fe-Si-Al flakes and polymer. These composite sheets exhibit a peculiar dual dispersion in the frequency characteristics of the imaginary part of the permeability. For use as a noise countermeasure, a steeply rising profile accompanying wide dispersion is required for the frequency characteristic of μ”. In this work, the mechanism that brings about the dual dispersion and a method for controlling it are investigated with respect to the micro-forging process that changes the raw granules of Fe-Si-Al alloy into very thin flakes. In the initial stage of forging, the granules and the flakes coexist and the skin depth is estimated to be between the sizes of the granules and the flakes. Subsequently, the coexistence of eddy current loss in the granules and magnetic resonance loss in the flakes leads to a dual-peak profile of μ” in the frequency range from about 10 MHz to over 1 GHz. The improved performance of conduction noise suppression for these sheets was verified by setting them on a micro-strip line and measuring the variation of the transmission parameters S11 and S12 associated with the dual peak-loss.
Fine flakes of Fe-Si-Al fabricated by forging for a long-duration exhibit a peculiar dual-peak dispersion in the frequency characteristics of the imaginary part of the permeability. This dual dispersion is composed of the dispersions D II and D III can be used to suppress noise in the quasi-microwave band. In a previous paper we claimed that the dispersion D II, which appears in the lower frequency range, is correlated to a shape anisotropy of the flakes. In this paper, the origin of the dispersion D III is studied by analyses of crystalline structure changes during the forging (100 h to 180 h) and annealing processes. X-ray diffraction study and measurements of Moessbauer spectra strongly suggest that the annealed flakes have a two-phase structure consisting of an Fe-rich mother alloy and a Si/Al rich surface oxide layer. From the development of this structure accompanied by enhancement of the dual-peak profile of μ” as the forging time increases, it is concluded that the dispersion D III is caused by the magneto-elastic effect near the surface layer. High performance of conduction noise suppression in the microwave band for this composite was verified by setting them on a micro-strip line and measuring the transmission loss characteristics associated with the μ” dispersion D III.