Microstructure, soft magnetic properties, and applications of high resistive Fe-M-O (M=Hf, Zr, rare-earth metals) were studied. The Fe-M-O films are composed of bcc nanograins and amorphous phases with larger amounts of M and O elements which chemically combine each other. Consequently, the amorphous phases have high electrical resistivity. The compositional dependence of magnetic properties, electrical resistivity, and structure have been almost clarified. For example, the high magnetization of 1.3 T, high permeability of 1400 at 100 MHz and the high electrical resistivity of 4.1 μΩ m are simultaneously obtained for as-deposited Fe62Hf11O27 nanostructured film fabricated by rf reactive sputtering in a static magnetic field. Furthermore, Co addition to Fe-M-O films improves the frequency characteristics mainly by the increase in the crystalline anisotropy of the nanograins. The Co44.3Fe19.1Hf14.5O22.1 film exhibits the quality factor (Q=μ′/μ′′) of 61 and the μ′ of 170 at 100 MHz as well as the high Is of 1.1 T. This frequency characteristics is considered to be superior to the other films already reported. The films also exhibit high corrosion resistance in an isotonic sodium chloride solution. Therefore, these films enable us to realize the high-frequency magnetic devices, such as thin-film inductors and transformers for microswitching converters and ultrahigh-density recording heads.

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