High-frequency permeability of soft-magnetic Fe–Hf–O films with high resistivity

A series of Fe–Hf–O films has been prepared by means of rf sputter deposition. The composition for optimal permeability was found to be Fe55Hf17O28. These films exhibit a combination of soft-magnetic properties (μ>103, Hc<50 A/m, μ0Ms⩾1.0 T) and a high resistivity (ρ>1 mΩ cm) after rapid thermal annealing in flowing argon at 400 °C in a dc magnetic field. The structure of the films consists of α-Fe nanocrystallites, randomly dispersed in an amorphous matrix. The amorphous phase is essential in achieving the advantageous combination of properties. In contrast to conventional soft-magnetic films, the properties of the Fe55Hf17O28 films do not show a strong thickness dependence for thicknesses up to 10 μm. Therefore, the roll-off frequency of the initial relative permeability for 10 μm thick films is high, but not as high as expected from only the resistivity. It is shown that, besides electromagnetic induction, ferromagnetic resonance is also essential in explaining the observed frequency dependence of the permeability.