Enhancement of high-frequency permeability of FeCoHf films by surface oxidation

Nanocrystalline FeCoHf films with crystal size about 10 nm were prepared by radio-frequency magnetron sputtering method at room temperature. The films were annealed at 400degC for 1 h in the presence of magnetic field of 2 kG. Then the films were cooled in vacuum or floating Ar/O mixed gas (referred to as AV and AO, respectively). Bright-argentine and light-brown surfaces were observed for AV and AO, respectively. The average O composition of samples AV and AO was 5.3 and 14.6 at%, respectively. Comparing with sample AV, evidently enhanced anisotropy field, resistivity and ferromagnetic resonance frequency over 3 GHz were achieved in AO. It is revealed that the surface oxidation is an effective way to increase the resonance frequency of high-frequency ferromagnetic films.