The high-frequency properties of soft ferromagnetic Fe-Co-Hf-N films with an in-plane uniaxial anisotropy show temperature and stress induced ferromagnetic resonance frequency changes. Therefore, the films allow to monitor thermal and mechanical induced stress which is promising for high frequency sensor applications. One sensor application is to use these films in combination with wear resistant coatings to measure the temperature and stress of cutting tools in metal processing. Consequently it is necessary to deposit the film system on WC-Co substrates, a material which is used for cutting tools. The problem hereby is a hard ferromagnetic Co phase of about 10 wt%. This leads to magnetic exchange interactions between the substrate and the soft ferromagnetic film. Due to the exchange effects the in-plane uniaxial anisotropy is disturbed and the ferromagnetic resonance absorption in the frequency range from 50 MHz up to 5 GHz cannot be observed, which leads to a loss of the sensor functionality.
The Fe-Co-Hf-N film can be magnetically decoupled from the WC-Co substrate by using a non-ferromagnetic interlayer. For this purpose two buffer layer materials Ti-Al-N and Si-O were investigated. Ti-Al-N is a non-ferromagnetic, electric conductive hard coating which is used as a wear resistant protective layer. In contrast to Ti-Al-N, Si-O is not resistant to wear but it is electrically insulating and also non-ferromagnetic. The buffer layer is deposited by r.f. magnetron sputtering on WC-Co substrates. On top of the interlayer a 200 nm thick Fe-Co-Hf-N film was deposited which exhibits a saturation polarization of 1.4 T and an in-plane uniaxial anisotropy field of 4.5 mT after annealing it in a static magnetic field. In order to determine the complex permeability, the film system was measured by a strip-line permeameter setup. The results show that the buffer layers provide a decoupling which leads into a measurable ferromagnetic resonance absorbance at a frequency of 2.2 GHz. The decoupling with Si-O instead of Ti-Al-N leads to well defined resonance lines. The explanation for this behavior was attributed to the formation of eddy-currents in the electrically conductive material causing a magnetic field which perturbs the r.f. field of the strip-line.