High-frequency magnetoelastic measurements on Fe–Co–Hf–N/Ti–N multilayer coatings

Abstract Fe 32 Co 44 Hf 12 N 12 /Ti 50 N 50 thin multilayer coatings are realized as wear resistant coatings with an integrated sensor function, in order to detect e.g. external mechanical stress. A setup was designed to monitor changes in the frequency-dependent permeability due to the mechanically induced stress in the ferromagnetic Fe 32 Co 44 Hf 12 N 12 single layers. The change of the ferromagnetic cut-off frequency is described by an effective in-plane anisotropy field H → u , e f f which accounts for a thermally induced uniaxial anisotropy field H → u and an effective biaxial magnetoelastic anisotropy field H → m e . Fe 32 Co 44 Hf 12 N 12 /Ti 50 N 50 multilayer films with seven bilayers and a total thickness of about 1 μm were grown by a sequential deposition of Fe 32 Co 44 Hf 12 N 12 and Ti 50 N 50 individual layers by means of reactive DC and RF magnetron sputtering, respectively. Additionally, a Fe 32 Co 44 Hf 12 N 12 monolayer coating with a thickness of about 500 nm was deposited in order to determine the magnetic properties of the single layer. The films were investigated with respect to their static and dynamic ferromagnetic properties. A uniaxial anisotropy field of 5 mT could be induced after annealing the multilayer films for 1 h at 400 °C in vacuum in a static magnetic field of 50 mT. Additionally, magnetic softness ( μ 0 H c =0.6 mT) and cut-off frequencies above 2 GHz were observed. Inducing compressive stress increases the cut-off frequency of the Fe 32 Co 44 Hf 12 N 12 single layer as well as of the Fe 32 Co 44 Hf 12 N 12 /Ti 50 N 50 multilayer films whereas tensile stress causes a decrease in the cut-off frequency. Accordingly, the dependence of the cut-off frequency on external mechanical stress is theoretically described. Furthermore, the possibility of detecting mechanically induced stress in a ferromagnetic layer was demonstrated by the shift of the cut-off frequency with a resolution of 1.0 MPa.