Thermal Stability of the Ferromagnetic In-Plane Uniaxial Anisotropy of Fe-Co-Hf-N/Ti-N Multilayer Films for High-Frequency Sensor Applications

In this study, the thermal stability of the in-plane uniaxial anisotropy field of Fe₃₂Co₄₄Hf₁₂N₁₂/Ti₅₀N₅₀ multilayer films was investigated from room temperature up to 500 °C with regard to contactless high-frequency sensor applications. The uniaxial anisotropy field of about μ_oH_u ≈ 5 mT was thermally induced by annealing the films in a static magnetic field of 50 mT in vacuum for one hour either at an annealing temperature of T_α = 400 °C or T_α = 600 °C. The films which were annealed at T_α = 600 °C show a clear distinction between the easy and hard axis of polarization up to temperatures of 500 °C. Hence, the uniaxial anisotropy, thermally induced at T_α = 600 °C, is stable at working temperatures up to 500 °C. In contrast, films with a uniaxial anisotropy induced at T_α = 400 °C show a different behavior. Here, hysteresis loop measurements indicate that above a working temperature of 200 °C the uniaxial anisotropy field turns away from its direction at room temperature. In conclusion, only the Fe₃₂Co₄₄Hf₁₂N₁₂/Ti₅₀N₅₀ multilayer films with a uniaxial anisotropy field induced at T_α = 600 °C promise a great potential for contactless high-frequency sensor applications up to 500 °C.