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 Fe
₃₂
Co
₄₄
Hf
₁₂
N
₁₂
/Ti
₅₀
N
₅₀
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
_o
H
_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 Fe
₃₂
Co
₄₄
Hf
₁₂
N
₁₂
/Ti
₅₀
N
₅₀
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.