Sensitivity Enhancement of a Pd/Co Bilayer Film for Hydrogen Gas Sensing Using a Perpendicular-to-Plane Ferromagnetic Resonance Configuration

Previously, it has been shown that the strength of the perpendicular magnetic anisotropy (PMA) of thin palladium (Pd)–cobalt (Co) bilayer films can be modified when hydrogen gas is absorbed by the Pd layer. In our recent work, we showed that the ferromagnetic resonance (FMR) response of this material is sensitive to the changes in PMA upon exposure of Pd to hydrogen gas. As such, a simple, compact, and contactless hydrogen gas sensor could exploit the FMR-based detection of the reversible hydrogen-gas induced changes in PMA. The magnitude of the FMR peak shift is critical to determining detection sensitivity: the higher the FMR peak shift at a given hydrogen gas concentration, the higher the sensitivity. Here, we demonstrate that the detection sensitivity is enhanced when the static magnetic field is applied perpendicular to the film plane. This is due to the frequency large shift, which is eight times larger than in the in-plane FMR configuration studied previously. An analysis based on the Kittel equation
for FMR frequencies of an FM film is carried out in order to understand the mechanism of sensitivity enhancement. The result is important for optimizing Pd/Co bilayered thin films for use in novel platforms for hydrogen gas sensing.