  Previously, it has been shown that the strength of the perpendicular magnetic
anisotropy (PMA) of thin Palladium-Cobalt bi-layer films can be modified when
hydrogen gas is absorbed by the Palladium (Pd) layer. In our recent work we
showed that the ferromagnetic resonance (FMR) response of this material is
sensitive to changes in PMA upon exposure of Pd to hydrogen gas. As such, a
simple, compact and contactless hydrogen gas sensor could exploit FMR-based
detection, of the reversible hydrogen-gas-induced changes in PMA. The magnitude
of the FMR peak shift is critical in determining the sensor's sensitivity: the
higher the FMR peak shift at a given hydrogen gas concentration, the higher the
sensitivity. In the present work we demonstrate that the detection sensitivity
is enhanced when the static magnetic field is applied perpendicular to the film
plane. A factor of eight times improvement is observed with respect to the
in-plane FMR configuration studied previously. An analysis based on the Kittel
equation for FMR frequencies of a ferromagnetic film is carried out in order to
understand the mechanism of sensitivity enhancement. The result is important
for optimizing Pd/Co bi-layered thin films for use in novel platforms for
hydrogen gas sensing.
