Pd/Co bi-layer films for microwave-frequency hydrogen gas sensing applications

We investigate the ferromagnetic resonance response of palladium-cobalt bi-layer thin films to hydrogen charging at atmospheric pressure. Recently it has been found that hydrogen absorption by a palladium capping layer results in the narrowing and shifting of the ferromagnetic resonance peak of the underlying cobalt layer. This effect was explained as originating from a reduction in the spin pumping effect and a variation in the magnetic anisotropy of the cobalt layer. In this work we study the dependence of the shift of the resonance peak on the cobalt layer thickness. We observe a strong increase in the resonance peak field shift with a decrease in the cobalt layer thickness. In contrast, thicker Cobalt layers exhibit stronger changes in the absorption amplitude in the presence of hydrogen gas. Our results are important for optimization of Pd/Cobalt multi-layers as a novel platform for hydrogen gas sensing applications.