  Effect of hydrogen adsorption on the extraordinary Hall phenomenon (EHE) in
ferromagnetic CoPd films is studied as a function of composition, thickness,
substrate and hydrogen concentration in atmosphere. Adsorption of hydrogen adds
a positive term in the extraordinary Hall effect coefficient and modifies the
perpendicular magnetic anisotropy with the respective changes in coercivity and
remanence of hysteresis loops. Hydrogen sensitive compositions are within the
Co concentration range 20% < x < 50% with the strongest response near the EHE
polarity reversal point x_0 ~ 38%. Depending on the film composition and field
of operation the EHE response of CoPd to low concentration hydrogen can reach
hundreds percent, which makes the method and the material attractive for
hydrogen sensing.
