Isothermal-desorption-rate measurements in the vicinity of the Curie temperature for H2 chemisorbed on nickel films

Measurements of the isothermal desorption rate of H2 chemisorbed onto polycrystalline nickel films made for temperatures spanning the Curie temperature of the nickel film are presented. Desorption kinetics were followed by measuring the decay of the change in resistance of the nickel film brought about by hydrogen chemisorption after gas-phase H2 had been rapidly evacuated. The desorption rate is found to undergo an anomalous decrease in the vicinity of the Curie temperature, accompanied by an increase in the desorption activation energy and the equilibrium constant for the chemisorbed hydrogen. The results are interpreted in terms of anomalous variations in rate constants for the formation of the precursor molecular adsorbed state and the chemisorbed atomic state due to the phase transition in the nickel. The changes in rate constants are also considered to be in qualitative agreement with theoretical predictions based on a spin coupling between the adatom and the magnetic substrate.