Magnetic Structures and Stresses in Thin Films II. Thermodynamical Treatment and Discussion

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Abstract
Using a thermodynamic treatment of thin film magnetism, UHV‐experiments on the ferromagnetic resonance of Fe‐, NiFe‐, and Ni‐thin films in the 2– 50 monolayer range can be evaluated. – The discussion of the results about clean surfaces leads to the following conclusions: (a) Magnetic “dead layers” of Ni can be excluded with an accuracy of 0.2 monolayers. b) Results from the spin wave theory are applicable to explain the temperature and thickness dependence of the saturation magnetization of Ni although the films are not ideal on a microscopical scale, c) Magnetic surface anisotropies can be explained by Néel's phenomenological approach, d) Due to the high magnetostriction of Ni the data are affected by mechanical stresses. Therefore mechanical stresses of thin films can be estimated from experimentally determined magnetic anisotropic fields. Tentatively we describe these effects by simple models including stresses from interfacial energies. thermal expansions and grain boundaries. – Chemisorption of H2 and CO on Ni at T±500 K leads to changes in the surface magnetism: a)“Dead layers” are induced by chemisorption. One hydrogen atom diminishes the contribution of one nickel atom to the thin film ferromagnetism. Twice the effect is found for CO adsorption. b) Magnetic surface anisotropies are reduced. c) Mechanical stresses are reduced too. This effect depends on the coverage of chemisorbed particles, the film thickness and the concentration of crystallite boundaries. Only its lower limit can be estimated using a thermodynamic treatment of reduced surface free energies.

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