Range of Ferromagnetic Exchange Interaction in Nonmagnetic Films on Iron Substrates

Abstract

An experiment has been performed to measure the penetration of conduction electron spin polarization into nonmagnetic metallic films deposited on an iron substrate. The technique was to evaporate the metal in question on the iron and then to probe the effective exchange field, produced by the electron spin polarization, as a function of the distance from the substrate by evaporating an intermediate partial layer of 57Co. The hyperfine field of the 57Fe was measured by the Mössbauer method and the exchange was calculated from the hyperfine field under the assumption that the hyperfine field is related to the exchange field through the Brillouin function of the local spin. Measurements were carried out at 300°, 78°, 4.20°, and 1.22°K. Samples using palladium on an iron substrate were investigated with the 57Fe placed from 20 to 120 lattice spacings away from the substrate with the following results: (1) The size of the exchange field varies from 0° to ∼150°K in the thinnest sample and from 0° to 25°K in the thickest. (2) For samples of given thickness, an empirical probability distribution function for the exchange field was found which gave a good fit to the hyperfine spectra at all temperatures. (3) The probability function is maximum at zero exchange field, decays initially quite rapidly, and then tails off slowly. A few measurements have been made with copper and silver as the intermediate metal. The indication is that the effective exchange field in copper is greater than in silver but considerably less than in palladium.