PHYSICAL BASIS AND BREAK DOWN OF HYPERFINE FIELD DISTRIBUTION ANALYSIS IN FCC FE-NI (5-70 AT-PERCENT-FE)

Room temperature Mossbauer spectra of 15 splat-quenched Fe-Ni alloys in the composition range of 5-70 at%Fe and low and high temperature spectra of an Fe65Ni35 Invar sample have been analyzed by a new hyperfine field distribution analysis method in which the arbitrary shape distributions are expanded in terms of Gaussian components. At compositions C less-than-or-equal-to 45 at%Fe, where collinear ferromagnetism occurs, we observe a linear relation between the average room temperature hyperfine field and the alloy composition and find that the extracted hyperfine field distributions are accurately modelled by random distributions of near neighbour environments, with no free parameters. At C > 45 at%Fe, this simple near neighbour model breaks down because of partial antiferromagnetism (or spin canting). At C > 60 at%Fe, the hyperfine field distribution analysis itself breaks down - as indicated by unphysical changes in some of the hyperfine parameters. The latter anomalies are removed at low temperatures and are argued to be caused by dynamic lineshape effects that become significant as the temperature is increased towards the Curie point (at constant C, or by increasing C --> 70 at%Fe at constant T).