Resolving the magnetic and structure anomalies in face-centred cubic iron-nickel alloys: A solution to the Invar and anti-Invar problems.

We have studied the magnetic and structural properties of synthetic and meteoritic face centred cubic (FCC) Fe-Ni alloys in an effort to understand the underlying mechanisms responsible for the Invar and anti-Invar effects. We have performed a detailed 57Fe Mossbauer spectroscopic and electronic imaging study of the Santa Catharina meteorite, an FCC Fe-Ni meteorite with a bulk composition close to that of Invar (Fe65Ni 35). Its microstructure consists of islands of tetrataenite in a matrix of Fe-rich antitaenite. Using Mossbauer spectroscopy (MS) and electron probe microanalysis (EPMA), we have determined that antitaenite is a low moment alloy with an Fe content of 88 +/- 2 at. %. Antitaenite appears to be stabilized in the FCC phase by its epitaxial relation with tetrataenite. Its Neel temperature is ∼60 K and its isomer shift (IS) is close to that of gamma-Fe. The effect of the magnetic interaction with tetrataenite has been studied by Monte Carlo (MC) simulations and have been shown to be consistent with the anomalous temperature dependence of the width of the antitaenite's Mossbauer spectrum. Using electronic structure calculations (ESCs), we have studied the T = 0 K properties of chemically ordered and disordered FCC Fe-Ni alloys in the ferromagnetic (FM), non-magnetic (NM) and disordered local moment (DLM) states. A significant drop in the IS and in the atomic volume are shown to occur at the transition from the high moment (HM) FM phase to the NM phase which occurs, according to our calculations, at 74.5 at. % Fe. Our calculations of the DLM and FM states show that in Fe-rich Invar alloys, the FM order contributes to stabilizing large moment magnitudes. Our systematic study of the hyperfine fields (HF) by ESCs supports the phenomenological model of Dang and Rancourt (1996a). Calculations on a supercell of Fe11Ni5 show the effects of nearest neighbour (NN) chemical and magnetic environments on the local magnetic moment magnitude and the HF distribution, thereby explaining the main observed features in Fe-rich alloys. Using the IS as a direct probe of the electronic structure, a HM/LM (low moment) transition has been unambiguously observed to occur at ∼70 at. % Fe. Our results have allowed us to conclude that the Invar effect in Fe-Ni is not directly caused by HM/LM admixture or so-called 2-gamma-state like excitations. (Abstract shortened by UMI.