Abstract
Iron-aluminum alloys with less than 50 atomic percent aluminum are known to exhibit an interesting phase diagram, including paramagnetic, ferromagnetic, and spin-glass regions. We present a microscopic model of this behavior, involving competition between direct ferromagnetic Fe-Fe interactions and indirect antiferromagnetic Fe-Al-Fe superexchange interactions. The model incorporates the experimentally observed fact that only Fe atoms with four or more Fe nearest neighbors have fully developed magnetic moments. By performing an approximate decimation-type pretransformation, we are able to map this model onto an equivalent nearest-neighbor simple-cubic bond-random Ising model, the properties of which have been previously studied by mean-field and renormalization-group methods. We are, thus, able to derive theoretical phase diagrams in the temperature-concentration plane based on three model parameters: the two elementary interaction strengths (Fe-Fe and Fe-Al-Fe) and the (weak) moment of a triply coordinated iron atom. Assignment of physically reasonable values to these parameters leads to a phase diagram in semi-quantitative agreement with experiment. We treat alloys with both Fe3Al and FeAl-type structure. Research supported in part by NSF grants DMR-77-23999 and DMR-75-22241.
Published Version
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