Mössbauer Study and Molecular Field Theory of the Magnetic Properties of Fe–Al Alloys

Abstract

Mössbauer studies of the antiferromagnetic alloys, Fe_1−xAl_x (x≈0.33 to 0.47), have been made from 1.8° to 295°K. Supplementary low‐temperature susceptibility measurements were also carried out. The alloys have the CsCl structure and are best described as two interpenetrating simple cubic lattices, one of pure Fe, the other having composition Al_1−cFe_c (c=1−2x). Fe atoms on the Al_1−cFe_c sublattice (Fe_A) have nearly the full moment of pure Fe, while most atoms on the Fe sublattice (Fe_F) have zero moment. The hyperfine fields at both sites have a T² temperature dependence for ${\mathrm{(T/T}}_N{)}^2\text{≲0.6}$ . At T = 0, the Fe_A field consists of a −145‐kG core polarization field plus an RKKY conduction electron polarization (cep) field, while the Fe_F field is given primarily by an RKKY cep field. The RKKY cep fields and Néel temperatures increase linearly with c. A molecular field theory is outlined which considers RKKY exchange interactions between Fe spins out to fifth nearest‐neighbor distances and a direct d electron exchange interaction between nearest‐neighbor spins. The theory explains the observed properties of the antiferromagnetic state and predicts with reasonable accuracy the magnetic phase diagram of the whole Fe–Al system.