Magnetic order and defect structure ofFexAl1−xalloys aroundx=0.5: An experimental and theoretical study

Abstract

High-field ${}^{57}\mathrm{Fe}$ M\ossbauer investigations up to 13.5 T on a series of ${\mathrm{Fe}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}$ alloys around $x=0.5$ and magnetic measurements on a 51.8% sample are performed between 4.2 and 295 K. The experimental results are complemented with augmented spherical-wave (ASW) and linear augmented plane-wave (LAPW) band structure as well as thermodynamic model calculations. For ideally ordered FeAl $(B2$ structure) both types of band-structure calculations yield ${\ensuremath{\mu}}_{\mathrm{Fe}}=0.71{\ensuremath{\mu}}_{B}.$ The ferromagnetic ground state is 0.7 mRy per formula unit below the nonmagnetic state. In experiment it was found that only approximately 25% of the Fe atoms carry a magnetic moment. This discrepancy can be explained if noncollinear spin ordering is allowed and a high density of defects is taken into account, which is typical for real alloys and destroys translational periodicity. Experimentally magnetic moments are only observed for Fe antistructure atoms and their eight Fe neighbors. This nine-atom cluster has a mean moment of $0.4{\ensuremath{\mu}}_{B}$ per atom, which is in fair agreement with the results from supercell (16 and 54 atoms) calculations $(0.6{\ensuremath{\mu}}_{B}).$ For the M\ossbauer analysis four subspectra are used, which are allocated to (i) Fe in the completely ordered $B2$ structure, (ii) Fe antistructure atoms, and (iii) their nearest Fe neighbors, as well as (iv) Fe atoms around a vacancy in the Fe sublattice. This analysis allows us to obtain simultaneously the concentration of both the Fe vacancies and the Fe antistructure atoms. The derived temperature dependence for both defect types corresponds well with thermodynamic model calculations, which account for all possible kinds of point defects.