Magnetic structure map for face-centered tetragonal iron: Appearance of a collinear spin structure

For fcc and tetragonal distorted fct iron a large number of magnetic configurations as a function of crystal structural parameters were studied by means of density functional theory concepts. The stability of magnetic structures was defined by the magnetic reorientation energy "Ereori as the difference of the total energy of configuration i and that of the fcc ferromagnetic state. The cluster expansion technique was applied to six volumes deriving "Ereor for more than 90 000 collinear spin structures at each volume. Structures with low "Ereor were tetragonally distorted according to a two-dimensional mesh defined by volume per atom V and c/a ratio. At each mesh point "Ereor for all collinear structures were compared to results for spin spirals (SSs) which were calculated on a grid of propagation directions, and then the lowest "Ereor defined the magnetic structure map. Three local minima were identified and for each of the minima SSs were calculated on a fine grid of propagation vectors. At the minimum with V=10.6 Å3 and 0.94c/a1.01 a hitherto unknown simple collinear spin structure with four atoms per fct unit cell was the most stable one. It consists of two atoms with antiferromagnetically ordered local moments of ±1.8ÎB and of two atoms with zero local moment.