Interplay between magnetic properties and Fermi surface nesting in iron pnictides

Abstract

The wave-vector $(\mathbf{q})$ and doping $(x,y)$ dependences of the magnetic energy, iron moment, and effective exchange interactions in ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ and ${\text{Ba}}_{1\ensuremath{-}2y}{\text{K}}_{2y}{\text{Fe}}_{2}{\text{As}}_{2}$ are studied by self-consistent LSDA calculations for co-planar spin spirals. For the undoped compounds $(x=0,y=0)$, the minimum of the calculated total energy, $E(\mathbf{q})$, is for $\mathbf{q}$ corresponding to stripe antiferromagnetic order. Already at low levels of electron doping $(x)$, this minimum becomes flat in ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ and for $x\ensuremath{\gtrsim}5%$, it shifts to an incommensurate $\mathbf{q}$. In ${\text{Ba}}_{1\ensuremath{-}2y}{\text{K}}_{2y}{\text{Fe}}_{2}{\text{As}}_{2}$, stripe order remains stable for hole doping up to $y=0.3$. These results are explained in terms of the band structure. The magnetic interactions cannot be accurately described by a simple classical Heisenberg model and the effective exchange interactions fitted to $E(\mathbf{q})$ depend strongly on doping. The doping dependence of the $E(\mathbf{q})$ curves is compared with that of the noninteracting magnetic susceptibility for which similar trends are found.