Spin dynamics in antiferromagnetic oxypnictides and fluoropnictides: LaMnAsO, LaMnSbO, and BaMnAsF

Abstract

Inelastic neutron scattering (INS) from polycrystalline antiferromagnetic LaMnAsO, LaMnSbO, and BaMnAsF are analyzed using a ${J}_{1}\text{\ensuremath{-}}{J}_{2}\text{\ensuremath{-}}{J}_{c}$ Heisenberg model in the framework of the linear spin-wave theory. All three systems show clear evidence that the nearest- and next-nearest-neighbor interactions within the Mn square lattice layer (${J}_{1}$ and ${J}_{2}$) are both antiferromagnetic (AFM). However, for all compounds studied the competing interactions have a ratio of $2{J}_{2}/{J}_{1}<1$, which favors the square lattice checkerboard AFM structure over the stripe AFM structure. The interplane coupling ${J}_{c}$ in all three systems is on the order of $\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}{J}_{1}$, rendering the magnetic properties of these systems with quasi-two-dimensional character. The substitution of Sb for As significantly lowers the in-plane exchange coupling, which is also reflected in the decrease of the N\'eel temperature, ${T}_{\mathrm{N}}$. Although BaMnAsF shares the same MnAs sheets as LaMnAsO, their ${J}_{1}$ and ${J}_{2}$ values are substantially different. Using density functional theory, we calculate exchange parameters ${J}_{ij}$ to rationalize the differences among these systems.