Spin Dynamics in the Antiferromagnetic Oxy- and Fluoro- Pnictides: LaMnAsO, LaMnSbO, and BaMnAsF*

Inelastic neutron scattering (INS) from polycrystalline LaMnAsO, LaMnSbO, and BaMnAsF are analyzed using J₁-J₂-J_c Heisenberg model in the framework of the linear spin-wave theory. All three systems show clear evidence that J₁ and J₂ are both antiferromagnetic (AFM) with a ratio of 2J₂/J₁ < 1. Thus, the competing interactions do not lead to the stripe structure, preserving the square lattice checkerboard AFM structure. The inter-plane coupling J_c in all three systems is on the order of ≈ 3×10^-4J₁, rendering the magnetic properties of these systems quasi-two-dimensional. The substitution of Sb for As significantly lowers the in-plane exchange coupling, which is also reflected in the Néel temperature T_N. Although BaMnAsF shares similar MnAs sheets as LaMnAsO, their J₁-J₂ values are substantially different. Using density functional theory, we calculate exchange parameters J_ij to rationalize the differences between these systems.