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

Inelastic neutron scattering (INS) from polycrystalline antiferromagnetic LaMnAsO, LaMnSbO, and BaMnAsF are analyzed using a J1-J2-Jc 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 (J1 and J2) are both antiferromagnetic (AFM). However, for all compounds studied the competing interactions have a ratio of 2J2/J1<1, which favors the square lattice checkerboard AFM structure over the stripe AFM structure. The interplane coupling Jc in all three systems is on the order of ∼3×10-4J1, 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 Néel temperature, TN. Although BaMnAsF shares the same MnAs sheets as LaMnAsO, their J1 and J2 values are substantially different. Using density functional theory, we calculate exchange parameters Jij to rationalize the differences among these systems.