Antiferromagnetic Exchange, Hunds Coupling and the Origin of the Charge Gap in LaMnPO

LaMnPO is an antiferromagnetic insulator with an ordering temperature T$_{N}$ = 375 K, ordered moment of 3.2 $\mu_{B}$/Mn and a charge gap $\Delta$ = 1 eV. We present inelastic neutron scattering and magnetization data that are well described by a Heisenberg model of magnetic interactions with nearest neighbour exchange SJ$_{1}$$\sim$39 meV and next-nearest exchange SJ$_{2}$$\sim$12 meV. These measurements also show magnetic correlations persist up to T$_{max}$$\sim$700 K, significantly larger than T$_{N}$ due to the effectively decoupled MnP layers. High temperature optical transmission measurements show the charge gap has decreased by $\approx$ 10\% by T$_{max}$ suggesting the rather small exchange interactions J $\ll$ $\Delta$ have only a small effect on the gap. Density functional theory and dynamical mean field theory DFT+DMFT reproduce the observed gap in the paramagnetic state of LaMnPO only in the presence of strong Hunds coupling J$_{H}$, as well as onsite Coulomb interactions U. In light of these experimental and theoretical results, LaMnPO should be considered a Mott-Hunds insulator.