Magnetic structure and spin excitations in BaMn$_2$Bi$_2$*

The magnetic structure and associated spin wave excitations of the recently synthesized BaMn$_2$Bi$_2$ have been studied using neutron scattering. BaMn$_2$Bi$_2$ exhibits the same ThCr$_2$Si$_2$ crystal structure as the 122 iron superconductors (AFe$_2$As$_2$). Single crystal neutron diffraction reveals that the ordered state below T$_N$ $\sim$ 390 K is consistent with G-type antiferromagnetic order and suggests the presence of a structural phase transition at 100 K. Inelastic neutron scattering reveals anisotropic spin waves characterized by a gap of 16 meV, in-plane excitations with a maximum energy of 55 meV and a c-axis dispersion extending to about 35 meV. The observed magnetic excitations are well described by a J$_1$-J$_2$-J$_c$ Heisenberg Hamiltonian and the relevant exchange interactions are extracted. The results will be compared to other related materials such as BaFe$_2$As$_2$ and BaMn$_2$As$_2$.