Magnetic structure and spin excitations in BaMn2Bi2

We present a single-crystal neutron scattering study of BaMn2Bi2, a recently synthesized material with the same ThCr2Si2-type structure found in several Fe-based unconventional superconducting materials. We show long-range magnetic order, in the form of a G-type antiferromagnetic structure, exists up to 390 K with an indication of a structural transition at 100 K. Utilizing inelastic neutron scattering, we observe a spin gap of 16 meV, with spin waves extending up to 55 meV. We find these magnetic excitations are well fit to a J(1)-J(2)-J(c) Heisenberg model and present values for the exchange interactions. The spin-wave spectrum appears to be unchanged by the 100 K structural phase transition.