Anisotropic itinerant magnetism and spin fluctuations in BaFe2 As2: A neutron scattering study

Neutron scattering measurements were performed to investigate magnetic excitations in a single-crystal sample of the ternary iron arsenide BaFe2 As2, a parent compound of a recently discovered family of Fe-based superconductors. In the ordered state, we observe low energy spin-wave excitations with a gap energy Δ=9.8 (4) meV. The in-plane spin-wave velocity vab and out-of-plane spin-wave velocity vc measured at 12 meV are 280(150) and 57 (7) me, respectively. At high energy, we observe anisotropic scattering centered at the antiferromagnetic wave vectors. This scattering indicates two-dimensional spin dynamics, which possibly exist inside the Stoner continuum. At TN =136 (1) K, the gap closes and quasielastic scattering is observed above TN, indicative of short-range spin fluctuations. In the paramagnetic state, the scattering intensity along the L direction becomes "rodlike," characteristic of uncorrelated out-of-plane spins, attesting to the two-dimensionality of the system.